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ddassl.f

      SUBROUTINE DDAINI (X, Y, YPRIME, NEQ, RES, JAC, H, WT, IDID, RPAR,
     +   IPAR, PHI, DELTA, E, WM, IWM, HMIN, UROUND, NONNEG, NTEMP)
      common/ierode/iero

C***BEGIN PROLOGUE  DDAINI
C***SUBSIDIARY
C***PURPOSE  Initialization routine for DDASSL.
C***LIBRARY   SLATEC (DASSL)
C***TYPE      DOUBLE PRECISION (SDAINI-S, DDAINI-D)
C***AUTHOR  PETZOLD, LINDA R., (LLNL)
C***DESCRIPTION
C-----------------------------------------------------------------
C     DDAINI TAKES ONE STEP OF SIZE H OR SMALLER
C     WITH THE BACKWARD EULER METHOD, TO
C     FIND YPRIME.  X AND Y ARE UPDATED TO BE CONSISTENT WITH THE
C     NEW STEP.  A MODIFIED DAMPED NEWTON ITERATION IS USED TO
C     SOLVE THE CORRECTOR ITERATION.
C
C     THE INITIAL GUESS FOR YPRIME IS USED IN THE
C     PREDICTION, AND IN FORMING THE ITERATION
C     MATRIX, BUT IS NOT INVOLVED IN THE
C     ERROR TEST. THIS MAY HAVE TROUBLE
C     CONVERGING IF THE INITIAL GUESS IS NO
C     GOOD, OR IF G(X,Y,YPRIME) DEPENDS
C     NONLINEARLY ON YPRIME.
C
C     THE PARAMETERS REPRESENT:
C     X --         INDEPENDENT VARIABLE
C     Y --         SOLUTION VECTOR AT X
C     YPRIME --    DERIVATIVE OF SOLUTION VECTOR
C     NEQ --       NUMBER OF EQUATIONS
C     H --         STEPSIZE. IMDER MAY USE A STEPSIZE
C                  SMALLER THAN H.
C     WT --        VECTOR OF WEIGHTS FOR ERROR
C                  CRITERION
C     IDID --      COMPLETION CODE WITH THE FOLLOWING MEANINGS
C                  IDID= 1 -- YPRIME WAS FOUND SUCCESSFULLY
C                  IDID=-12 -- DDAINI FAILED TO FIND YPRIME
C     RPAR,IPAR -- REAL AND INTEGER PARAMETER ARRAYS
C                  THAT ARE NOT ALTERED BY DDAINI
C     PHI --       WORK SPACE FOR DDAINI
C     DELTA,E --   WORK SPACE FOR DDAINI
C     WM,IWM --    REAL AND INTEGER ARRAYS STORING
C                  MATRIX INFORMATION
C
C-----------------------------------------------------------------
C***ROUTINES CALLED  DDAJAC, DDANRM, DDASLV
C***REVISION HISTORY  (YYMMDD)
C   830315  DATE WRITTEN
C   901009  Finished conversion to SLATEC 4.0 format (F.N.Fritsch)
C   901019  Merged changes made by C. Ulrich with SLATEC 4.0 format.
C   901026  Added explicit declarations for all variables and minor
C           cosmetic changes to prologue.  (FNF)
C   901030  Minor corrections to declarations.  (FNF)
C***END PROLOGUE  DDAINI
C
      INTEGER  NEQ, IDID, IPAR(*), IWM(*), NONNEG, NTEMP
      DOUBLE PRECISION
     *   X, Y(*), YPRIME(*), H, WT(*), RPAR(*), PHI(NEQ,*), DELTA(*),
     *   E(*), WM(*), HMIN, UROUND
      EXTERNAL  RES, JAC
C
      EXTERNAL  DDAJAC, DDANRM, DDASLV
      DOUBLE PRECISION  DDANRM
C
      INTEGER  I, IER, IRES, JCALC, LNJE, LNRE, M, MAXIT, MJAC, NCF,
     *   NEF, NSF
      DOUBLE PRECISION
     *   CJ, DAMP, DELNRM, ERR, OLDNRM, R, RATE, S, XOLD, YNORM
      LOGICAL  CONVGD
C
      PARAMETER (LNRE=12)
      PARAMETER (LNJE=13)
C
      DATA MAXIT/10/,MJAC/5/
      DATA DAMP/0.75D0/
C
C
C---------------------------------------------------
C     BLOCK 1.
C     INITIALIZATIONS.
C---------------------------------------------------
C
C***FIRST EXECUTABLE STATEMENT  DDAINI
      IDID=1
      NEF=0
      NCF=0
      NSF=0
      XOLD=X
      YNORM=DDANRM(NEQ,Y,WT,RPAR,IPAR)
C
C     SAVE Y AND YPRIME IN PHI
      DO 100 I=1,NEQ
         PHI(I,1)=Y(I)
100      PHI(I,2)=YPRIME(I)
C
C
C----------------------------------------------------
C     BLOCK 2.
C     DO ONE BACKWARD EULER STEP.
C----------------------------------------------------
C
C     SET UP FOR START OF CORRECTOR ITERATION
200   CJ=1.0D0/H
      X=X+H
C
C     PREDICT SOLUTION AND DERIVATIVE
      DO 250 I=1,NEQ
250     Y(I)=Y(I)+H*YPRIME(I)
C
      JCALC=-1
      M=0
      CONVGD=.TRUE.
C
C
C     CORRECTOR LOOP.
300   IWM(LNRE)=IWM(LNRE)+1
      IRES=0
C
      CALL RES(X,Y,YPRIME,DELTA,IRES,RPAR,IPAR)
      if(iero.ne.0) return
      IF (IRES.LT.0) GO TO 430
C
C
C     EVALUATE THE ITERATION MATRIX
      IF (JCALC.NE.-1) GO TO 310
      IWM(LNJE)=IWM(LNJE)+1
      JCALC=0
      CALL DDAJAC(NEQ,X,Y,YPRIME,DELTA,CJ,H,
     *   IER,WT,E,WM,IWM,RES,IRES,
     *   UROUND,JAC,RPAR,IPAR,NTEMP)
      if(iero.ne.0) return
C
      S=1000000.D0
      IF (IRES.LT.0) GO TO 430
      IF (IER.NE.0) GO TO 430
      NSF=0
C
C
C
C     MULTIPLY RESIDUAL BY DAMPING FACTOR
310   CONTINUE
      DO 320 I=1,NEQ
320      DELTA(I)=DELTA(I)*DAMP
C
C     COMPUTE A NEW ITERATE (BACK SUBSTITUTION)
C     STORE THE CORRECTION IN DELTA
C
      CALL DDASLV(NEQ,DELTA,WM,IWM)
C
C     UPDATE Y AND YPRIME
      DO 330 I=1,NEQ
         Y(I)=Y(I)-DELTA(I)
330      YPRIME(I)=YPRIME(I)-CJ*DELTA(I)
C
C     TEST FOR CONVERGENCE OF THE ITERATION.
C
      DELNRM=DDANRM(NEQ,DELTA,WT,RPAR,IPAR)
      IF (DELNRM.LE.100.D0*UROUND*YNORM)
     *   GO TO 400
C
      IF (M.GT.0) GO TO 340
         OLDNRM=DELNRM
         GO TO 350
C
340   RATE=(DELNRM/OLDNRM)**(1.0D0/M)
      IF (RATE.GT.0.90D0) GO TO 430
      S=RATE/(1.0D0-RATE)
C
350   IF (S*DELNRM .LE. 0.33D0) GO TO 400
C
C
C     THE CORRECTOR HAS NOT YET CONVERGED. UPDATE
C     M AND AND TEST WHETHER THE MAXIMUM
C     NUMBER OF ITERATIONS HAVE BEEN TRIED.
C     EVERY MJAC ITERATIONS, GET A NEW
C     ITERATION MATRIX.
C
      M=M+1
      IF (M.GE.MAXIT) GO TO 430
C
      IF ((M/MJAC)*MJAC.EQ.M) JCALC=-1
      GO TO 300
C
C
C     THE ITERATION HAS CONVERGED.
C     CHECK NONNEGATIVITY CONSTRAINTS
400   IF (NONNEG.EQ.0) GO TO 450
      DO 410 I=1,NEQ
410      DELTA(I)=MIN(Y(I),0.0D0)
C
      DELNRM=DDANRM(NEQ,DELTA,WT,RPAR,IPAR)
      IF (DELNRM.GT.0.33D0) GO TO 430
C
      DO 420 I=1,NEQ
         Y(I)=Y(I)-DELTA(I)
420      YPRIME(I)=YPRIME(I)-CJ*DELTA(I)
      GO TO 450
C
C
C     EXITS FROM CORRECTOR LOOP.
430   CONVGD=.FALSE.
450   IF (.NOT.CONVGD) GO TO 600
C
C
C
C-----------------------------------------------------
C     BLOCK 3.
C     THE CORRECTOR ITERATION CONVERGED.
C     DO ERROR TEST.
C-----------------------------------------------------
C
      DO 510 I=1,NEQ
510      E(I)=Y(I)-PHI(I,1)
      ERR=DDANRM(NEQ,E,WT,RPAR,IPAR)
C
      IF (ERR.LE.1.0D0) RETURN
C
C
C
C--------------------------------------------------------
C     BLOCK 4.
C     THE BACKWARD EULER STEP FAILED. RESTORE X, Y
C     AND YPRIME TO THEIR ORIGINAL VALUES.
C     REDUCE STEPSIZE AND TRY AGAIN, IF
C     POSSIBLE.
C---------------------------------------------------------
C
600   CONTINUE
      X = XOLD
      DO 610 I=1,NEQ
         Y(I)=PHI(I,1)
610      YPRIME(I)=PHI(I,2)
C
      IF (CONVGD) GO TO 640
      IF (IER.EQ.0) GO TO 620
         NSF=NSF+1
         H=H*0.25D0
         IF (NSF.LT.3.AND.ABS(H).GE.HMIN) GO TO 690
         IDID=-12
         RETURN
620   IF (IRES.GT.-2) GO TO 630
         IDID=-12
         RETURN
630   NCF=NCF+1
      H=H*0.25D0
      IF (NCF.LT.10.AND.ABS(H).GE.HMIN) GO TO 690
         IDID=-12
         RETURN
C
640   NEF=NEF+1
      R=0.90D0/(2.0D0*ERR+0.0001D0)
      R=MAX(0.1D0,MIN(0.5D0,R))
      H=H*R
      IF (ABS(H).GE.HMIN.AND.NEF.LT.10) GO TO 690
         IDID=-12
         RETURN
690      GO TO 200
C
C-------------END OF SUBROUTINE DDAINI----------------------
      END
      SUBROUTINE DDAJAC (NEQ, X, Y, YPRIME, DELTA, CJ, H,
     +   IER, WT, E, WM, IWM, RES, IRES, UROUND, JAC, RPAR,
     +   IPAR, NTEMP)
      common/ierode/iero

C***BEGIN PROLOGUE  DDAJAC
C***SUBSIDIARY
C***PURPOSE  Compute the iteration matrix for DDASSL and form the
C            LU-decomposition.
C***LIBRARY   SLATEC (DASSL)
C***TYPE      DOUBLE PRECISION (SDAJAC-S, DDAJAC-D)
C***AUTHOR  PETZOLD, LINDA R., (LLNL)
C***DESCRIPTION
C-----------------------------------------------------------------------
C     THIS ROUTINE COMPUTES THE ITERATION MATRIX
C     PD=DG/DY+CJ*DG/DYPRIME (WHERE G(X,Y,YPRIME)=0).
C     HERE PD IS COMPUTED BY THE USER-SUPPLIED
C     ROUTINE JAC IF IWM(MTYPE) IS 1 OR 4, AND
C     IT IS COMPUTED BY NUMERICAL FINITE DIFFERENCING
C     IF IWM(MTYPE)IS 2 OR 5
C     THE PARAMETERS HAVE THE FOLLOWING MEANINGS.
C     Y        = ARRAY CONTAINING PREDICTED VALUES
C     YPRIME   = ARRAY CONTAINING PREDICTED DERIVATIVES
C     DELTA    = RESIDUAL EVALUATED AT (X,Y,YPRIME)
C                (USED ONLY IF IWM(MTYPE)=2 OR 5)
C     CJ       = SCALAR PARAMETER DEFINING ITERATION MATRIX
C     H        = CURRENT STEPSIZE IN INTEGRATION
C     IER      = VARIABLE WHICH IS .NE. 0
C                IF ITERATION MATRIX IS SINGULAR,
C                AND 0 OTHERWISE.
C     WT       = VECTOR OF WEIGHTS FOR COMPUTING NORMS
C     E        = WORK SPACE (TEMPORARY) OF LENGTH NEQ
C     WM       = REAL WORK SPACE FOR MATRICES. ON
C                OUTPUT IT CONTAINS THE LU DECOMPOSITION
C                OF THE ITERATION MATRIX.
C     IWM      = INTEGER WORK SPACE CONTAINING
C                MATRIX INFORMATION
C     RES      = NAME OF THE EXTERNAL USER-SUPPLIED ROUTINE
C                TO EVALUATE THE RESIDUAL FUNCTION G(X,Y,YPRIME)
C     IRES     = FLAG WHICH IS EQUAL TO ZERO IF NO ILLEGAL VALUES
C                IN RES, AND LESS THAN ZERO OTHERWISE.  (IF IRES
C                IS LESS THAN ZERO, THE MATRIX WAS NOT COMPLETED)
C                IN THIS CASE (IF IRES .LT. 0), THEN IER = 0.
C     UROUND   = THE UNIT ROUNDOFF ERROR OF THE MACHINE BEING USED.
C     JAC      = NAME OF THE EXTERNAL USER-SUPPLIED ROUTINE
C                TO EVALUATE THE ITERATION MATRIX (THIS ROUTINE
C                IS ONLY USED IF IWM(MTYPE) IS 1 OR 4)
C-----------------------------------------------------------------------
C***ROUTINES CALLED  DGBFA, DGEFA
C***REVISION HISTORY  (YYMMDD)
C   830315  DATE WRITTEN
C   901009  Finished conversion to SLATEC 4.0 format (F.N.Fritsch)
C   901010  Modified three MAX calls to be all on one line.  (FNF)
C   901019  Merged changes made by C. Ulrich with SLATEC 4.0 format.
C   901026  Added explicit declarations for all variables and minor
C           cosmetic changes to prologue.  (FNF)
C   901101  Corrected PURPOSE.  (FNF)
C***END PROLOGUE  DDAJAC
C
      INTEGER  NEQ, IER, IWM(*), IRES, IPAR(*), NTEMP
      DOUBLE PRECISION
     *   X, Y(*), YPRIME(*), DELTA(*), CJ, H, WT(*), E(*), WM(*),
     *   UROUND, RPAR(*)
      EXTERNAL  RES, JAC
C
      EXTERNAL  DGBFA, DGEFA
C
      INTEGER  I, I1, I2, II, IPSAVE, ISAVE, J, K, L, LENPD, LIPVT,
     *   LML, LMTYPE, LMU, MBA, MBAND, MEB1, MEBAND, MSAVE, MTYPE, N,
     *   NPD, NPDM1, NROW
      DOUBLE PRECISION  DEL, DELINV, SQUR, YPSAVE, YSAVE
C
      PARAMETER (NPD=1)
      PARAMETER (LML=1)
      PARAMETER (LMU=2)
      PARAMETER (LMTYPE=4)
      PARAMETER (LIPVT=21)
C
C***FIRST EXECUTABLE STATEMENT  DDAJAC
      IER = 0
      NPDM1=NPD-1
      MTYPE=IWM(LMTYPE)
      GO TO (100,200,300,400,500),MTYPE
C
C
C     DENSE USER-SUPPLIED MATRIX
100   LENPD=NEQ*NEQ
      DO 110 I=1,LENPD
110      WM(NPDM1+I)=0.0D0
      CALL JAC(X,Y,YPRIME,WM(NPD),CJ,RPAR,IPAR)
      if(iero.ne.0) return
      GO TO 230
C
C
C     DENSE FINITE-DIFFERENCE-GENERATED MATRIX
200   IRES=0
      NROW=NPDM1
      SQUR = SQRT(UROUND)
      DO 210 I=1,NEQ
         DEL=SQUR*MAX(ABS(Y(I)),ABS(H*YPRIME(I)),ABS(WT(I)))
         DEL=SIGN(DEL,H*YPRIME(I))
         DEL=(Y(I)+DEL)-Y(I)
         YSAVE=Y(I)
         YPSAVE=YPRIME(I)
         Y(I)=Y(I)+DEL
         YPRIME(I)=YPRIME(I)+CJ*DEL
         CALL RES(X,Y,YPRIME,E,IRES,RPAR,IPAR)
         if(iero.ne.0) return
         IF (IRES .LT. 0) RETURN
         DELINV=1.0D0/DEL
         DO 220 L=1,NEQ
220      WM(NROW+L)=(E(L)-DELTA(L))*DELINV
      NROW=NROW+NEQ
      Y(I)=YSAVE
      YPRIME(I)=YPSAVE
210   CONTINUE
C
C
C     DO DENSE-MATRIX LU DECOMPOSITION ON PD
230      CALL DGEFA(WM(NPD),NEQ,NEQ,IWM(LIPVT),IER)
      RETURN
C
C
C     DUMMY SECTION FOR IWM(MTYPE)=3
300   RETURN
C
C
C     BANDED USER-SUPPLIED MATRIX
400   LENPD=(2*IWM(LML)+IWM(LMU)+1)*NEQ
      DO 410 I=1,LENPD
410      WM(NPDM1+I)=0.0D0
      CALL JAC(X,Y,YPRIME,WM(NPD),CJ,RPAR,IPAR)
      if(iero.ne.0) return
      MEBAND=2*IWM(LML)+IWM(LMU)+1
      GO TO 550
C
C
C     BANDED FINITE-DIFFERENCE-GENERATED MATRIX
500   MBAND=IWM(LML)+IWM(LMU)+1
      MBA=MIN(MBAND,NEQ)
      MEBAND=MBAND+IWM(LML)
      MEB1=MEBAND-1
      MSAVE=(NEQ/MBAND)+1
      ISAVE=NTEMP-1
      IPSAVE=ISAVE+MSAVE
      IRES=0
      SQUR=SQRT(UROUND)
      DO 540 J=1,MBA
         DO 510 N=J,NEQ,MBAND
          K= (N-J)/MBAND + 1
          WM(ISAVE+K)=Y(N)
          WM(IPSAVE+K)=YPRIME(N)
          DEL=SQUR*MAX(ABS(Y(N)),ABS(H*YPRIME(N)),ABS(WT(N)))
          DEL=SIGN(DEL,H*YPRIME(N))
          DEL=(Y(N)+DEL)-Y(N)
          Y(N)=Y(N)+DEL
510       YPRIME(N)=YPRIME(N)+CJ*DEL
      CALL RES(X,Y,YPRIME,E,IRES,RPAR,IPAR)
      if(iero.ne.0) return
      IF (IRES .LT. 0) RETURN
      DO 530 N=J,NEQ,MBAND
          K= (N-J)/MBAND + 1
          Y(N)=WM(ISAVE+K)
          YPRIME(N)=WM(IPSAVE+K)
          DEL=SQUR*MAX(ABS(Y(N)),ABS(H*YPRIME(N)),ABS(WT(N)))
          DEL=SIGN(DEL,H*YPRIME(N))
          DEL=(Y(N)+DEL)-Y(N)
          DELINV=1.0D0/DEL
          I1=MAX(1,(N-IWM(LMU)))
          I2=MIN(NEQ,(N+IWM(LML)))
          II=N*MEB1-IWM(LML)+NPDM1
          DO 520 I=I1,I2
520         WM(II+I)=(E(I)-DELTA(I))*DELINV
530      CONTINUE
540   CONTINUE
C
C
C     DO LU DECOMPOSITION OF BANDED PD
550   CALL DGBFA(WM(NPD),MEBAND,NEQ,
     *    IWM(LML),IWM(LMU),IWM(LIPVT),IER)
      RETURN
C------END OF SUBROUTINE DDAJAC------
      END
      DOUBLE PRECISION FUNCTION DDANRM (NEQ, V, WT, RPAR, IPAR)
C***BEGIN PROLOGUE  DDANRM
C***SUBSIDIARY
C***PURPOSE  Compute vector norm for DDASSL.
C***LIBRARY   SLATEC (DASSL)
C***TYPE      DOUBLE PRECISION (SDANRM-S, DDANRM-D)
C***AUTHOR  PETZOLD, LINDA R., (LLNL)
C***DESCRIPTION
C-----------------------------------------------------------------------
C     THIS FUNCTION ROUTINE COMPUTES THE WEIGHTED
C     ROOT-MEAN-SQUARE NORM OF THE VECTOR OF LENGTH
C     NEQ CONTAINED IN THE ARRAY V,WITH WEIGHTS
C     CONTAINED IN THE ARRAY WT OF LENGTH NEQ.
C        DDANRM=SQRT((1/NEQ)*SUM(V(I)/WT(I))**2)
C-----------------------------------------------------------------------
C***ROUTINES CALLED  (NONE)
C***REVISION HISTORY  (YYMMDD)
C   830315  DATE WRITTEN
C   901009  Finished conversion to SLATEC 4.0 format (F.N.Fritsch)
C   901019  Merged changes made by C. Ulrich with SLATEC 4.0 format.
C   901026  Added explicit declarations for all variables and minor
C           cosmetic changes to prologue.  (FNF)
C***END PROLOGUE  DDANRM
C
      INTEGER  NEQ, IPAR(*)
      DOUBLE PRECISION  V(NEQ), WT(NEQ), RPAR(*)
C
      INTEGER  I
      DOUBLE PRECISION  SUM, VMAX
C
C***FIRST EXECUTABLE STATEMENT  DDANRM
      DDANRM = 0.0D0
      VMAX = 0.0D0
      DO 10 I = 1,NEQ
        IF(ABS(V(I)/WT(I)) .GT. VMAX) VMAX = ABS(V(I)/WT(I))
10      CONTINUE
      IF(VMAX .LE. 0.0D0) GO TO 30
      SUM = 0.0D0
      DO 20 I = 1,NEQ
20      SUM = SUM + ((V(I)/WT(I))/VMAX)**2
      DDANRM = VMAX*SQRT(SUM/NEQ)
30    CONTINUE
      RETURN
C------END OF FUNCTION DDANRM------
      END
      SUBROUTINE DDASLV (NEQ, DELTA, WM, IWM)
C***BEGIN PROLOGUE  DDASLV
C***SUBSIDIARY
C***PURPOSE  Linear system solver for DDASSL.
C***LIBRARY   SLATEC (DASSL)
C***TYPE      DOUBLE PRECISION (SDASLV-S, DDASLV-D)
C***AUTHOR  PETZOLD, LINDA R., (LLNL)
C***DESCRIPTION
C-----------------------------------------------------------------------
C     THIS ROUTINE MANAGES THE SOLUTION OF THE LINEAR
C     SYSTEM ARISING IN THE NEWTON ITERATION.
C     MATRICES AND REAL TEMPORARY STORAGE AND
C     REAL INFORMATION ARE STORED IN THE ARRAY WM.
C     INTEGER MATRIX INFORMATION IS STORED IN
C     THE ARRAY IWM.
C     FOR A DENSE MATRIX, THE LINPACK ROUTINE
C     DGESL IS CALLED.
C     FOR A BANDED MATRIX,THE LINPACK ROUTINE
C     DGBSL IS CALLED.
C-----------------------------------------------------------------------
C***ROUTINES CALLED  DGBSL, DGESL
C***REVISION HISTORY  (YYMMDD)
C   830315  DATE WRITTEN
C   901009  Finished conversion to SLATEC 4.0 format (F.N.Fritsch)
C   901019  Merged changes made by C. Ulrich with SLATEC 4.0 format.
C   901026  Added explicit declarations for all variables and minor
C           cosmetic changes to prologue.  (FNF)
C***END PROLOGUE  DDASLV
C
      INTEGER  NEQ, IWM(*)
      DOUBLE PRECISION  DELTA(*), WM(*)
C
      EXTERNAL  DGBSL, DGESL
C
      INTEGER  LIPVT, LML, LMU, LMTYPE, MEBAND, MTYPE, NPD
      PARAMETER (NPD=1)
      PARAMETER (LML=1)
      PARAMETER (LMU=2)
      PARAMETER (LMTYPE=4)
      PARAMETER (LIPVT=21)
C
C***FIRST EXECUTABLE STATEMENT  DDASLV
      MTYPE=IWM(LMTYPE)
      GO TO(100,100,300,400,400),MTYPE
C
C     DENSE MATRIX
100   CALL DGESL(WM(NPD),NEQ,NEQ,IWM(LIPVT),DELTA,0)
      RETURN
C
C     DUMMY SECTION FOR MTYPE=3
300   CONTINUE
      RETURN
C
C     BANDED MATRIX
400   MEBAND=2*IWM(LML)+IWM(LMU)+1
      CALL DGBSL(WM(NPD),MEBAND,NEQ,IWM(LML),
     *  IWM(LMU),IWM(LIPVT),DELTA,0)
      RETURN
C------END OF SUBROUTINE DDASLV------
      END
      SUBROUTINE DDASSL (RES, NEQ, T, Y, YPRIME, TOUT, INFO, RTOL, ATOL,
     +   IDID, RWORK, LRW, IWORK, LIW, RPAR, IPAR, JAC)
      common/ierode/iero
C***BEGIN PROLOGUE  DDASSL
C***PURPOSE  This code solves a system of differential/algebraic
C            equations of the form G(T,Y,YPRIME) = 0.
C***LIBRARY   SLATEC (DASSL)
C***CATEGORY  I1A2
C***TYPE      DOUBLE PRECISION (SDASSL-S, DDASSL-D)
C***KEYWORDS  DIFFERENTIAL/ALGEBRAIC, BACKWARD DIFFERENTIATION FORMULAS,
C             IMPLICIT DIFFERENTIAL SYSTEMS
C***AUTHOR  PETZOLD, LINDA R., (LLNL)
C             COMPUTING AND MATHEMATICS RESEARCH DIVISION
C             LAWRENCE LIVERMORE NATIONAL LABORATORY
C             L - 316, P.O. BOX 808,
C             LIVERMORE, CA.    94550
C***DESCRIPTION
C
C *Usage:
C
C      EXTERNAL RES, JAC
C      INTEGER NEQ, INFO(N), IDID, LRW, LIW, IWORK(LIW), IPAR
C      DOUBLE PRECISION T, Y(NEQ), YPRIME(NEQ), TOUT, RTOL, ATOL,
C     *   RWORK(LRW), RPAR
C
C      CALL DDASSL (RES, NEQ, T, Y, YPRIME, TOUT, INFO, RTOL, ATOL,
C     *   IDID, RWORK, LRW, IWORK, LIW, RPAR, IPAR, JAC)
C
C
C *Arguments:
C  (In the following, all real arrays should be type DOUBLE PRECISION.)
C
C  RES:EXT     This is a subroutine which you provide to define the
C              differential/algebraic system.
C
C  NEQ:IN      This is the number of equations to be solved.
C
C  T:INOUT     This is the current value of the independent variable.
C
C  Y(*):INOUT  This array contains the solution components at T.
C
C  YPRIME(*):INOUT  This array contains the derivatives of the solution
C              components at T.
C
C  TOUT:IN     This is a point at which a solution is desired.
C
C  INFO(N):IN  The basic task of the code is to solve the system from T
C              to TOUT and return an answer at TOUT.  INFO is an integer
C              array which is used to communicate exactly how you want
C              this task to be carried out.  (See below for details.)
C              N must be greater than or equal to 15.
C
C  RTOL,ATOL:INOUT  These quantities represent relative and absolute
C              error tolerances which you provide to indicate how
C              accurately you wish the solution to be computed.  You
C              may choose them to be both scalars or else both vectors.
C              Caution:  In Fortran 77, a scalar is not the same as an
C                        array of length 1.  Some compilers may object
C                        to using scalars for RTOL,ATOL.
C
C  IDID:OUT    This scalar quantity is an indicator reporting what the
C              code did.  You must monitor this integer variable to
C              decide  what action to take next.
C
C  RWORK:WORK  A real work array of length LRW which provides the
C              code with needed storage space.
C
C  LRW:IN      The length of RWORK.  (See below for required length.)
C
C  IWORK:WORK  An integer work array of length LIW which probides the
C              code with needed storage space.
C
C  LIW:IN      The length of IWORK.  (See below for required length.)
C
C  RPAR,IPAR:IN  These are real and integer parameter arrays which
C              you can use for communication between your calling
C              program and the RES subroutine (and the JAC subroutine)
C
C  JAC:EXT     This is the name of a subroutine which you may choose
C              to provide for defining a matrix of partial derivatives
C              described below.
C
C  Quantities which may be altered by DDASSL are:
C     T, Y(*), YPRIME(*), INFO(1), RTOL, ATOL,
C     IDID, RWORK(*) AND IWORK(*)
C
C *Description
C
C  Subroutine DDASSL uses the backward differentiation formulas of
C  orders one through five to solve a system of the above form for Y and
C  YPRIME.  Values for Y and YPRIME at the initial time must be given as
C  input.  These values must be consistent, (that is, if T,Y,YPRIME are
C  the given initial values, they must satisfy G(T,Y,YPRIME) = 0.).  The
C  subroutine solves the system from T to TOUT.  It is easy to continue
C  the solution to get results at additional TOUT.  This is the interval
C  mode of operation.  Intermediate results can also be obtained easily
C  by using the intermediate-output capability.
C
C  The following detailed description is divided into subsections:
C    1. Input required for the first call to DDASSL.
C    2. Output after any return from DDASSL.
C    3. What to do to continue the integration.
C    4. Error messages.
C
C
C  -------- INPUT -- WHAT TO DO ON THE FIRST CALL TO DDASSL ------------
C
C  The first call of the code is defined to be the start of each new
C  problem. Read through the descriptions of all the following items,
C  provide sufficient storage space for designated arrays, set
C  appropriate variables for the initialization of the problem, and
C  give information about how you want the problem to be solved.
C
C
C  RES -- Provide a subroutine of the form
C             SUBROUTINE RES(T,Y,YPRIME,DELTA,IRES,RPAR,IPAR)
C         to define the system of differential/algebraic
C         equations which is to be solved. For the given values
C         of T,Y and YPRIME, the subroutine should
C         return the residual of the defferential/algebraic
C         system
C             DELTA = G(T,Y,YPRIME)
C         (DELTA(*) is a vector of length NEQ which is
C         output for RES.)
C
C         Subroutine RES must not alter T,Y or YPRIME.
C         You must declare the name RES in an external
C         statement in your program that calls DDASSL.
C         You must dimension Y,YPRIME and DELTA in RES.
C
C         IRES is an integer flag which is always equal to
C         zero on input. Subroutine RES should alter IRES
C         only if it encounters an illegal value of Y or
C         a stop condition. Set IRES = -1 if an input value
C         is illegal, and DDASSL will try to solve the problem
C         without getting IRES = -1. If IRES = -2, DDASSL
C         will return control to the calling program
C         with IDID = -11.
C
C         RPAR and IPAR are real and integer parameter arrays which
C         you can use for communication between your calling program
C         and subroutine RES. They are not altered by DDASSL. If you
C         do not need RPAR or IPAR, ignore these parameters by treat-
C         ing them as dummy arguments. If you do choose to use them,
C         dimension them in your calling program and in RES as arrays
C         of appropriate length.
C
C  NEQ -- Set it to the number of differential equations.
C         (NEQ .GE. 1)
C
C  T -- Set it to the initial point of the integration.
C         T must be defined as a variable.
C
C  Y(*) -- Set this vector to the initial values of the NEQ solution
C         components at the initial point. You must dimension Y of
C         length at least NEQ in your calling program.
C
C  YPRIME(*) -- Set this vector to the initial values of the NEQ
C         first derivatives of the solution components at the initial
C         point.  You must dimension YPRIME at least NEQ in your
C         calling program. If you do not know initial values of some
C         of the solution components, see the explanation of INFO(11).
C
C  TOUT -- Set it to the first point at which a solution
C         is desired. You can not take TOUT = T.
C         integration either forward in T (TOUT .GT. T) or
C         backward in T (TOUT .LT. T) is permitted.
C
C         The code advances the solution from T to TOUT using
C         step sizes which are automatically selected so as to
C         achieve the desired accuracy. If you wish, the code will
C         return with the solution and its derivative at
C         intermediate steps (intermediate-output mode) so that
C         you can monitor them, but you still must provide TOUT in
C         accord with the basic aim of the code.
C
C         The first step taken by the code is a critical one
C         because it must reflect how fast the solution changes near
C         the initial point. The code automatically selects an
C         initial step size which is practically always suitable for
C         the problem. By using the fact that the code will not step
C         past TOUT in the first step, you could, if necessary,
C         restrict the length of the initial step size.
C
C         For some problems it may not be permissible to integrate
C         past a point TSTOP because a discontinuity occurs there
C         or the solution or its derivative is not defined beyond
C         TSTOP. When you have declared a TSTOP point (SEE INFO(4)
C         and RWORK(1)), you have told the code not to integrate
C         past TSTOP. In this case any TOUT beyond TSTOP is invalid
C         input.
C
C  INFO(*) -- Use the INFO array to give the code more details about
C         how you want your problem solved.  This array should be
C         dimensioned of length 15, though DDASSL uses only the first
C         eleven entries.  You must respond to all of the following
C         items, which are arranged as questions.  The simplest use
C         of the code corresponds to answering all questions as yes,
C         i.e. setting all entries of INFO to 0.
C
C       INFO(1) - This parameter enables the code to initialize
C              itself. You must set it to indicate the start of every
C              new problem.
C
C          **** Is this the first call for this problem ...
C                Yes - Set INFO(1) = 0
C                 No - Not applicable here.
C                      See below for continuation calls.  ****
C
C       INFO(2) - How much accuracy you want of your solution
C              is specified by the error tolerances RTOL and ATOL.
C              The simplest use is to take them both to be scalars.
C              To obtain more flexibility, they can both be vectors.
C              The code must be told your choice.
C
C          **** Are both error tolerances RTOL, ATOL scalars ...
C                Yes - Set INFO(2) = 0
C                      and input scalars for both RTOL and ATOL
C                 No - Set INFO(2) = 1
C                      and input arrays for both RTOL and ATOL ****
C
C       INFO(3) - The code integrates from T in the direction
C              of TOUT by steps. If you wish, it will return the
C              computed solution and derivative at the next
C              intermediate step (the intermediate-output mode) or
C              TOUT, whichever comes first. This is a good way to
C              proceed if you want to see the behavior of the solution.
C              If you must have solutions at a great many specific
C              TOUT points, this code will compute them efficiently.
C
C          **** Do you want the solution only at
C                TOUT (and not at the next intermediate step) ...
C                 Yes - Set INFO(3) = 0
C                  No - Set INFO(3) = 1 ****
C
C       INFO(4) - To handle solutions at a great many specific
C              values TOUT efficiently, this code may integrate past
C              TOUT and interpolate to obtain the result at TOUT.
C              Sometimes it is not possible to integrate beyond some
C              point TSTOP because the equation changes there or it is
C              not defined past TSTOP. Then you must tell the code
C              not to go past.
C
C           **** Can the integration be carried out without any
C                restrictions on the independent variable T ...
C                 Yes - Set INFO(4)=0
C                  No - Set INFO(4)=1
C                       and define the stopping point TSTOP by
C                       setting RWORK(1)=TSTOP ****
C
C       INFO(5) - To solve differential/algebraic problems it is
C              necessary to use a matrix of partial derivatives of the
C              system of differential equations. If you do not
C              provide a subroutine to evaluate it analytically (see
C              description of the item JAC in the call list), it will
C              be approximated by numerical differencing in this code.
C              although it is less trouble for you to have the code
C              compute partial derivatives by numerical differencing,
C              the solution will be more reliable if you provide the
C              derivatives via JAC. Sometimes numerical differencing
C              is cheaper than evaluating derivatives in JAC and
C              sometimes it is not - this depends on your problem.
C
C           **** Do you want the code to evaluate the partial
C                derivatives automatically by numerical differences ...
C                   Yes - Set INFO(5)=0
C                    No - Set INFO(5)=1
C                  and provide subroutine JAC for evaluating the
C                  matrix of partial derivatives ****
C
C       INFO(6) - DDASSL will perform much better if the matrix of
C              partial derivatives, DG/DY + CJ*DG/DYPRIME,
C              (here CJ is a scalar determined by DDASSL)
C              is banded and the code is told this. In this
C              case, the storage needed will be greatly reduced,
C              numerical differencing will be performed much cheaper,
C              and a number of important algorithms will execute much
C              faster. The differential equation is said to have
C              half-bandwidths ML (lower) and MU (upper) if equation i
C              involves only unknowns Y(J) with
C                             I-ML .LE. J .LE. I+MU
C              for all I=1,2,...,NEQ. Thus, ML and MU are the widths
C              of the lower and upper parts of the band, respectively,
C              with the main diagonal being excluded. If you do not
C              indicate that the equation has a banded matrix of partial
C              derivatives, the code works with a full matrix of NEQ**2
C              elements (stored in the conventional way). Computations
C              with banded matrices cost less time and storage than with
C              full matrices if 2*ML+MU .LT. NEQ. If you tell the
C              code that the matrix of partial derivatives has a banded
C              structure and you want to provide subroutine JAC to
C              compute the partial derivatives, then you must be careful
C              to store the elements of the matrix in the special form
C              indicated in the description of JAC.
C
C          **** Do you want to solve the problem using a full
C               (dense) matrix (and not a special banded
C               structure) ...
C                Yes - Set INFO(6)=0
C                 No - Set INFO(6)=1
C                       and provide the lower (ML) and upper (MU)
C                       bandwidths by setting
C                       IWORK(1)=ML
C                       IWORK(2)=MU ****
C
C
C        INFO(7) -- You can specify a maximum (absolute value of)
C              stepsize, so that the code
C              will avoid passing over very
C              large regions.
C
C          ****  Do you want the code to decide
C                on its own maximum stepsize?
C                Yes - Set INFO(7)=0
C                 No - Set INFO(7)=1
C                      and define HMAX by setting
C                      RWORK(2)=HMAX ****
C
C        INFO(8) -- Differential/algebraic problems
C              may occaisionally suffer from
C              severe scaling difficulties on the
C              first step. If you know a great deal
C              about the scaling of your problem, you can
C              help to alleviate this problem by
C              specifying an initial stepsize HO.
C
C          ****  Do you want the code to define
C                its own initial stepsize?
C                Yes - Set INFO(8)=0
C                 No - Set INFO(8)=1
C                      and define HO by setting
C                      RWORK(3)=HO ****
C
C        INFO(9) -- If storage is a severe problem,
C              you can save some locations by
C              restricting the maximum order MAXORD.
C              the default value is 5. for each
C              order decrease below 5, the code
C              requires NEQ fewer locations, however
C              it is likely to be slower. In any
C              case, you must have 1 .LE. MAXORD .LE. 5
C          ****  Do you want the maximum order to
C                default to 5?
C                Yes - Set INFO(9)=0
C                 No - Set INFO(9)=1
C                      and define MAXORD by setting
C                      IWORK(3)=MAXORD ****
C
C        INFO(10) --If you know that the solutions to your equations
C               will always be nonnegative, it may help to set this
C               parameter. However, it is probably best to
C               try the code without using this option first,
C               and only to use this option if that doesn't
C               work very well.
C           ****  Do you want the code to solve the problem without
C                 invoking any special nonnegativity constraints?
C                  Yes - Set INFO(10)=0
C                   No - Set INFO(10)=1
C
C        INFO(11) --DDASSL normally requires the initial T,
C               Y, and YPRIME to be consistent. That is,
C               you must have G(T,Y,YPRIME) = 0 at the initial
C               time. If you do not know the initial
C               derivative precisely, you can let DDASSL try
C               to compute it.
C          ****   Are the initialHE INITIAL T, Y, YPRIME consistent?
C                 Yes - Set INFO(11) = 0
C                  No - Set INFO(11) = 1,
C                       and set YPRIME to an initial approximation
C                       to YPRIME.  (If you have no idea what
C                       YPRIME should be, set it to zero. Note
C                       that the initial Y should be such
C                       that there must exist a YPRIME so that
C                       G(T,Y,YPRIME) = 0.)
C
C  RTOL, ATOL -- You must assign relative (RTOL) and absolute (ATOL
C         error tolerances to tell the code how accurately you
C         want the solution to be computed.  They must be defined
C         as variables because the code may change them.  You
C         have two choices --
C               Both RTOL and ATOL are scalars. (INFO(2)=0)
C               Both RTOL and ATOL are vectors. (INFO(2)=1)
C         in either case all components must be non-negative.
C
C         The tolerances are used by the code in a local error
C         test at each step which requires roughly that
C               ABS(LOCAL ERROR) .LE. RTOL*ABS(Y)+ATOL
C         for each vector component.
C         (More specifically, a root-mean-square norm is used to
C         measure the size of vectors, and the error test uses the
C         magnitude of the solution at the beginning of the step.)
C
C         The true (global) error is the difference between the
C         true solution of the initial value problem and the
C         computed approximation.  Practically all present day
C         codes, including this one, control the local error at
C         each step and do not even attempt to control the global
C         error directly.
C         Usually, but not always, the true accuracy of the
C         computed Y is comparable to the error tolerances. This
C         code will usually, but not always, deliver a more
C         accurate solution if you reduce the tolerances and
C         integrate again.  By comparing two such solutions you
C         can get a fairly reliable idea of the true error in the
C         solution at the bigger tolerances.
C
C         Setting ATOL=0. results in a pure relative error test on
C         that component.  Setting RTOL=0. results in a pure
C         absolute error test on that component.  A mixed test
C         with non-zero RTOL and ATOL corresponds roughly to a
C         relative error test when the solution component is much
C         bigger than ATOL and to an absolute error test when the
C         solution component is smaller than the threshhold ATOL.
C
C         The code will not attempt to compute a solution at an
C         accuracy unreasonable for the machine being used.  It will
C         advise you if you ask for too much accuracy and inform
C         you as to the maximum accuracy it believes possible.
C
C  RWORK(*) --  Dimension this real work array of length LRW in your
C         calling program.
C
C  LRW -- Set it to the declared length of the RWORK array.
C               You must have
C                    LRW .GE. 40+(MAXORD+4)*NEQ+NEQ**2
C               for the full (dense) JACOBIAN case (when INFO(6)=0), or
C                    LRW .GE. 40+(MAXORfD+4)*NEQ+(2*ML+MU+1)*NEQ
C               for the banded user-defined JACOBIAN case
C               (when INFO(5)=1 and INFO(6)=1), or
C                     LRW .GE. 40+(MAXORD+4)*NEQ+(2*ML+MU+1)*NEQ
C                           +2*(NEQ/(ML+MU+1)+1)
C               for the banded finite-difference-generated JACOBIAN case
C               (when INFO(5)=0 and INFO(6)=1)
C
C  IWORK(*) --  Dimension this integer work array of length LIW in
C         your calling program.
C
C  LIW -- Set it to the declared length of the IWORK array.
C               You must have LIW .GE. 20+NEQ
C
C  RPAR, IPAR -- These are parameter arrays, of real and integer
C         type, respectively.  You can use them for communication
C         between your program that calls DDASSL and the
C         RES subroutine (and the JAC subroutine).  They are not
C         altered by DDASSL.  If you do not need RPAR or IPAR,
C         ignore these parameters by treating them as dummy
C         arguments.  If you do choose to use them, dimension
C         them in your calling program and in RES (and in JAC)
C         as arrays of appropriate length.
C
C  JAC -- If you have set INFO(5)=0, you can ignore this parameter
C         by treating it as a dummy argument.  Otherwise, you must
C         provide a subroutine of the form
C             SUBROUTINE JAC(T,Y,YPRIME,PD,CJ,RPAR,IPAR)
C         to define the matrix of partial derivatives
C             PD=DG/DY+CJ*DG/DYPRIME
C         CJ is a scalar which is input to JAC.
C         For the given values of T,Y,YPRIME, the
C         subroutine must evaluate the non-zero partial
C         derivatives for each equation and each solution
C         component, and store these values in the
C         matrix PD.  The elements of PD are set to zero
C         before each call to JAC so only non-zero elements
C         need to be defined.
C
C         Subroutine JAC must not alter T,Y,(*),YPRIME(*), or CJ.
C         You must declare the name JAC in an EXTERNAL statement in
C         your program that calls DDASSL.  You must dimension Y,
C         YPRIME and PD in JAC.
C
C         The way you must store the elements into the PD matrix
C         depends on the structure of the matrix which you
C         indicated by INFO(6).
C               *** INFO(6)=0 -- Full (dense) matrix ***
C                   Give PD a first dimension of NEQ.
C                   When you evaluate the (non-zero) partial derivative
C                   of equation I with respect to variable J, you must
C                   store it in PD according to
C                   PD(I,J) = "DG(I)/DY(J)+CJ*DG(I)/DYPRIME(J)"
C               *** INFO(6)=1 -- Banded JACOBIAN with ML lower and MU
C                   upper diagonal bands (refer to INFO(6) description
C                   of ML and MU) ***
C                   Give PD a first dimension of 2*ML+MU+1.
C                   when you evaluate the (non-zero) partial derivative
C                   of equation I with respect to variable J, you must
C                   store it in PD according to
C                   IROW = I - J + ML + MU + 1
C                   PD(IROW,J) = "DG(I)/DY(J)+CJ*DG(I)/DYPRIME(J)"
C
C         RPAR and IPAR are real and integer parameter arrays
C         which you can use for communication between your calling
C         program and your JACOBIAN subroutine JAC. They are not
C         altered by DDASSL. If you do not need RPAR or IPAR,
C         ignore these parameters by treating them as dummy
C         arguments. If you do choose to use them, dimension
C         them in your calling program and in JAC as arrays of
C         appropriate length.
C
C
C  OPTIONALLY REPLACEABLE NORM ROUTINE:
C
C     DDASSL uses a weighted norm DDANRM to measure the size
C     of vectors such as the estimated error in each step.
C     A FUNCTION subprogram
C       DOUBLE PRECISION FUNCTION DDANRM(NEQ,V,WT,RPAR,IPAR)
C       DIMENSION V(NEQ),WT(NEQ)
C     is used to define this norm. Here, V is the vector
C     whose norm is to be computed, and WT is a vector of
C     weights.  A DDANRM routine has been included with DDASSL
C     which computes the weighted root-mean-square norm
C     given by
C       DDANRM=SQRT((1/NEQ)*SUM(V(I)/WT(I))**2)
C     this norm is suitable for most problems. In some
C     special cases, it may be more convenient and/or
C     efficient to define your own norm by writing a function
C     subprogram to be called instead of DDANRM. This should,
C     however, be attempted only after careful thought and
C     consideration.
C
C
C  -------- OUTPUT -- AFTER ANY RETURN FROM DDASSL ---------------------
C
C  The principal aim of the code is to return a computed solution at
C  TOUT, although it is also possible to obtain intermediate results
C  along the way. To find out whether the code achieved its goal
C  or if the integration process was interrupted before the task was
C  completed, you must check the IDID parameter.
C
C
C  T -- The solution was successfully advanced to the
C               output value of T.
C
C  Y(*) -- Contains the computed solution approximation at T.
C
C  YPRIME(*) -- Contains the computed derivative
C               approximation at T.
C
C  IDID -- Reports what the code did.
C
C                     *** Task completed ***
C                Reported by positive values of IDID
C
C           IDID = 1 -- A step was successfully taken in the
C                   intermediate-output mode. The code has not
C                   yet reached TOUT.
C
C           IDID = 2 -- The integration to TSTOP was successfully
C                   completed (T=TSTOP) by stepping exactly to TSTOP.
C
C           IDID = 3 -- The integration to TOUT was successfully
C                   completed (T=TOUT) by stepping past TOUT.
C                   Y(*) is obtained by interpolation.
C                   YPRIME(*) is obtained by interpolation.
C
C                    *** Task interrupted ***
C                Reported by negative values of IDID
C
C           IDID = -1 -- A large amount of work has been expended.
C                   (About 500 steps)
C
C           IDID = -2 -- The error tolerances are too stringent.
C
C           IDID = -3 -- The local error test cannot be satisfied
C                   because you specified a zero component in ATOL
C                   and the corresponding computed solution
C                   component is zero. Thus, a pure relative error
C                   test is impossible for this component.
C
C           IDID = -6 -- DDASSL had repeated error test
C                   failures on the last attempted step.
C
C           IDID = -7 -- The corrector could not converge.
C
C           IDID = -8 -- The matrix of partial derivatives
C                   is singular.
C
C           IDID = -9 -- The corrector could not converge.
C                   there were repeated error test failures
C                   in this step.
C
C           IDID =-10 -- The corrector could not converge
C                   because IRES was equal to minus one.
C
C           IDID =-11 -- IRES equal to -2 was encountered
C                   and control is being returned to the
C                   calling program.
C
C           IDID =-12 -- DDASSL failed to compute the initial
C                   YPRIME.
C
C
C
C           IDID = -13,..,-32 -- Not applicable for this code
C
C                    *** Task terminated ***
C                Reported by the value of IDID=-33
C
C           IDID = -33 -- The code has encountered trouble from which
C                   it cannot recover. A message is printed
C                   explaining the trouble and control is returned
C                   to the calling program. For example, this occurs
C                   when invalid input is detected.
C
C  RTOL, ATOL -- These quantities remain unchanged except when
C               IDID = -2. In this case, the error tolerances have been
C               increased by the code to values which are estimated to
C               be appropriate for continuing the integration. However,
C               the reported solution at T was obtained using the input
C               values of RTOL and ATOL.
C
C  RWORK, IWORK -- Contain information which is usually of no
C               interest to the user but necessary for subsequent calls.
C               However, you may find use for
C
C               RWORK(3)--Which contains the step size H to be
C                       attempted on the next step.
C
C               RWORK(4)--Which contains the current value of the
C                       independent variable, i.e., the farthest point
C                       integration has reached. This will be different
C                       from T only when interpolation has been
C                       performed (IDID=3).
C
C               RWORK(7)--Which contains the stepsize used
C                       on the last successful step.
C
C               IWORK(7)--Which contains the order of the method to
C                       be attempted on the next step.
C
C               IWORK(8)--Which contains the order of the method used
C                       on the last step.
C
C               IWORK(11)--Which contains the number of steps taken so
C                        far.
C
C               IWORK(12)--Which contains the number of calls to RES
C                        so far.
C
C               IWORK(13)--Which contains the number of evaluations of
C                        the matrix of partial derivatives needed so
C                        far.
C
C               IWORK(14)--Which contains the total number
C                        of error test failures so far.
C
C               IWORK(15)--Which contains the total number
C                        of convergence test failures so far.
C                        (includes singular iteration matrix
C                        failures.)
C
C
C  -------- INPUT -- WHAT TO DO TO CONTINUE THE INTEGRATION ------------
C                    (CALLS AFTER THE FIRST)
C
C  This code is organized so that subsequent calls to continue the
C  integration involve little (if any) additional effort on your
C  part. You must monitor the IDID parameter in order to determine
C  what to do next.
C
C  Recalling that the principal task of the code is to integrate
C  from T to TOUT (the interval mode), usually all you will need
C  to do is specify a new TOUT upon reaching the current TOUT.
C
C  Do not alter any quantity not specifically permitted below,
C  in particular do not alter NEQ,T,Y(*),YPRIME(*),RWORK(*),IWORK(*)
C  or the differential equation in subroutine RES. Any such
C  alteration constitutes a new problem and must be treated as such,
C  i.e., you must start afresh.
C
C  You cannot change from vector to scalar error control or vice
C  versa (INFO(2)), but you can change the size of the entries of
C  RTOL, ATOL. Increasing a tolerance makes the equation easier
C  to integrate. Decreasing a tolerance will make the equation
C  harder to integrate and should generally be avoided.
C
C  You can switch from the intermediate-output mode to the
C  interval mode (INFO(3)) or vice versa at any time.
C
C  If it has been necessary to prevent the integration from going
C  past a point TSTOP (INFO(4), RWORK(1)), keep in mind that the
C  code will not integrate to any TOUT beyond the currently
C  specified TSTOP. Once TSTOP has been reached you must change
C  the value of TSTOP or set INFO(4)=0. You may change INFO(4)
C  or TSTOP at any time but you must supply the value of TSTOP in
C  RWORK(1) whenever you set INFO(4)=1.
C
C  Do not change INFO(5), INFO(6), IWORK(1), or IWORK(2)
C  unless you are going to restart the code.
C
C                 *** Following a completed task ***
C  If
C     IDID = 1, call the code again to continue the integration
C                  another step in the direction of TOUT.
C
C     IDID = 2 or 3, define a new TOUT and call the code again.
C                  TOUT must be different from T. You cannot change
C                  the direction of integration without restarting.
C
C                 *** Following an interrupted task ***
C               To show the code that you realize the task was
C               interrupted and that you want to continue, you
C               must take appropriate action and set INFO(1) = 1
C  If
C    IDID = -1, The code has taken about 500 steps.
C                  If you want to continue, set INFO(1) = 1 and
C                  call the code again. An additional 500 steps
C                  will be allowed.
C
C    IDID = -2, The error tolerances RTOL, ATOL have been
C                  increased to values the code estimates appropriate
C                  for continuing. You may want to change them
C                  yourself. If you are sure you want to continue
C                  with relaxed error tolerances, set INFO(1)=1 and
C                  call the code again.
C
C    IDID = -3, A solution component is zero and you set the
C                  corresponding component of ATOL to zero. If you
C                  are sure you want to continue, you must first
C                  alter the error criterion to use positive values
C                  for those components of ATOL corresponding to zero
C                  solution components, then set INFO(1)=1 and call
C                  the code again.
C
C    IDID = -4,-5  --- Cannot occur with this code.
C
C    IDID = -6, Repeated error test failures occurred on the
C                  last attempted step in DDASSL. A singularity in the
C                  solution may be present. If you are absolutely
C                  certain you want to continue, you should restart
C                  the integration. (Provide initial values of Y and
C                  YPRIME which are consistent)
C
C    IDID = -7, Repeated convergence test failures occurred
C                  on the last attempted step in DDASSL. An inaccurate
C                  or ill-conditioned JACOBIAN may be the problem. If
C                  you are absolutely certain you want to continue, you
C                  should restart the integration.
C
C    IDID = -8, The matrix of partial derivatives is singular.
C                  Some of your equations may be redundant.
C                  DDASSL cannot solve the problem as stated.
C                  It is possible that the redundant equations
C                  could be removed, and then DDASSL could
C                  solve the problem. It is also possible
C                  that a solution to your problem either
C                  does not exist or is not unique.
C
C    IDID = -9, DDASSL had multiple convergence test
C                  failures, preceeded by multiple error
C                  test failures, on the last attempted step.
C                  It is possible that your problem
C                  is ill-posed, and cannot be solved
C                  using this code. Or, there may be a
C                  discontinuity or a singularity in the
C                  solution. If you are absolutely certain
C                  you want to continue, you should restart
C                  the integration.
C
C    IDID =-10, DDASSL had multiple convergence test failures
C                  because IRES was equal to minus one.
C                  If you are absolutely certain you want
C                  to continue, you should restart the
C                  integration.
C
C    IDID =-11, IRES=-2 was encountered, and control is being
C                  returned to the calling program.
C
C    IDID =-12, DDASSL failed to compute the initial YPRIME.
C                  This could happen because the initial
C                  approximation to YPRIME was not very good, or
C                  if a YPRIME consistent with the initial Y
C                  does not exist. The problem could also be caused
C                  by an inaccurate or singular iteration matrix.
C
C    IDID = -13,..,-32  --- Cannot occur with this code.
C
C
C                 *** Following a terminated task ***
C
C  If IDID= -33, you cannot continue the solution of this problem.
C                  An attempt to do so will result in your
C                  run being terminated.
C
C
C  -------- ERROR MESSAGES ---------------------------------------------
C
C      The SLATEC error print routine XERMSG is called in the event of
C   unsuccessful completion of a task.  Most of these are treated as
C   "recoverable errors", which means that (unless the user has directed
C   otherwise) control will be returned to the calling program for
C   possible action after the message has been printed.
C
C   In the event of a negative value of IDID other than -33, an appro-
C   priate message is printed and the "error number" printed by XERMSG
C   is the value of IDID.  There are quite a number of illegal input
C   errors that can lead to a returned value IDID=-33.  The conditions
C   and their printed "error numbers" are as follows:
C
C   Error number       Condition
C
C        1       Some element of INFO vector is not zero or one.
C        2       NEQ .le. 0
C        3       MAXORD not in range.
C        4       LRW is less than the required length for RWORK.
C        5       LIW is less than the required length for IWORK.
C        6       Some element of RTOL is .lt. 0
C        7       Some element of ATOL is .lt. 0
C        8       All elements of RTOL and ATOL are zero.
C        9       INFO(4)=1 and TSTOP is behind TOUT.
C       10       HMAX .lt. 0.0
C       11       TOUT is behind T.
C       12       INFO(8)=1 and H0=0.0
C       13       Some element of WT is .le. 0.0
C       14       TOUT is too close to T to start integration.
C       15       INFO(4)=1 and TSTOP is behind T.
C       16       --( Not used in this version )--
C       17       ML illegal.  Either .lt. 0 or .gt. NEQ
C       18       MU illegal.  Either .lt. 0 or .gt. NEQ
C       19       TOUT = T.
C
C   If DDASSL is called again without any action taken to remove the
C   cause of an unsuccessful return, XERMSG will be called with a fatal
C   error flag, which will cause unconditional termination of the
C   program.  There are two such fatal errors:
C
C   Error number -998:  The last step was terminated with a negative
C       value of IDID other than -33, and no appropriate action was
C       taken.
C
C   Error number -999:  The previous call was terminated because of
C       illegal input (IDID=-33) and there is illegal input in the
C       present call, as well.  (Suspect infinite loop.)
C
C  ---------------------------------------------------------------------
C
C***REFERENCES  A DESCRIPTION OF DASSL: A DIFFERENTIAL/ALGEBRAIC
C                 SYSTEM SOLVER, L. R. PETZOLD, SAND82-8637,
C                 SANDIA NATIONAL LABORATORIES, SEPTEMBER 1982.
C***ROUTINES CALLED  DLAMCH, DDAINI, DDANRM, DDASTP, DDATRP, DDAWTS,
C                    XERMSG
C***REVISION HISTORY  (YYMMDD)
C   830315  DATE WRITTEN
C   880387  Code changes made.  All common statements have been
C           replaced by a DATA statement, which defines pointers into
C           RWORK, and PARAMETER statements which define pointers
C           into IWORK.  As well the documentation has gone through
C           grammatical changes.
C   881005  The prologue has been changed to mixed case.
C           The subordinate routines had revision dates changed to
C           this date, although the documentation for these routines
C           is all upper case.  No code changes.
C   890511  Code changes made.  The DATA statement in the declaration
C           section of DDASSL was replaced with a PARAMETER
C           statement.  Also the statement S = 100.D0 was removed
C           from the top of the Newton iteration in DDASTP.
C           The subordinate routines had revision dates changed to
C           this date.
C   890517  The revision date syntax was replaced with the revision
C           history syntax.  Also the "DECK" comment was added to
C           the top of all subroutines.  These changes are consistent
C           with new SLATEC guidelines.
C           The subordinate routines had revision dates changed to
C           this date.  No code changes.
C   891013  Code changes made.
C           Removed all occurrances of FLOAT or DBLE.  All operations
C           are now performed with "mixed-mode" arithmetic.
C           Also, specific function names were replaced with generic
C           function names to be consistent with new SLATEC guidelines.
C           In particular:
C              Replaced DSQRT with SQRT everywhere.
C              Replaced DABS with ABS everywhere.
C              Replaced DMIN1 with MIN everywhere.
C              Replaced MIN0 with MIN everywhere.
C              Replaced DMAX1 with MAX everywhere.
C              Replaced MAX0 with MAX everywhere.
C              Replaced DSIGN with SIGN everywhere.
C           Also replaced REVISION DATE with REVISION HISTORY in all
C           subordinate routines.
C  901004  Miscellaneous changes to prologue to complete conversion
C          to SLATEC 4.0 format.  No code changes.  (F.N.Fritsch)
C  901009  Corrected GAMS classification code and converted subsidiary
C          routines to 4.0 format.  No code changes.  (F.N.Fritsch)
C  901010  Converted XERRWV calls to XERMSG calls.  (R.Clemens,AFWL)
C  901019  Code changes made.
C          Merged SLATEC 4.0 changes with previous changes made
C          by C. Ulrich.  Below is a history of the changes made by
C          C. Ulrich. (Changes in subsidiary routines are implied
C          by this history)
C          891228  Bug was found and repaired inside the DDASSL
C                  and DDAINI routines.  DDAINI was incorrectly
C                  returning the initial T with Y and YPRIME
C                  computed at T+H.  The routine now returns T+H
C                  rather than the initial T.
C                  Cosmetic changes made to DDASTP.
C          900904  Three modifications were made to fix a bug (inside
C                  DDASSL) re interpolation for continuation calls and
C                  cases where TN is very close to TSTOP:
C
C                  1) In testing for whether H is too large, just
C                     compare H to (TSTOP - TN), rather than
C                     (TSTOP - TN) * (1-4*UROUND), and set H to
C                     TSTOP - TN.  This will force DDASTP to step
C                     exactly to TSTOP under certain situations
C                     (i.e. when H returned from DDASTP would otherwise
C                     take TN beyond TSTOP).
C
C                  2) Inside the DDASTP loop, interpolate exactly to
C                     TSTOP if TN is very close to TSTOP (rather than
C                     interpolating to within roundoff of TSTOP).
C
C                  3) Modified IDID description for IDID = 2 to say that
C                     the solution is returned by stepping exactly to
C                     TSTOP, rather than TOUT.  (In some cases the
C                     solution is actually obtained by extrapolating
C                     over a distance near unit roundoff to TSTOP,
C                     but this small distance is deemed acceptable in
C                     these circumstances.)
C   901026  Added explicit declarations for all variables and minor
C           cosmetic changes to prologue, removed unreferenced labels,
C           and improved XERMSG calls.  (FNF)
C   901030  Added ERROR MESSAGES section and reworked other sections to
C           be of more uniform format.  (FNF)
C   910624  Fixed minor bug related to HMAX (five lines ending in
C           statement 526 in DDASSL).   (LRP)
C
C***END PROLOGUE  DDASSL
C
C**End
C
C     Declare arguments.
C
      INTEGER  NEQ, INFO(15), IDID, LRW, IWORK(*), LIW, IPAR(*)
      DOUBLE PRECISION
     *   T, Y(*), YPRIME(*), TOUT, RTOL(*), ATOL(*), RWORK(*),
     *   RPAR(*)
      EXTERNAL  RES, JAC
C
C     Declare externals.
C
      EXTERNAL  DLAMCH, DDAINI, DDANRM, DDASTP, DDATRP, DDAWTS, XERMSG
      DOUBLE PRECISION  DLAMCH, DDANRM
C
C     Declare local variables.
C
      INTEGER  I, ITEMP, LALPHA, LBETA, LCJ, LCJOLD, LCTF, LDELTA,
     *   LENIW, LENPD, LENRW, LE, LETF, LGAMMA, LH, LHMAX, LHOLD, LIPVT,
     *   LJCALC, LK, LKOLD, LIWM, LML, LMTYPE, LMU, LMXORD, LNJE, LNPD,
     *   LNRE, LNS, LNST, LNSTL, LPD, LPHASE, LPHI, LPSI, LROUND, LS,
     *   LSIGMA, LTN, LTSTOP, LWM, LWT, MBAND, MSAVE, MXORD, NPD, NTEMP,
     *   NZFLG
      DOUBLE PRECISION
     *   ATOLI, H, HMAX, HMIN, HO, R, RH, RTOLI, TDIST, TN, TNEXT,
     *   TSTOP, UROUND, YPNORM
      LOGICAL  DONE
C       Auxiliary variables for conversion of values to be included in
C       error messages.
      CHARACTER*8  XERN1, XERN2
      CHARACTER*16 XERN3, XERN4
C
C     SET POINTERS INTO IWORK
      PARAMETER (LML=1, LMU=2, LMXORD=3, LMTYPE=4, LNST=11,
     *  LNRE=12, LNJE=13, LETF=14, LCTF=15, LNPD=16,
     *  LIPVT=21, LJCALC=5, LPHASE=6, LK=7, LKOLD=8,
     *  LNS=9, LNSTL=10, LIWM=1)
C
C     SET RELATIVE OFFSET INTO RWORK
      PARAMETER (NPD=1)
C
C     SET POINTERS INTO RWORK
      PARAMETER (LTSTOP=1, LHMAX=2, LH=3, LTN=4,
     *  LCJ=5, LCJOLD=6, LHOLD=7, LS=8, LROUND=9,
     *  LALPHA=11, LBETA=17, LGAMMA=23,
     *  LPSI=29, LSIGMA=35, LDELTA=41)
C
C***FIRST EXECUTABLE STATEMENT  DDASSL
      IF(INFO(1).NE.0)GO TO 100
C
C-----------------------------------------------------------------------
C     THIS BLOCK IS EXECUTED FOR THE INITIAL CALL ONLY.
C     IT CONTAINS CHECKING OF INPUTS AND INITIALIZATIONS.
C-----------------------------------------------------------------------
C
C     FIRST CHECK INFO ARRAY TO MAKE SURE ALL ELEMENTS OF INFO
C     ARE EITHER ZERO OR ONE.
      DO 10 I=2,11
         IF(INFO(I).NE.0.AND.INFO(I).NE.1)GO TO 701
10       CONTINUE
C
      IF(NEQ.LE.0)GO TO 702
C
C     CHECK AND COMPUTE MAXIMUM ORDER
      MXORD=5
      IF(INFO(9).EQ.0)GO TO 20
         MXORD=IWORK(LMXORD)
         IF(MXORD.LT.1.OR.MXORD.GT.5)GO TO 703
20       IWORK(LMXORD)=MXORD
C
C     COMPUTE MTYPE,LENPD,LENRW.CHECK ML AND MU.
      IF(INFO(6).NE.0)GO TO 40
         LENPD=NEQ**2
         LENRW=40+(IWORK(LMXORD)+4)*NEQ+LENPD
         IF(INFO(5).NE.0)GO TO 30
            IWORK(LMTYPE)=2
            GO TO 60
30          IWORK(LMTYPE)=1
            GO TO 60
40    IF(IWORK(LML).LT.0.OR.IWORK(LML).GE.NEQ)GO TO 717
      IF(IWORK(LMU).LT.0.OR.IWORK(LMU).GE.NEQ)GO TO 718
      LENPD=(2*IWORK(LML)+IWORK(LMU)+1)*NEQ
      IF(INFO(5).NE.0)GO TO 50
         IWORK(LMTYPE)=5
         MBAND=IWORK(LML)+IWORK(LMU)+1
         MSAVE=(NEQ/MBAND)+1
         LENRW=40+(IWORK(LMXORD)+4)*NEQ+LENPD+2*MSAVE
         GO TO 60
50       IWORK(LMTYPE)=4
         LENRW=40+(IWORK(LMXORD)+4)*NEQ+LENPD
C
C     CHECK LENGTHS OF RWORK AND IWORK
60    LENIW=20+NEQ
      IWORK(LNPD)=LENPD
      IF(LRW.LT.LENRW)GO TO 704
      IF(LIW.LT.LENIW)GO TO 705
C
C     CHECK TO SEE THAT TOUT IS DIFFERENT FROM T
      IF(TOUT .EQ. T)GO TO 719
C
C     CHECK HMAX
      IF(INFO(7).EQ.0)GO TO 70
         HMAX=RWORK(LHMAX)
         IF(HMAX.LE.0.0D0)GO TO 710
70    CONTINUE
C
C     INITIALIZE COUNTERS
      IWORK(LNST)=0
      IWORK(LNRE)=0
      IWORK(LNJE)=0
C
      IWORK(LNSTL)=0
      IDID=1
      GO TO 200
C
C-----------------------------------------------------------------------
C     THIS BLOCK IS FOR CONTINUATION CALLS
C     ONLY. HERE WE CHECK INFO(1),AND IF THE
C     LAST STEP WAS INTERRUPTED WE CHECK WHETHER
C     APPROPRIATE ACTION WAS TAKEN.
C-----------------------------------------------------------------------
C
100   CONTINUE
      IF(INFO(1).EQ.1)GO TO 110
      IF(INFO(1).NE.-1)GO TO 701
C
C     IF WE ARE HERE, THE LAST STEP WAS INTERRUPTED
C     BY AN ERROR CONDITION FROM DDASTP,AND
C     APPROPRIATE ACTION WAS NOT TAKEN. THIS
C     IS A FATAL ERROR.
      WRITE (XERN1, '(I8)') IDID
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'THE LAST STEP TERMINATED WITH A NEGATIVE VALUE OF IDID = ' //
     *   XERN1 // ' AND NO APPROPRIATE ACTION WAS TAKEN.  ' //
     *   'RUN TERMINATED', -998, 2)
      RETURN
110   CONTINUE
      IWORK(LNSTL)=IWORK(LNST)
C
C-----------------------------------------------------------------------
C     THIS BLOCK IS EXECUTED ON ALL CALLS.
C     THE ERROR TOLERANCE PARAMETERS ARE
C     CHECKED, AND THE WORK ARRAY POINTERS
C     ARE SET.
C-----------------------------------------------------------------------
C
200   CONTINUE
C     CHECK RTOL,ATOL
      NZFLG=0
      RTOLI=RTOL(1)
      ATOLI=ATOL(1)
      DO 210 I=1,NEQ
         IF(INFO(2).EQ.1)RTOLI=RTOL(I)
         IF(INFO(2).EQ.1)ATOLI=ATOL(I)
         IF(RTOLI.GT.0.0D0.OR.ATOLI.GT.0.0D0)NZFLG=1
         IF(RTOLI.LT.0.0D0)GO TO 706
         IF(ATOLI.LT.0.0D0)GO TO 707
210      CONTINUE
      IF(NZFLG.EQ.0)GO TO 708
C
C     SET UP RWORK STORAGE.IWORK STORAGE IS FIXED
C     IN DATA STATEMENT.
      LE=LDELTA+NEQ
      LWT=LE+NEQ
      LPHI=LWT+NEQ
      LPD=LPHI+(IWORK(LMXORD)+1)*NEQ
      LWM=LPD
      NTEMP=NPD+IWORK(LNPD)
      IF(INFO(1).EQ.1)GO TO 400
C
C-----------------------------------------------------------------------
C     THIS BLOCK IS EXECUTED ON THE INITIAL CALL
C     ONLY. SET THE INITIAL STEP SIZE, AND
C     THE ERROR WEIGHT VECTOR, AND PHI.
C     COMPUTE INITIAL YPRIME, IF NECESSARY.
C-----------------------------------------------------------------------
C
      TN=T
      IDID=1
C
C     SET ERROR WEIGHT VECTOR WT
      CALL DDAWTS(NEQ,INFO(2),RTOL,ATOL,Y,RWORK(LWT),RPAR,IPAR)
      DO 305 I = 1,NEQ
         IF(RWORK(LWT+I-1).LE.0.0D0) GO TO 713
305      CONTINUE
C
C     COMPUTE UNIT ROUNDOFF AND HMIN
      UROUND = DLAMCH('P')
      RWORK(LROUND) = UROUND
      HMIN = 4.0D0*UROUND*MAX(ABS(T),ABS(TOUT))
C
C     CHECK INITIAL INTERVAL TO SEE THAT IT IS LONG ENOUGH
      TDIST = ABS(TOUT - T)
      IF(TDIST .LT. HMIN) GO TO 714
C
C     CHECK HO, IF THIS WAS INPUT
      IF (INFO(8) .EQ. 0) GO TO 310
         HO = RWORK(LH)
         IF ((TOUT - T)*HO .LT. 0.0D0) GO TO 711
         IF (HO .EQ. 0.0D0) GO TO 712
         GO TO 320
310    CONTINUE
C
C     COMPUTE INITIAL STEPSIZE, TO BE USED BY EITHER
C     DDASTP OR DDAINI, DEPENDING ON INFO(11)
      HO = 0.001D0*TDIST
      YPNORM = DDANRM(NEQ,YPRIME,RWORK(LWT),RPAR,IPAR)
      IF (YPNORM .GT. 0.5D0/HO) HO = 0.5D0/YPNORM
      HO = SIGN(HO,TOUT-T)
C     ADJUST HO IF NECESSARY TO MEET HMAX BOUND
320   IF (INFO(7) .EQ. 0) GO TO 330
         RH = ABS(HO)/RWORK(LHMAX)
         IF (RH .GT. 1.0D0) HO = HO/RH
C     COMPUTE TSTOP, IF APPLICABLE
330   IF (INFO(4) .EQ. 0) GO TO 340
         TSTOP = RWORK(LTSTOP)
         IF ((TSTOP - T)*HO .LT. 0.0D0) GO TO 715
         IF ((T + HO - TSTOP)*HO .GT. 0.0D0) HO = TSTOP - T
         IF ((TSTOP - TOUT)*HO .LT. 0.0D0) GO TO 709
C
C     COMPUTE INITIAL DERIVATIVE, UPDATING TN AND Y, IF APPLICABLE
340   IF (INFO(11) .EQ. 0) GO TO 350
      CALL DDAINI(TN,Y,YPRIME,NEQ,
     *  RES,JAC,HO,RWORK(LWT),IDID,RPAR,IPAR,
     *  RWORK(LPHI),RWORK(LDELTA),RWORK(LE),
     *  RWORK(LWM),IWORK(LIWM),HMIN,RWORK(LROUND),
     *  INFO(10),NTEMP)
      if(iero.ne.0) return
      IF (IDID .LT. 0) GO TO 390
C
C     LOAD H WITH HO.  STORE H IN RWORK(LH)
350   H = HO
      RWORK(LH) = H
C
C     LOAD Y AND H*YPRIME INTO PHI(*,1) AND PHI(*,2)
      ITEMP = LPHI + NEQ
      DO 370 I = 1,NEQ
         RWORK(LPHI + I - 1) = Y(I)
370      RWORK(ITEMP + I - 1) = H*YPRIME(I)
C
390   GO TO 500
C
C-------------------------------------------------------
C     THIS BLOCK IS FOR CONTINUATION CALLS ONLY. ITS
C     PURPOSE IS TO CHECK STOP CONDITIONS BEFORE
C     TAKING A STEP.
C     ADJUST H IF NECESSARY TO MEET HMAX BOUND
C-------------------------------------------------------
C
400   CONTINUE
      UROUND=RWORK(LROUND)
      DONE = .FALSE.
      TN=RWORK(LTN)
      H=RWORK(LH)
      IF(INFO(7) .EQ. 0) GO TO 410
         RH = ABS(H)/RWORK(LHMAX)
         IF(RH .GT. 1.0D0) H = H/RH
410   CONTINUE
      IF(T .EQ. TOUT) GO TO 719
      IF((T - TOUT)*H .GT. 0.0D0) GO TO 711
      IF(INFO(4) .EQ. 1) GO TO 430
      IF(INFO(3) .EQ. 1) GO TO 420
      IF((TN-TOUT)*H.LT.0.0D0)GO TO 490
      CALL DDATRP(TN,TOUT,Y,YPRIME,NEQ,IWORK(LKOLD),
     *  RWORK(LPHI),RWORK(LPSI))
      T=TOUT
      IDID = 3
      DONE = .TRUE.
      GO TO 490
420   IF((TN-T)*H .LE. 0.0D0) GO TO 490
      IF((TN - TOUT)*H .GT. 0.0D0) GO TO 425
      CALL DDATRP(TN,TN,Y,YPRIME,NEQ,IWORK(LKOLD),
     *  RWORK(LPHI),RWORK(LPSI))
      T = TN
      IDID = 1
      DONE = .TRUE.
      GO TO 490
425   CONTINUE
      CALL DDATRP(TN,TOUT,Y,YPRIME,NEQ,IWORK(LKOLD),
     *  RWORK(LPHI),RWORK(LPSI))
      T = TOUT
      IDID = 3
      DONE = .TRUE.
      GO TO 490
430   IF(INFO(3) .EQ. 1) GO TO 440
      TSTOP=RWORK(LTSTOP)
      IF((TN-TSTOP)*H.GT.0.0D0) GO TO 715
      IF((TSTOP-TOUT)*H.LT.0.0D0)GO TO 709
      IF((TN-TOUT)*H.LT.0.0D0)GO TO 450
      CALL DDATRP(TN,TOUT,Y,YPRIME,NEQ,IWORK(LKOLD),
     *   RWORK(LPHI),RWORK(LPSI))
      T=TOUT
      IDID = 3
      DONE = .TRUE.
      GO TO 490
440   TSTOP = RWORK(LTSTOP)
      IF((TN-TSTOP)*H .GT. 0.0D0) GO TO 715
      IF((TSTOP-TOUT)*H .LT. 0.0D0) GO TO 709
      IF((TN-T)*H .LE. 0.0D0) GO TO 450
      IF((TN - TOUT)*H .GT. 0.0D0) GO TO 445
      CALL DDATRP(TN,TN,Y,YPRIME,NEQ,IWORK(LKOLD),
     *  RWORK(LPHI),RWORK(LPSI))
      T = TN
      IDID = 1
      DONE = .TRUE.
      GO TO 490
445   CONTINUE
      CALL DDATRP(TN,TOUT,Y,YPRIME,NEQ,IWORK(LKOLD),
     *  RWORK(LPHI),RWORK(LPSI))
      T = TOUT
      IDID = 3
      DONE = .TRUE.
      GO TO 490
450   CONTINUE
C     CHECK WHETHER WE ARE WITHIN ROUNDOFF OF TSTOP
      IF(ABS(TN-TSTOP).GT.100.0D0*UROUND*
     *   (ABS(TN)+ABS(H)))GO TO 460
      CALL DDATRP(TN,TSTOP,Y,YPRIME,NEQ,IWORK(LKOLD),
     *  RWORK(LPHI),RWORK(LPSI))
      IDID=2
      T=TSTOP
      DONE = .TRUE.
      GO TO 490
460   TNEXT=TN+H
      IF((TNEXT-TSTOP)*H.LE.0.0D0)GO TO 490
      H=TSTOP-TN
      RWORK(LH)=H
C
490   IF (DONE) GO TO 580
C
C-------------------------------------------------------
C     THE NEXT BLOCK CONTAINS THE CALL TO THE
C     ONE-STEP INTEGRATOR DDASTP.
C     THIS IS A LOOPING POINT FOR THE INTEGRATION STEPS.
C     CHECK FOR TOO MANY STEPS.
C     UPDATE WT.
C     CHECK FOR TOO MUCH ACCURACY REQUESTED.
C     COMPUTE MINIMUM STEPSIZE.
C-------------------------------------------------------
C
500   CONTINUE
C     CHECK FOR FAILURE TO COMPUTE INITIAL YPRIME
      IF (IDID .EQ. -12) GO TO 527
C
C     CHECK FOR TOO MANY STEPS
      IF((IWORK(LNST)-IWORK(LNSTL)).LT.500)
     *   GO TO 510
           IDID=-1
           GO TO 527
C
C     UPDATE WT
510   CALL DDAWTS(NEQ,INFO(2),RTOL,ATOL,RWORK(LPHI),
     *  RWORK(LWT),RPAR,IPAR)
      DO 520 I=1,NEQ
         IF(RWORK(I+LWT-1).GT.0.0D0)GO TO 520
           IDID=-3
           GO TO 527
520   CONTINUE
C
C     TEST FOR TOO MUCH ACCURACY REQUESTED.
      R=DDANRM(NEQ,RWORK(LPHI),RWORK(LWT),RPAR,IPAR)*
     *   100.0D0*UROUND
      IF(R.LE.1.0D0)GO TO 525
C     MULTIPLY RTOL AND ATOL BY R AND RETURN
      IF(INFO(2).EQ.1)GO TO 523
           RTOL(1)=R*RTOL(1)
           ATOL(1)=R*ATOL(1)
           IDID=-2
           GO TO 527
523   DO 524 I=1,NEQ
           RTOL(I)=R*RTOL(I)
524        ATOL(I)=R*ATOL(I)
      IDID=-2
      GO TO 527
525   CONTINUE
C
C     COMPUTE MINIMUM STEPSIZE
      HMIN=4.0D0*UROUND*MAX(ABS(TN),ABS(TOUT))
C
C     TEST H VS. HMAX
      IF (INFO(7) .EQ. 0) GO TO 526
         RH = ABS(H)/RWORK(LHMAX)
         IF (RH .GT. 1.0D0) H = H/RH
526   CONTINUE           
C
      CALL DDASTP(TN,Y,YPRIME,NEQ,
     *   RES,JAC,H,RWORK(LWT),INFO(1),IDID,RPAR,IPAR,
     *   RWORK(LPHI),RWORK(LDELTA),RWORK(LE),
     *   RWORK(LWM),IWORK(LIWM),
     *   RWORK(LALPHA),RWORK(LBETA),RWORK(LGAMMA),
     *   RWORK(LPSI),RWORK(LSIGMA),
     *   RWORK(LCJ),RWORK(LCJOLD),RWORK(LHOLD),
     *   RWORK(LS),HMIN,RWORK(LROUND),
     *   IWORK(LPHASE),IWORK(LJCALC),IWORK(LK),
     *   IWORK(LKOLD),IWORK(LNS),INFO(10),NTEMP)
      if(iero.ne.0) return
527   IF(IDID.LT.0)GO TO 600
C
C--------------------------------------------------------
C     THIS BLOCK HANDLES THE CASE OF A SUCCESSFUL RETURN
C     FROM DDASTP (IDID=1).  TEST FOR STOP CONDITIONS.
C--------------------------------------------------------
C
      IF(INFO(4).NE.0)GO TO 540
           IF(INFO(3).NE.0)GO TO 530
             IF((TN-TOUT)*H.LT.0.0D0)GO TO 500
             CALL DDATRP(TN,TOUT,Y,YPRIME,NEQ,
     *         IWORK(LKOLD),RWORK(LPHI),RWORK(LPSI))
             IDID=3
             T=TOUT
             GO TO 580
530          IF((TN-TOUT)*H.GE.0.0D0)GO TO 535
             T=TN
             IDID=1
             GO TO 580
535          CALL DDATRP(TN,TOUT,Y,YPRIME,NEQ,
     *         IWORK(LKOLD),RWORK(LPHI),RWORK(LPSI))
             IDID=3
             T=TOUT
             GO TO 580
540   IF(INFO(3).NE.0)GO TO 550
      IF((TN-TOUT)*H.LT.0.0D0)GO TO 542
         CALL DDATRP(TN,TOUT,Y,YPRIME,NEQ,
     *     IWORK(LKOLD),RWORK(LPHI),RWORK(LPSI))
         T=TOUT
         IDID=3
         GO TO 580
542   IF(ABS(TN-TSTOP).LE.100.0D0*UROUND*
     *   (ABS(TN)+ABS(H)))GO TO 545
      TNEXT=TN+H
      IF((TNEXT-TSTOP)*H.LE.0.0D0)GO TO 500
      H=TSTOP-TN
      GO TO 500
545   CALL DDATRP(TN,TSTOP,Y,YPRIME,NEQ,
     *  IWORK(LKOLD),RWORK(LPHI),RWORK(LPSI))
      IDID=2
      T=TSTOP
      GO TO 580
550   IF((TN-TOUT)*H.GE.0.0D0)GO TO 555
      IF(ABS(TN-TSTOP).LE.100.0D0*UROUND*(ABS(TN)+ABS(H)))GO TO 552
      T=TN
      IDID=1
      GO TO 580
552   CALL DDATRP(TN,TSTOP,Y,YPRIME,NEQ,
     *  IWORK(LKOLD),RWORK(LPHI),RWORK(LPSI))
      IDID=2
      T=TSTOP
      GO TO 580
555   CALL DDATRP(TN,TOUT,Y,YPRIME,NEQ,
     *   IWORK(LKOLD),RWORK(LPHI),RWORK(LPSI))
      T=TOUT
      IDID=3
      GO TO 580
C
C--------------------------------------------------------
C     ALL SUCCESSFUL RETURNS FROM DDASSL ARE MADE FROM
C     THIS BLOCK.
C--------------------------------------------------------
C
580   CONTINUE
      RWORK(LTN)=TN
      RWORK(LH)=H
      RETURN
C
C-----------------------------------------------------------------------
C     THIS BLOCK HANDLES ALL UNSUCCESSFUL
C     RETURNS OTHER THAN FOR ILLEGAL INPUT.
C-----------------------------------------------------------------------
C
600   CONTINUE
      ITEMP=-IDID
      GO TO (610,620,630,690,690,640,650,660,670,675,
     *  680,685), ITEMP
C
C     THE MAXIMUM NUMBER OF STEPS WAS TAKEN BEFORE
C     REACHING TOUT
610   WRITE (XERN3, '(1P,D15.6)') TN
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'AT CURRENT T = ' // XERN3 // ' 500 STEPS TAKEN ON THIS ' //
     *   'CALL BEFORE REACHING TOUT', IDID, 1)
      GO TO 690
C
C     TOO MUCH ACCURACY FOR MACHINE PRECISION
620   WRITE (XERN3, '(1P,D15.6)') TN
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'AT T = ' // XERN3 // ' TOO MUCH ACCURACY REQUESTED FOR ' //
     *   'PRECISION OF MACHINE. RTOL AND ATOL WERE INCREASED TO ' //
     *   'APPROPRIATE VALUES', IDID, 1)
      GO TO 690
C
C     WT(I) .LE. 0.0 FOR SOME I (NOT AT START OF PROBLEM)
630   WRITE (XERN3, '(1P,D15.6)') TN
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'AT T = ' // XERN3 // ' SOME ELEMENT OF WT HAS BECOME .LE. ' //
     *   '0.0', IDID, 1)
      GO TO 690
C
C     ERROR TEST FAILED REPEATEDLY OR WITH H=HMIN
640   WRITE (XERN3, '(1P,D15.6)') TN
      WRITE (XERN4, '(1P,D15.6)') H
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'AT T = ' // XERN3 // ' AND STEPSIZE H = ' // XERN4 //
     *   ' THE ERROR TEST FAILED REPEATEDLY OR WITH ABS(H)=HMIN',
     *   IDID, 1)
      GO TO 690
C
C     CORRECTOR CONVERGENCE FAILED REPEATEDLY OR WITH H=HMIN
650   WRITE (XERN3, '(1P,D15.6)') TN
      WRITE (XERN4, '(1P,D15.6)') H
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'AT T = ' // XERN3 // ' AND STEPSIZE H = ' // XERN4 //
     *   ' THE CORRECTOR FAILED TO CONVERGE REPEATEDLY OR WITH ' //
     *   'ABS(H)=HMIN', IDID, 1)
      GO TO 690
C
C     THE ITERATION MATRIX IS SINGULAR
660   WRITE (XERN3, '(1P,D15.6)') TN
      WRITE (XERN4, '(1P,D15.6)') H
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'AT T = ' // XERN3 // ' AND STEPSIZE H = ' // XERN4 //
     *   ' THE ITERATION MATRIX IS SINGULAR', IDID, 1)
      GO TO 690
C
C     CORRECTOR FAILURE PRECEEDED BY ERROR TEST FAILURES.
670   WRITE (XERN3, '(1P,D15.6)') TN
      WRITE (XERN4, '(1P,D15.6)') H
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'AT T = ' // XERN3 // ' AND STEPSIZE H = ' // XERN4 //
     *   ' THE CORRECTOR COULD NOT CONVERGE.  ALSO, THE ERROR TEST ' //
     *   'FAILED REPEATEDLY.', IDID, 1)
      GO TO 690
C
C     CORRECTOR FAILURE BECAUSE IRES = -1
675   WRITE (XERN3, '(1P,D15.6)') TN
      WRITE (XERN4, '(1P,D15.6)') H
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'AT T = ' // XERN3 // ' AND STEPSIZE H = ' // XERN4 //
     *   ' THE CORRECTOR COULD NOT CONVERGE BECAUSE IRES WAS EQUAL ' //
     *   'TO MINUS ONE', IDID, 1)
      GO TO 690
C
C     FAILURE BECAUSE IRES = -2
680   WRITE (XERN3, '(1P,D15.6)') TN
      WRITE (XERN4, '(1P,D15.6)') H
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'AT T = ' // XERN3 // ' AND STEPSIZE H = ' // XERN4 //
     *   ' IRES WAS EQUAL TO MINUS TWO', IDID, 1)
      GO TO 690
C
C     FAILED TO COMPUTE INITIAL YPRIME
685   WRITE (XERN3, '(1P,D15.6)') TN
      WRITE (XERN4, '(1P,D15.6)') HO
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'AT T = ' // XERN3 // ' AND STEPSIZE H = ' // XERN4 //
     *   ' THE INITIAL YPRIME COULD NOT BE COMPUTED', IDID, 1)
      GO TO 690
C
690   CONTINUE
      INFO(1)=-1
      T=TN
      RWORK(LTN)=TN
      RWORK(LH)=H
      RETURN
C
C-----------------------------------------------------------------------
C     THIS BLOCK HANDLES ALL ERROR RETURNS DUE
C     TO ILLEGAL INPUT, AS DETECTED BEFORE CALLING
C     DDASTP. FIRST THE ERROR MESSAGE ROUTINE IS
C     CALLED. IF THIS HAPPENS TWICE IN
C     SUCCESSION, EXECUTION IS TERMINATED
C
C-----------------------------------------------------------------------
701   CALL XERMSG ('SLATEC', 'DDASSL',
     *   'SOME ELEMENT OF INFO VECTOR IS NOT ZERO OR ONE', 1, 1)
      GO TO 750
C
702   WRITE (XERN1, '(I8)') NEQ
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'NEQ = ' // XERN1 // ' .LE. 0', 2, 1)
      GO TO 750
C
703   WRITE (XERN1, '(I8)') MXORD
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'MAXORD = ' // XERN1 // ' NOT IN RANGE', 3, 1)
      GO TO 750
C
704   WRITE (XERN1, '(I8)') LENRW
      WRITE (XERN2, '(I8)') LRW
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'RWORK LENGTH NEEDED, LENRW = ' // XERN1 //
     *   ', EXCEEDS LRW = ' // XERN2, 4, 1)
      GO TO 750
C
705   WRITE (XERN1, '(I8)') LENIW
      WRITE (XERN2, '(I8)') LIW
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'IWORK LENGTH NEEDED, LENIW = ' // XERN1 //
     *   ', EXCEEDS LIW = ' // XERN2, 5, 1)
      GO TO 750
C
706   CALL XERMSG ('SLATEC', 'DDASSL',
     *   'SOME ELEMENT OF RTOL IS .LT. 0', 6, 1)
      GO TO 750
C
707   CALL XERMSG ('SLATEC', 'DDASSL',
     *   'SOME ELEMENT OF ATOL IS .LT. 0', 7, 1)
      GO TO 750
C
708   CALL XERMSG ('SLATEC', 'DDASSL',
     *   'ALL ELEMENTS OF RTOL AND ATOL ARE ZERO', 8, 1)
      GO TO 750
C
709   WRITE (XERN3, '(1P,D15.6)') TSTOP
      WRITE (XERN4, '(1P,D15.6)') TOUT
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'INFO(4) = 1 AND TSTOP = ' // XERN3 // ' BEHIND TOUT = ' //
     *   XERN4, 9, 1)
      GO TO 750
C
710   WRITE (XERN3, '(1P,D15.6)') HMAX
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'HMAX = ' // XERN3 // ' .LT. 0.0', 10, 1)
      GO TO 750
C
711   WRITE (XERN3, '(1P,D15.6)') TOUT
      WRITE (XERN4, '(1P,D15.6)') T
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'TOUT = ' // XERN3 // ' BEHIND T = ' // XERN4, 11, 1)
      GO TO 750
C
712   CALL XERMSG ('SLATEC', 'DDASSL',
     *   'INFO(8)=1 AND H0=0.0', 12, 1)
      GO TO 750
C
713   CALL XERMSG ('SLATEC', 'DDASSL',
     *   'SOME ELEMENT OF WT IS .LE. 0.0', 13, 1)
      GO TO 750
C
714   WRITE (XERN3, '(1P,D15.6)') TOUT
      WRITE (XERN4, '(1P,D15.6)') T
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'TOUT = ' // XERN3 // ' TOO CLOSE TO T = ' // XERN4 //
     *   ' TO START INTEGRATION', 14, 1)
      GO TO 750
C
715   WRITE (XERN3, '(1P,D15.6)') TSTOP
      WRITE (XERN4, '(1P,D15.6)') T
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'INFO(4)=1 AND TSTOP = ' // XERN3 // ' BEHIND T = ' // XERN4,
     *   15, 1)
      GO TO 750
C
717   WRITE (XERN1, '(I8)') IWORK(LML)
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'ML = ' // XERN1 // ' ILLEGAL.  EITHER .LT. 0 OR .GT. NEQ',
     *   17, 1)
      GO TO 750
C
718   WRITE (XERN1, '(I8)') IWORK(LMU)
      CALL XERMSG ('SLATEC', 'DDASSL',
     *   'MU = ' // XERN1 // ' ILLEGAL.  EITHER .LT. 0 OR .GT. NEQ',
     *   18, 1)
      GO TO 750
C
719   WRITE (XERN3, '(1P,D15.6)') TOUT
      CALL XERMSG ('SLATEC', 'DDASSL',
     *  'TOUT = T = ' // XERN3, 19, 1)
      GO TO 750
C
750   IDID=-33
      IF(INFO(1).EQ.-1) THEN
         CALL XERMSG ('SLATEC', 'DDASSL',
     *      'REPEATED OCCURRENCES OF ILLEGAL INPUT$$' //
     *      'RUN TERMINATED. APPARENT INFINITE LOOP', -999, 2)
      ENDIF
C
      INFO(1)=-1
      RETURN
C-----------END OF SUBROUTINE DDASSL------------------------------------
      END

      SUBROUTINE DDASTP (X, Y, YPRIME, NEQ, RES, JAC, H, WT, JSTART,
     +   IDID, RPAR, IPAR, PHI, DELTA, E, WM, IWM, ALPHA, BETA, GAMMA,
     +   PSI, SIGMA, CJ, CJOLD, HOLD, S, HMIN, UROUND, IPHASE, JCALC,
     +   K, KOLD, NS, NONNEG, NTEMP)
      common/ierode/iero

C***BEGIN PROLOGUE  DDASTP
C***SUBSIDIARY
C***PURPOSE  Perform one step of the DDASSL integration.
C***LIBRARY   SLATEC (DASSL)
C***TYPE      DOUBLE PRECISION (SDASTP-S, DDASTP-D)
C***AUTHOR  PETZOLD, LINDA R., (LLNL)
C***DESCRIPTION
C-----------------------------------------------------------------------
C     DDASTP SOLVES A SYSTEM OF DIFFERENTIAL/
C     ALGEBRAIC EQUATIONS OF THE FORM
C     G(X,Y,YPRIME) = 0,  FOR ONE STEP (NORMALLY
C     FROM X TO X+H).
C
C     THE METHODS USED ARE MODIFIED DIVIDED
C     DIFFERENCE,FIXED LEADING COEFFICIENT
C     FORMS OF BACKWARD DIFFERENTIATION
C     FORMULAS. THE CODE ADJUSTS THE STEPSIZE
C     AND ORDER TO CONTROL THE LOCAL ERROR PER
C     STEP.
C
C
C     THE PARAMETERS REPRESENT
C     X  --        INDEPENDENT VARIABLE
C     Y  --        SOLUTION VECTOR AT X
C     YPRIME --    DERIVATIVE OF SOLUTION VECTOR
C                  AFTER SUCCESSFUL STEP
C     NEQ --       NUMBER OF EQUATIONS TO BE INTEGRATED
C     RES --       EXTERNAL USER-SUPPLIED SUBROUTINE
C                  TO EVALUATE THE RESIDUAL.  THE CALL IS
C                  CALL RES(X,Y,YPRIME,DELTA,IRES,RPAR,IPAR)
C                  X,Y,YPRIME ARE INPUT.  DELTA IS OUTPUT.
C                  ON INPUT, IRES=0.  RES SHOULD ALTER IRES ONLY
C                  IF IT ENCOUNTERS AN ILLEGAL VALUE OF Y OR A
C                  STOP CONDITION.  SET IRES=-1 IF AN INPUT VALUE
C                  OF Y IS ILLEGAL, AND DDASTP WILL TRY TO SOLVE
C                  THE PROBLEM WITHOUT GETTING IRES = -1.  IF
C                  IRES=-2, DDASTP RETURNS CONTROL TO THE CALLING
C                  PROGRAM WITH IDID = -11.
C     JAC --       EXTERNAL USER-SUPPLIED ROUTINE TO EVALUATE
C                  THE ITERATION MATRIX (THIS IS OPTIONAL)
C                  THE CALL IS OF THE FORM
C                  CALL JAC(X,Y,YPRIME,PD,CJ,RPAR,IPAR)
C                  PD IS THE MATRIX OF PARTIAL DERIVATIVES,
C                  PD=DG/DY+CJ*DG/DYPRIME
C     H --         APPROPRIATE STEP SIZE FOR NEXT STEP.
C                  NORMALLY DETERMINED BY THE CODE
C     WT --        VECTOR OF WEIGHTS FOR ERROR CRITERION.
C     JSTART --    INTEGER VARIABLE SET 0 FOR
C                  FIRST STEP, 1 OTHERWISE.
C     IDID --      COMPLETION CODE WITH THE FOLLOWING MEANINGS:
C                  IDID= 1 -- THE STEP WAS COMPLETED SUCCESSFULLY
C                  IDID=-6 -- THE ERROR TEST FAILED REPEATEDLY
C                  IDID=-7 -- THE CORRECTOR COULD NOT CONVERGE
C                  IDID=-8 -- THE ITERATION MATRIX IS SINGULAR
C                  IDID=-9 -- THE CORRECTOR COULD NOT CONVERGE.
C                             THERE WERE REPEATED ERROR TEST
C                             FAILURES ON THIS STEP.
C                  IDID=-10-- THE CORRECTOR COULD NOT CONVERGE
C                             BECAUSE IRES WAS EQUAL TO MINUS ONE
C                  IDID=-11-- IRES EQUAL TO -2 WAS ENCOUNTERED,
C                             AND CONTROL IS BEING RETURNED TO
C                             THE CALLING PROGRAM
C     RPAR,IPAR -- REAL AND INTEGER PARAMETER ARRAYS THAT
C                  ARE USED FOR COMMUNICATION BETWEEN THE
C                  CALLING PROGRAM AND EXTERNAL USER ROUTINES
C                  THEY ARE NOT ALTERED BY DDASTP
C     PHI --       ARRAY OF DIVIDED DIFFERENCES USED BY
C                  DDASTP. THE LENGTH IS NEQ*(K+1),WHERE
C                  K IS THE MAXIMUM ORDER
C     DELTA,E --   WORK VECTORS FOR DDASTP OF LENGTH NEQ
C     WM,IWM --    REAL AND INTEGER ARRAYS STORING
C                  MATRIX INFORMATION SUCH AS THE MATRIX
C                  OF PARTIAL DERIVATIVES,PERMUTATION
C                  VECTOR,AND VARIOUS OTHER INFORMATION.
C
C     THE OTHER PARAMETERS ARE INFORMATION
C     WHICH IS NEEDED INTERNALLY BY DDASTP TO
C     CONTINUE FROM STEP TO STEP.
C
C-----------------------------------------------------------------------
C***ROUTINES CALLED  DDAJAC, DDANRM, DDASLV, DDATRP
C***REVISION HISTORY  (YYMMDD)
C   830315  DATE WRITTEN
C   901009  Finished conversion to SLATEC 4.0 format (F.N.Fritsch)
C   901019  Merged changes made by C. Ulrich with SLATEC 4.0 format.
C   901026  Added explicit declarations for all variables and minor
C           cosmetic changes to prologue.  (FNF)
C***END PROLOGUE  DDASTP
C
      INTEGER  NEQ, JSTART, IDID, IPAR(*), IWM(*), IPHASE, JCALC, K,
     *   KOLD, NS, NONNEG, NTEMP
      DOUBLE PRECISION
     *   X, Y(*), YPRIME(*), H, WT(*), RPAR(*), PHI(NEQ,*), DELTA(*),
     *   E(*), WM(*), ALPHA(*), BETA(*), GAMMA(*), PSI(*), SIGMA(*), CJ,
     *   CJOLD, HOLD, S, HMIN, UROUND
      EXTERNAL  RES, JAC
C
      EXTERNAL  DDAJAC, DDANRM, DDASLV, DDATRP
      DOUBLE PRECISION  DDANRM
C
      INTEGER  I, IER, IRES, J, J1, KDIFF, KM1, KNEW, KP1, KP2, LCTF,
     *   LETF, LMXORD, LNJE, LNRE, LNST, M, MAXIT, NCF, NEF, NSF, NSP1
      DOUBLE PRECISION
     *   ALPHA0, ALPHAS, CJLAST, CK, DELNRM, ENORM, ERK, ERKM1,
     *   ERKM2, ERKP1, ERR, EST, HNEW, OLDNRM, PNORM, R, RATE, TEMP1,
     *   TEMP2, TERK, TERKM1, TERKM2, TERKP1, XOLD, XRATE
      LOGICAL  CONVGD
C
      PARAMETER (LMXORD=3)
      PARAMETER (LNST=11)
      PARAMETER (LNRE=12)
      PARAMETER (LNJE=13)
      PARAMETER (LETF=14)
      PARAMETER (LCTF=15)
C
      DATA MAXIT/4/
      DATA XRATE/0.25D0/
C
C
C
C
C
C-----------------------------------------------------------------------
C     BLOCK 1.
C     INITIALIZE. ON THE FIRST CALL,SET
C     THE ORDER TO 1 AND INITIALIZE
C     OTHER VARIABLES.
C-----------------------------------------------------------------------
C
C     INITIALIZATIONS FOR ALL CALLS
C***FIRST EXECUTABLE STATEMENT  DDASTP
      IDID=1
      XOLD=X
      NCF=0
      NSF=0
      NEF=0
      IF(JSTART .NE. 0) GO TO 120
C
C     IF THIS IS THE FIRST STEP,PERFORM
C     OTHER INITIALIZATIONS
      IWM(LETF) = 0
      IWM(LCTF) = 0
      K=1
      KOLD=0
      HOLD=0.0D0
      JSTART=1
      PSI(1)=H
      CJOLD = 1.0D0/H
      CJ = CJOLD
      S = 100.D0
      JCALC = -1
      DELNRM=1.0D0
      IPHASE = 0
      NS=0
120   CONTINUE
C
C
C
C
C
C-----------------------------------------------------------------------
C     BLOCK 2
C     COMPUTE COEFFICIENTS OF FORMULAS FOR
C     THIS STEP.
C-----------------------------------------------------------------------
200   CONTINUE
      KP1=K+1
      KP2=K+2
      KM1=K-1
      XOLD=X
      IF(H.NE.HOLD.OR.K .NE. KOLD) NS = 0
      NS=MIN(NS+1,KOLD+2)
      NSP1=NS+1
      IF(KP1 .LT. NS)GO TO 230
C
      BETA(1)=1.0D0
      ALPHA(1)=1.0D0
      TEMP1=H
      GAMMA(1)=0.0D0
      SIGMA(1)=1.0D0
      DO 210 I=2,KP1
         TEMP2=PSI(I-1)
         PSI(I-1)=TEMP1
         BETA(I)=BETA(I-1)*PSI(I-1)/TEMP2
         TEMP1=TEMP2+H
         ALPHA(I)=H/TEMP1
         SIGMA(I)=(I-1)*SIGMA(I-1)*ALPHA(I)
         GAMMA(I)=GAMMA(I-1)+ALPHA(I-1)/H
210      CONTINUE
      PSI(KP1)=TEMP1
230   CONTINUE
C
C     COMPUTE ALPHAS, ALPHA0
      ALPHAS = 0.0D0
      ALPHA0 = 0.0D0
      DO 240 I = 1,K
        ALPHAS = ALPHAS - 1.0D0/I
        ALPHA0 = ALPHA0 - ALPHA(I)
240     CONTINUE
C
C     COMPUTE LEADING COEFFICIENT CJ
      CJLAST = CJ
      CJ = -ALPHAS/H
C
C     COMPUTE VARIABLE STEPSIZE ERROR COEFFICIENT CK
      CK = ABS(ALPHA(KP1) + ALPHAS - ALPHA0)
      CK = MAX(CK,ALPHA(KP1))
C
C     DECIDE WHETHER NEW JACOBIAN IS NEEDED
      TEMP1 = (1.0D0 - XRATE)/(1.0D0 + XRATE)
      TEMP2 = 1.0D0/TEMP1
      IF (CJ/CJOLD .LT. TEMP1 .OR. CJ/CJOLD .GT. TEMP2) JCALC = -1
      IF (CJ .NE. CJLAST) S = 100.D0
C
C     CHANGE PHI TO PHI STAR
      IF(KP1 .LT. NSP1) GO TO 280
      DO 270 J=NSP1,KP1
         DO 260 I=1,NEQ
260         PHI(I,J)=BETA(J)*PHI(I,J)
270      CONTINUE
280   CONTINUE
C
C     UPDATE TIME
      X=X+H
C
C
C
C
C
C-----------------------------------------------------------------------
C     BLOCK 3
C     PREDICT THE SOLUTION AND DERIVATIVE,
C     AND SOLVE THE CORRECTOR EQUATION
C-----------------------------------------------------------------------
C
C     FIRST,PREDICT THE SOLUTION AND DERIVATIVE
300   CONTINUE
      DO 310 I=1,NEQ
         Y(I)=PHI(I,1)
310      YPRIME(I)=0.0D0
      DO 330 J=2,KP1
         DO 320 I=1,NEQ
            Y(I)=Y(I)+PHI(I,J)
320         YPRIME(I)=YPRIME(I)+GAMMA(J)*PHI(I,J)
330   CONTINUE
      PNORM = DDANRM (NEQ,Y,WT,RPAR,IPAR)
C
C
C
C     SOLVE THE CORRECTOR EQUATION USING A
C     MODIFIED NEWTON SCHEME.
      CONVGD= .TRUE.
      M=0
      IWM(LNRE)=IWM(LNRE)+1
      IRES = 0
      CALL RES(X,Y,YPRIME,DELTA,IRES,RPAR,IPAR)
      if(iero.ne.0) return
      IF (IRES .LT. 0) GO TO 380
C
C
C     IF INDICATED,REEVALUATE THE
C     ITERATION MATRIX PD = DG/DY + CJ*DG/DYPRIME
C     (WHERE G(X,Y,YPRIME)=0). SET
C     JCALC TO 0 AS AN INDICATOR THAT
C     THIS HAS BEEN DONE.
      IF(JCALC .NE. -1)GO TO 340
      IWM(LNJE)=IWM(LNJE)+1
      JCALC=0
      CALL DDAJAC(NEQ,X,Y,YPRIME,DELTA,CJ,H,
     * IER,WT,E,WM,IWM,RES,IRES,UROUND,JAC,RPAR,
     * IPAR,NTEMP)
      if(iero.ne.0) return
      CJOLD=CJ
      S = 100.D0
      IF (IRES .LT. 0) GO TO 380
      IF(IER .NE. 0)GO TO 380
      NSF=0
C
C
C     INITIALIZE THE ERROR ACCUMULATION VECTOR E.
340   CONTINUE
      DO 345 I=1,NEQ
345      E(I)=0.0D0
C
C
C     CORRECTOR LOOP.
350   CONTINUE
C
C     MULTIPLY RESIDUAL BY TEMP1 TO ACCELERATE CONVERGENCE
      TEMP1 = 2.0D0/(1.0D0 + CJ/CJOLD)
      DO 355 I = 1,NEQ
355     DELTA(I) = DELTA(I) * TEMP1
C
C     COMPUTE A NEW ITERATE (BACK-SUBSTITUTION).
C     STORE THE CORRECTION IN DELTA.
      CALL DDASLV(NEQ,DELTA,WM,IWM)
C
C     UPDATE Y,E,AND YPRIME
      DO 360 I=1,NEQ
         Y(I)=Y(I)-DELTA(I)
         E(I)=E(I)-DELTA(I)
360      YPRIME(I)=YPRIME(I)-CJ*DELTA(I)
C
C     TEST FOR CONVERGENCE OF THE ITERATION
      DELNRM=DDANRM(NEQ,DELTA,WT,RPAR,IPAR)
      IF (DELNRM .LE. 100.D0*UROUND*PNORM) GO TO 375
      IF (M .GT. 0) GO TO 365
         OLDNRM = DELNRM
         GO TO 367
365   RATE = (DELNRM/OLDNRM)**(1.0D0/M)
      IF (RATE .GT. 0.90D0) GO TO 370
      S = RATE/(1.0D0 - RATE)
367   IF (S*DELNRM .LE. 0.33D0) GO TO 375
C
C     THE CORRECTOR HAS NOT YET CONVERGED.
C     UPDATE M AND TEST WHETHER THE
C     MAXIMUM NUMBER OF ITERATIONS HAVE
C     BEEN TRIED.
      M=M+1
      IF(M.GE.MAXIT)GO TO 370
C
C     EVALUATE THE RESIDUAL
C     AND GO BACK TO DO ANOTHER ITERATION
      IWM(LNRE)=IWM(LNRE)+1
      IRES = 0
      CALL RES(X,Y,YPRIME,DELTA,IRES,
     *  RPAR,IPAR)
      if(iero.ne.0) return
      IF (IRES .LT. 0) GO TO 380
      GO TO 350
C
C
C     THE CORRECTOR FAILED TO CONVERGE IN MAXIT
C     ITERATIONS. IF THE ITERATION MATRIX
C     IS NOT CURRENT,RE-DO THE STEP WITH
C     A NEW ITERATION MATRIX.
370   CONTINUE
      IF(JCALC.EQ.0)GO TO 380
      JCALC=-1
      GO TO 300
C
C
C     THE ITERATION HAS CONVERGED.  IF NONNEGATIVITY OF SOLUTION IS
C     REQUIRED, SET THE SOLUTION NONNEGATIVE, IF THE PERTURBATION
C     TO DO IT IS SMALL ENOUGH.  IF THE CHANGE IS TOO LARGE, THEN
C     CONSIDER THE CORRECTOR ITERATION TO HAVE FAILED.
375   IF(NONNEG .EQ. 0) GO TO 390
      DO 377 I = 1,NEQ
377      DELTA(I) = MIN(Y(I),0.0D0)
      DELNRM = DDANRM(NEQ,DELTA,WT,RPAR,IPAR)
      IF(DELNRM .GT. 0.33D0) GO TO 380
      DO 378 I = 1,NEQ
378      E(I) = E(I) - DELTA(I)
      GO TO 390
C
C
C     EXITS FROM BLOCK 3
C     NO CONVERGENCE WITH CURRENT ITERATION
C     MATRIX,OR SINGULAR ITERATION MATRIX
380   CONVGD= .FALSE.
390   JCALC = 1
      IF(.NOT.CONVGD)GO TO 600
C
C
C
C
C
C-----------------------------------------------------------------------
C     BLOCK 4
C     ESTIMATE THE ERRORS AT ORDERS K,K-1,K-2
C     AS IF CONSTANT STEPSIZE WAS USED. ESTIMATE
C     THE LOCAL ERROR AT ORDER K AND TEST
C     WHETHER THE CURRENT STEP IS SUCCESSFUL.
C-----------------------------------------------------------------------
C
C     ESTIMATE ERRORS AT ORDERS K,K-1,K-2
      ENORM = DDANRM(NEQ,E,WT,RPAR,IPAR)
      ERK = SIGMA(K+1)*ENORM
      TERK = (K+1)*ERK
      EST = ERK
      KNEW=K
      IF(K .EQ. 1)GO TO 430
      DO 405 I = 1,NEQ
405     DELTA(I) = PHI(I,KP1) + E(I)
      ERKM1=SIGMA(K)*DDANRM(NEQ,DELTA,WT,RPAR,IPAR)
      TERKM1 = K*ERKM1
      IF(K .GT. 2)GO TO 410
      IF(TERKM1 .LE. 0.5D0*TERK)GO TO 420
      GO TO 430
410   CONTINUE
      DO 415 I = 1,NEQ
415     DELTA(I) = PHI(I,K) + DELTA(I)
      ERKM2=SIGMA(K-1)*DDANRM(NEQ,DELTA,WT,RPAR,IPAR)
      TERKM2 = (K-1)*ERKM2
      IF(MAX(TERKM1,TERKM2).GT.TERK)GO TO 430
C     LOWER THE ORDER
420   CONTINUE
      KNEW=K-1
      EST = ERKM1
C
C
C     CALCULATE THE LOCAL ERROR FOR THE CURRENT STEP
C     TO SEE IF THE STEP WAS SUCCESSFUL
430   CONTINUE
      ERR = CK * ENORM
      IF(ERR .GT. 1.0D0)GO TO 600
C
C
C
C
C
C-----------------------------------------------------------------------
C     BLOCK 5
C     THE STEP IS SUCCESSFUL. DETERMINE
C     THE BEST ORDER AND STEPSIZE FOR
C     THE NEXT STEP. UPDATE THE DIFFERENCES
C     FOR THE NEXT STEP.
C-----------------------------------------------------------------------
      IDID=1
      IWM(LNST)=IWM(LNST)+1
      KDIFF=K-KOLD
      KOLD=K
      HOLD=H
C
C
C     ESTIMATE THE ERROR AT ORDER K+1 UNLESS:
C        ALREADY DECIDED TO LOWER ORDER, OR
C        ALREADY USING MAXIMUM ORDER, OR
C        STEPSIZE NOT CONSTANT, OR
C        ORDER RAISED IN PREVIOUS STEP
      IF(KNEW.EQ.KM1.OR.K.EQ.IWM(LMXORD))IPHASE=1
      IF(IPHASE .EQ. 0)GO TO 545
      IF(KNEW.EQ.KM1)GO TO 540
      IF(K.EQ.IWM(LMXORD)) GO TO 550
      IF(KP1.GE.NS.OR.KDIFF.EQ.1)GO TO 550
      DO 510 I=1,NEQ
510      DELTA(I)=E(I)-PHI(I,KP2)
      ERKP1 = (1.0D0/(K+2))*DDANRM(NEQ,DELTA,WT,RPAR,IPAR)
      TERKP1 = (K+2)*ERKP1
      IF(K.GT.1)GO TO 520
      IF(TERKP1.GE.0.5D0*TERK)GO TO 550
      GO TO 530
520   IF(TERKM1.LE.MIN(TERK,TERKP1))GO TO 540
      IF(TERKP1.GE.TERK.OR.K.EQ.IWM(LMXORD))GO TO 550
C
C     RAISE ORDER
530   K=KP1
      EST = ERKP1
      GO TO 550
C
C     LOWER ORDER
540   K=KM1
      EST = ERKM1
      GO TO 550
C
C     IF IPHASE = 0, INCREASE ORDER BY ONE AND MULTIPLY STEPSIZE BY
C     FACTOR TWO
545   K = KP1
      HNEW = H*2.0D0
      H = HNEW
      GO TO 575
C
C
C     DETERMINE THE APPROPRIATE STEPSIZE FOR
C     THE NEXT STEP.
550   HNEW=H
      TEMP2=K+1
      R=(2.0D0*EST+0.0001D0)**(-1.0D0/TEMP2)
      IF(R .LT. 2.0D0) GO TO 555
      HNEW = 2.0D0*H
      GO TO 560
555   IF(R .GT. 1.0D0) GO TO 560
      R = MAX(0.5D0,MIN(0.9D0,R))
      HNEW = H*R
560   H=HNEW
C
C
C     UPDATE DIFFERENCES FOR NEXT STEP
575   CONTINUE
      IF(KOLD.EQ.IWM(LMXORD))GO TO 585
      DO 580 I=1,NEQ
580      PHI(I,KP2)=E(I)
585   CONTINUE
      DO 590 I=1,NEQ
590      PHI(I,KP1)=PHI(I,KP1)+E(I)
      DO 595 J1=2,KP1
         J=KP1-J1+1
         DO 595 I=1,NEQ
595      PHI(I,J)=PHI(I,J)+PHI(I,J+1)
      RETURN
C
C
C
C
C
C-----------------------------------------------------------------------
C     BLOCK 6
C     THE STEP IS UNSUCCESSFUL. RESTORE X,PSI,PHI
C     DETERMINE APPROPRIATE STEPSIZE FOR
C     CONTINUING THE INTEGRATION, OR EXIT WITH
C     AN ERROR FLAG IF THERE HAVE BEEN MANY
C     FAILURES.
C-----------------------------------------------------------------------
600   IPHASE = 1
C
C     RESTORE X,PHI,PSI
      X=XOLD
      IF(KP1.LT.NSP1)GO TO 630
      DO 620 J=NSP1,KP1
         TEMP1=1.0D0/BETA(J)
         DO 610 I=1,NEQ
610         PHI(I,J)=TEMP1*PHI(I,J)
620      CONTINUE
630   CONTINUE
      DO 640 I=2,KP1
640      PSI(I-1)=PSI(I)-H
C
C
C     TEST WHETHER FAILURE IS DUE TO CORRECTOR ITERATION
C     OR ERROR TEST
      IF(CONVGD)GO TO 660
      IWM(LCTF)=IWM(LCTF)+1
C
C
C     THE NEWTON ITERATION FAILED TO CONVERGE WITH
C     A CURRENT ITERATION MATRIX.  DETERMINE THE CAUSE
C     OF THE FAILURE AND TAKE APPROPRIATE ACTION.
      IF(IER.EQ.0)GO TO 650
C
C     THE ITERATION MATRIX IS SINGULAR. REDUCE
C     THE STEPSIZE BY A FACTOR OF 4. IF
C     THIS HAPPENS THREE TIMES IN A ROW ON
C     THE SAME STEP, RETURN WITH AN ERROR FLAG
      NSF=NSF+1
      R = 0.25D0
      H=H*R
      IF (NSF .LT. 3 .AND. ABS(H) .GE. HMIN) GO TO 690
      IDID=-8
      GO TO 675
C
C
C     THE NEWTON ITERATION FAILED TO CONVERGE FOR A REASON
C     OTHER THAN A SINGULAR ITERATION MATRIX.  IF IRES = -2, THEN
C     RETURN.  OTHERWISE, REDUCE THE STEPSIZE AND TRY AGAIN, UNLESS
C     TOO MANY FAILURES HAVE OCCURED.
650   CONTINUE
      IF (IRES .GT. -2) GO TO 655
      IDID = -11
      GO TO 675
655   NCF = NCF + 1
      R = 0.25D0
      H = H*R
      IF (NCF .LT. 10 .AND. ABS(H) .GE. HMIN) GO TO 690
      IDID = -7
      IF (IRES .LT. 0) IDID = -10
      IF (NEF .GE. 3) IDID = -9
      GO TO 675
C
C
C     THE NEWTON SCHEME CONVERGED,AND THE CAUSE
C     OF THE FAILURE WAS THE ERROR ESTIMATE
C     EXCEEDING THE TOLERANCE.
660   NEF=NEF+1
      IWM(LETF)=IWM(LETF)+1
      IF (NEF .GT. 1) GO TO 665
C
C     ON FIRST ERROR TEST FAILURE, KEEP CURRENT ORDER OR LOWER
C     ORDER BY ONE.  COMPUTE NEW STEPSIZE BASED ON DIFFERENCES
C     OF THE SOLUTION.
      K = KNEW
      TEMP2 = K + 1
      R = 0.90D0*(2.0D0*EST+0.0001D0)**(-1.0D0/TEMP2)
      R = MAX(0.25D0,MIN(0.9D0,R))
      H = H*R
      IF (ABS(H) .GE. HMIN) GO TO 690
      IDID = -6
      GO TO 675
C
C     ON SECOND ERROR TEST FAILURE, USE THE CURRENT ORDER OR
C     DECREASE ORDER BY ONE.  REDUCE THE STEPSIZE BY A FACTOR OF
C     FOUR.
665   IF (NEF .GT. 2) GO TO 670
      K = KNEW
      H = 0.25D0*H
      IF (ABS(H) .GE. HMIN) GO TO 690
      IDID = -6
      GO TO 675
C
C     ON THIRD AND SUBSEQUENT ERROR TEST FAILURES, SET THE ORDER TO
C     ONE AND REDUCE THE STEPSIZE BY A FACTOR OF FOUR.
670   K = 1
      H = 0.25D0*H
      IF (ABS(H) .GE. HMIN) GO TO 690
      IDID = -6
      GO TO 675
C
C
C
C
C     FOR ALL CRASHES, RESTORE Y TO ITS LAST VALUE,
C     INTERPOLATE TO FIND YPRIME AT LAST X, AND RETURN
675   CONTINUE
      CALL DDATRP(X,X,Y,YPRIME,NEQ,K,PHI,PSI)
      RETURN
C
C
C     GO BACK AND TRY THIS STEP AGAIN
690   GO TO 200
C
C------END OF SUBROUTINE DDASTP------
      END
      SUBROUTINE DDATRP (X, XOUT, YOUT, YPOUT, NEQ, KOLD, PHI, PSI)
C***BEGIN PROLOGUE  DDATRP
C***SUBSIDIARY
C***PURPOSE  Interpolation routine for DDASSL.
C***LIBRARY   SLATEC (DASSL)
C***TYPE      DOUBLE PRECISION (SDATRP-S, DDATRP-D)
C***AUTHOR  PETZOLD, LINDA R., (LLNL)
C***DESCRIPTION
C-----------------------------------------------------------------------
C     THE METHODS IN SUBROUTINE DDASTP USE POLYNOMIALS
C     TO APPROXIMATE THE SOLUTION. DDATRP APPROXIMATES THE
C     SOLUTION AND ITS DERIVATIVE AT TIME XOUT BY EVALUATING
C     ONE OF THESE POLYNOMIALS,AND ITS DERIVATIVE,THERE.
C     INFORMATION DEFINING THIS POLYNOMIAL IS PASSED FROM
C     DDASTP, SO DDATRP CANNOT BE USED ALONE.
C
C     THE PARAMETERS ARE:
C     X     THE CURRENT TIME IN THE INTEGRATION.
C     XOUT  THE TIME AT WHICH THE SOLUTION IS DESIRED
C     YOUT  THE INTERPOLATED APPROXIMATION TO Y AT XOUT
C           (THIS IS OUTPUT)
C     YPOUT THE INTERPOLATED APPROXIMATION TO YPRIME AT XOUT
C           (THIS IS OUTPUT)
C     NEQ   NUMBER OF EQUATIONS
C     KOLD  ORDER USED ON LAST SUCCESSFUL STEP
C     PHI   ARRAY OF SCALED DIVIDED DIFFERENCES OF Y
C     PSI   ARRAY OF PAST STEPSIZE HISTORY
C-----------------------------------------------------------------------
C***ROUTINES CALLED  (NONE)
C***REVISION HISTORY  (YYMMDD)
C   830315  DATE WRITTEN
C   901009  Finished conversion to SLATEC 4.0 format (F.N.Fritsch)
C   901019  Merged changes made by C. Ulrich with SLATEC 4.0 format.
C   901026  Added explicit declarations for all variables and minor
C           cosmetic changes to prologue.  (FNF)
C***END PROLOGUE  DDATRP
C
      INTEGER  NEQ, KOLD
      DOUBLE PRECISION  X, XOUT, YOUT(*), YPOUT(*), PHI(NEQ,*), PSI(*)
C
      INTEGER  I, J, KOLDP1
      DOUBLE PRECISION  C, D, GAMMA, TEMP1
C
C***FIRST EXECUTABLE STATEMENT  DDATRP
      KOLDP1=KOLD+1
      TEMP1=XOUT-X
      DO 10 I=1,NEQ
         YOUT(I)=PHI(I,1)
10       YPOUT(I)=0.0D0
      C=1.0D0
      D=0.0D0
      GAMMA=TEMP1/PSI(1)
      DO 30 J=2,KOLDP1
         D=D*GAMMA+C/PSI(J-1)
         C=C*GAMMA
         GAMMA=(TEMP1+PSI(J-1))/PSI(J)
         DO 20 I=1,NEQ
            YOUT(I)=YOUT(I)+C*PHI(I,J)
20          YPOUT(I)=YPOUT(I)+D*PHI(I,J)
30       CONTINUE
      RETURN
C
C------END OF SUBROUTINE DDATRP------
      END
      SUBROUTINE DDAWTS (NEQ, IWT, RTOL, ATOL, Y, WT, RPAR, IPAR)
C***BEGIN PROLOGUE  DDAWTS
C***SUBSIDIARY
C***PURPOSE  Set error weight vector for DDASSL.
C***LIBRARY   SLATEC (DASSL)
C***TYPE      DOUBLE PRECISION (SDAWTS-S, DDAWTS-D)
C***AUTHOR  PETZOLD, LINDA R., (LLNL)
C***DESCRIPTION
C-----------------------------------------------------------------------
C     THIS SUBROUTINE SETS THE ERROR WEIGHT VECTOR
C     WT ACCORDING TO WT(I)=RTOL(I)*ABS(Y(I))+ATOL(I),
C     I=1,-,N.
C     RTOL AND ATOL ARE SCALARS IF IWT = 0,
C     AND VECTORS IF IWT = 1.
C-----------------------------------------------------------------------
C***ROUTINES CALLED  (NONE)
C***REVISION HISTORY  (YYMMDD)
C   830315  DATE WRITTEN
C   901009  Finished conversion to SLATEC 4.0 format (F.N.Fritsch)
C   901019  Merged changes made by C. Ulrich with SLATEC 4.0 format.
C   901026  Added explicit declarations for all variables and minor
C           cosmetic changes to prologue.  (FNF)
C***END PROLOGUE  DDAWTS
C
      INTEGER  NEQ, IWT, IPAR(*)
      DOUBLE PRECISION  RTOL(*), ATOL(*), Y(*), WT(*), RPAR(*)
C
      INTEGER  I
      DOUBLE PRECISION  ATOLI, RTOLI
C
C***FIRST EXECUTABLE STATEMENT  DDAWTS
      RTOLI=RTOL(1)
      ATOLI=ATOL(1)
      DO 20 I=1,NEQ
         IF (IWT .EQ.0) GO TO 10
           RTOLI=RTOL(I)
           ATOLI=ATOL(I)
10         WT(I)=RTOLI*ABS(Y(I))+ATOLI
20         CONTINUE
      RETURN
C-----------END OF SUBROUTINE DDAWTS------------------------------------
      END

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