xref: /petsc/src/ts/tests/ex3.c (revision 4e8208cbcbc709572b8abe32f33c78b69c819375)

static char help[] = "Solves 1D heat equation with FEM formulation.\n\
Input arguments are\n\
  -useAlhs: solve Alhs*U' =  (Arhs*U + g) \n\
            otherwise, solve U' = inv(Alhs)*(Arhs*U + g) \n\n";

/*--------------------------------------------------------------------------
  Solves 1D heat equation U_t = U_xx with FEM formulation:
                          Alhs*U' = rhs (= Arhs*U + g)
  We thank Chris Cox <clcox@clemson.edu> for contributing the original code
----------------------------------------------------------------------------*/

#include <petscksp.h>
#include <petscts.h>

/* special variable - max size of all arrays  */
#define num_z 10

/*
   User-defined application context - contains data needed by the
   application-provided call-back routines.
*/
typedef struct {
  Mat          Amat;             /* left hand side matrix */
  Vec          ksp_rhs, ksp_sol; /* working vectors for formulating inv(Alhs)*(Arhs*U+g) */
  PetscInt     max_probsz;       /* max size of the problem */
  PetscBool    useAlhs;          /* flag (1 indicates solving Alhs*U' = Arhs*U+g */
  PetscInt     nz;               /* total number of grid points */
  PetscInt     m;                /* total number of interio grid points */
  Vec          solution;         /* global exact ts solution vector */
  PetscScalar *z;                /* array of grid points */
  PetscBool    debug;            /* flag (1 indicates activation of debugging printouts) */
} AppCtx;

extern PetscScalar    exact(PetscScalar, PetscReal);
extern PetscErrorCode Monitor(TS, PetscInt, PetscReal, Vec, void *);
extern PetscErrorCode Petsc_KSPSolve(AppCtx *);
extern PetscScalar    bspl(PetscScalar *, PetscScalar, PetscInt, PetscInt, PetscInt[][2], PetscInt);
extern PetscErrorCode femBg(PetscScalar[][3], PetscScalar *, PetscInt, PetscScalar *, PetscReal);
extern PetscErrorCode femA(AppCtx *, PetscInt, PetscScalar *);
extern PetscErrorCode rhs(AppCtx *, PetscScalar *, PetscInt, PetscScalar *, PetscReal);
extern PetscErrorCode RHSfunction(TS, PetscReal, Vec, Vec, void *);

int main(int argc, char **argv)
{
  PetscInt    i, m, nz, steps, max_steps, k, nphase = 1;
  PetscScalar zInitial, zFinal, val, *z;
  PetscReal   stepsz[4], T, ftime;
  TS          ts;
  SNES        snes;
  Mat         Jmat;
  AppCtx      appctx;   /* user-defined application context */
  Vec         init_sol; /* ts solution vector */
  PetscMPIInt size;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &argv, NULL, help));
  PetscCallMPI(MPI_Comm_size(PETSC_COMM_WORLD, &size));
  PetscCheck(size == 1, PETSC_COMM_SELF, PETSC_ERR_WRONG_MPI_SIZE, "This is a uniprocessor example only");

  /* initializations */
  zInitial  = 0.0;
  zFinal    = 1.0;
  nz        = num_z;
  m         = nz - 2;
  appctx.nz = nz;
  max_steps = 10000;

  appctx.m          = m;
  appctx.max_probsz = nz;
  appctx.debug      = PETSC_FALSE;
  appctx.useAlhs    = PETSC_FALSE;

  PetscOptionsBegin(PETSC_COMM_WORLD, NULL, "", "");
  PetscCall(PetscOptionsName("-debug", NULL, NULL, &appctx.debug));
  PetscCall(PetscOptionsName("-useAlhs", NULL, NULL, &appctx.useAlhs));
  PetscCall(PetscOptionsRangeInt("-nphase", NULL, NULL, nphase, &nphase, NULL, 1, 3));
  PetscOptionsEnd();
  T = 0.014 / nphase;

  /* create vector to hold ts solution */
  /*-----------------------------------*/
  PetscCall(VecCreate(PETSC_COMM_WORLD, &init_sol));
  PetscCall(VecSetSizes(init_sol, PETSC_DECIDE, m));
  PetscCall(VecSetFromOptions(init_sol));

  /* create vector to hold true ts soln for comparison */
  PetscCall(VecDuplicate(init_sol, &appctx.solution));

  /* create LHS matrix Amat */
  /*------------------------*/
  PetscCall(MatCreateSeqAIJ(PETSC_COMM_WORLD, m, m, 3, NULL, &appctx.Amat));
  PetscCall(MatSetFromOptions(appctx.Amat));
  PetscCall(MatSetUp(appctx.Amat));
  /* set space grid points - interio points only! */
  PetscCall(PetscMalloc1(nz + 1, &z));
  for (i = 0; i < nz; i++) z[i] = (i) * ((zFinal - zInitial) / (nz - 1));
  appctx.z = z;
  PetscCall(femA(&appctx, nz, z));

  /* create the jacobian matrix */
  /*----------------------------*/
  PetscCall(MatCreate(PETSC_COMM_WORLD, &Jmat));
  PetscCall(MatSetSizes(Jmat, PETSC_DECIDE, PETSC_DECIDE, m, m));
  PetscCall(MatSetFromOptions(Jmat));
  PetscCall(MatSetUp(Jmat));

  /* create working vectors for formulating rhs=inv(Alhs)*(Arhs*U + g) */
  PetscCall(VecDuplicate(init_sol, &appctx.ksp_rhs));
  PetscCall(VecDuplicate(init_sol, &appctx.ksp_sol));

  /* set initial guess */
  /*-------------------*/
  for (i = 0; i < nz - 2; i++) {
    val = exact(z[i + 1], 0.0);
    PetscCall(VecSetValue(init_sol, i, val, INSERT_VALUES));
  }
  PetscCall(VecAssemblyBegin(init_sol));
  PetscCall(VecAssemblyEnd(init_sol));

  /*create a time-stepping context and set the problem type */
  /*--------------------------------------------------------*/
  PetscCall(TSCreate(PETSC_COMM_WORLD, &ts));
  PetscCall(TSSetProblemType(ts, TS_NONLINEAR));

  /* set time-step method */
  PetscCall(TSSetType(ts, TSCN));

  /* Set optional user-defined monitoring routine */
  PetscCall(TSMonitorSet(ts, Monitor, &appctx, NULL));
  /* set the right-hand side of U_t = RHSfunction(U,t) */
  PetscCall(TSSetRHSFunction(ts, NULL, (PetscErrorCode (*)(TS, PetscScalar, Vec, Vec, void *))RHSfunction, &appctx));

  if (appctx.useAlhs) {
    /* set the left hand side matrix of Amat*U_t = rhs(U,t) */

    /* Note: this approach is incompatible with the finite differenced Jacobian set below because we can't restore the
     * Alhs matrix without making a copy.  Either finite difference the entire thing or use analytic Jacobians in both
     * places.
     */
    PetscCall(TSSetIFunction(ts, NULL, TSComputeIFunctionLinear, &appctx));
    PetscCall(TSSetIJacobian(ts, appctx.Amat, appctx.Amat, TSComputeIJacobianConstant, &appctx));
  }

  /* use PETSc to compute the jacobian by finite differences */
  PetscCall(TSGetSNES(ts, &snes));
  PetscCall(SNESSetJacobian(snes, Jmat, Jmat, SNESComputeJacobianDefault, NULL));

  /* get the command line options if there are any and set them */
  PetscCall(TSSetFromOptions(ts));

#if defined(PETSC_HAVE_SUNDIALS2)
  {
    TSType    type;
    PetscBool sundialstype = PETSC_FALSE;
    PetscCall(TSGetType(ts, &type));
    PetscCall(PetscObjectTypeCompare((PetscObject)ts, TSSUNDIALS, &sundialstype));
    PetscCheck(!sundialstype || !appctx.useAlhs, PETSC_COMM_SELF, PETSC_ERR_SUP, "Cannot use Alhs formulation for TSSUNDIALS type");
  }
#endif
  /* Sets the initial solution */
  PetscCall(TSSetSolution(ts, init_sol));

  stepsz[0] = 1.0 / (2.0 * (nz - 1) * (nz - 1)); /* (mesh_size)^2/2.0 */
  ftime     = 0.0;
  for (k = 0; k < nphase; k++) {
    if (nphase > 1) PetscCall(PetscPrintf(PETSC_COMM_WORLD, "Phase %" PetscInt_FMT " initial time %g, stepsz %g, duration: %g\n", k, (double)ftime, (double)stepsz[k], (double)((k + 1) * T)));
    PetscCall(TSSetTime(ts, ftime));
    PetscCall(TSSetTimeStep(ts, stepsz[k]));
    PetscCall(TSSetMaxSteps(ts, max_steps));
    PetscCall(TSSetMaxTime(ts, (k + 1) * T));
    PetscCall(TSSetExactFinalTime(ts, TS_EXACTFINALTIME_STEPOVER));

    /* loop over time steps */
    /*----------------------*/
    PetscCall(TSSolve(ts, init_sol));
    PetscCall(TSGetSolveTime(ts, &ftime));
    PetscCall(TSGetStepNumber(ts, &steps));
    stepsz[k + 1] = stepsz[k] * 1.5; /* change step size for the next phase */
  }

  /* free space */
  PetscCall(TSDestroy(&ts));
  PetscCall(MatDestroy(&appctx.Amat));
  PetscCall(MatDestroy(&Jmat));
  PetscCall(VecDestroy(&appctx.ksp_rhs));
  PetscCall(VecDestroy(&appctx.ksp_sol));
  PetscCall(VecDestroy(&init_sol));
  PetscCall(VecDestroy(&appctx.solution));
  PetscCall(PetscFree(z));

  PetscCall(PetscFinalize());
  return 0;
}

/*------------------------------------------------------------------------
  Set exact solution
  u(z,t) = sin(6*PI*z)*exp(-36.*PI*PI*t) + 3.*sin(2*PI*z)*exp(-4.*PI*PI*t)
--------------------------------------------------------------------------*/
PetscScalar exact(PetscScalar z, PetscReal t)
{
  PetscScalar val, ex1, ex2;

  ex1 = PetscExpReal(-36. * PETSC_PI * PETSC_PI * t);
  ex2 = PetscExpReal(-4. * PETSC_PI * PETSC_PI * t);
  val = PetscSinScalar(6 * PETSC_PI * z) * ex1 + 3. * PetscSinScalar(2 * PETSC_PI * z) * ex2;
  return val;
}

/*
   Monitor - User-provided routine to monitor the solution computed at
   each timestep.  This example plots the solution and computes the
   error in two different norms.

   Input Parameters:
   ts     - the timestep context
   step   - the count of the current step (with 0 meaning the
             initial condition)
   time   - the current time
   u      - the solution at this timestep
   ctx    - the user-provided context for this monitoring routine.
            In this case we use the application context which contains
            information about the problem size, workspace and the exact
            solution.
*/
PetscErrorCode Monitor(TS ts, PetscInt step, PetscReal time, Vec u, PetscCtx ctx)
{
  AppCtx      *appctx = (AppCtx *)ctx;
  PetscInt     i, m = appctx->m;
  PetscReal    norm_2, norm_max, h = 1.0 / (m + 1);
  PetscScalar *u_exact;

  PetscFunctionBeginUser;
  /* Compute the exact solution */
  PetscCall(VecGetArrayWrite(appctx->solution, &u_exact));
  for (i = 0; i < m; i++) u_exact[i] = exact(appctx->z[i + 1], time);
  PetscCall(VecRestoreArrayWrite(appctx->solution, &u_exact));

  /* Print debugging information if desired */
  if (appctx->debug) {
    PetscCall(PetscPrintf(PETSC_COMM_SELF, "Computed solution vector at time %g\n", (double)time));
    PetscCall(VecView(u, PETSC_VIEWER_STDOUT_SELF));
    PetscCall(PetscPrintf(PETSC_COMM_SELF, "Exact solution vector\n"));
    PetscCall(VecView(appctx->solution, PETSC_VIEWER_STDOUT_SELF));
  }

  /* Compute the 2-norm and max-norm of the error */
  PetscCall(VecAXPY(appctx->solution, -1.0, u));
  PetscCall(VecNorm(appctx->solution, NORM_2, &norm_2));

  norm_2 = PetscSqrtReal(h) * norm_2;
  PetscCall(VecNorm(appctx->solution, NORM_MAX, &norm_max));
  PetscCall(PetscPrintf(PETSC_COMM_SELF, "Timestep %" PetscInt_FMT ": time = %g, 2-norm error = %6.4f, max norm error = %6.4f\n", step, (double)time, (double)norm_2, (double)norm_max));

  /*
     Print debugging information if desired
  */
  if (appctx->debug) {
    PetscCall(PetscPrintf(PETSC_COMM_SELF, "Error vector\n"));
    PetscCall(VecView(appctx->solution, PETSC_VIEWER_STDOUT_SELF));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      Function to solve a linear system using KSP
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/

PetscErrorCode Petsc_KSPSolve(AppCtx *obj)
{
  KSP ksp;
  PC  pc;

  PetscFunctionBeginUser;
  /*create the ksp context and set the operators,that is, associate the system matrix with it*/
  PetscCall(KSPCreate(PETSC_COMM_WORLD, &ksp));
  PetscCall(KSPSetOperators(ksp, obj->Amat, obj->Amat));

  /*get the preconditioner context, set its type and the tolerances*/
  PetscCall(KSPGetPC(ksp, &pc));
  PetscCall(PCSetType(pc, PCLU));
  PetscCall(KSPSetTolerances(ksp, 1.e-7, PETSC_CURRENT, PETSC_CURRENT, PETSC_CURRENT));

  /*get the command line options if there are any and set them*/
  PetscCall(KSPSetFromOptions(ksp));

  /*get the linear system (ksp) solve*/
  PetscCall(KSPSolve(ksp, obj->ksp_rhs, obj->ksp_sol));

  PetscCall(KSPDestroy(&ksp));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/***********************************************************************
  Function to return value of basis function or derivative of basis function.
 ***********************************************************************

        Arguments:
          x       = array of xpoints or nodal values
          xx      = point at which the basis function is to be
                      evaluated.
          il      = interval containing xx.
          iq      = indicates which of the two basis functions in
                      interval intrvl should be used
          nll     = array containing the endpoints of each interval.
          id      = If id ~= 2, the value of the basis function
                      is calculated; if id = 2, the value of the
                      derivative of the basis function is returned.
 ***********************************************************************/

PetscScalar bspl(PetscScalar *x, PetscScalar xx, PetscInt il, PetscInt iq, PetscInt nll[][2], PetscInt id)
{
  PetscScalar x1, x2, bfcn;
  PetscInt    i1, i2, iq1, iq2;

  /* Determine which basis function in interval intrvl is to be used in */
  iq1 = iq;
  if (iq1 == 0) iq2 = 1;
  else iq2 = 0;

  /*    Determine endpoint of the interval intrvl   */
  i1 = nll[il][iq1];
  i2 = nll[il][iq2];

  /*   Determine nodal values at the endpoints of the interval intrvl   */
  x1 = x[i1];
  x2 = x[i2];

  /*   Evaluate basis function   */
  if (id == 2) bfcn = (1.0) / (x1 - x2);
  else bfcn = (xx - x2) / (x1 - x2);
  return bfcn;
}

/*---------------------------------------------------------
  Function called by rhs function to get B and g
---------------------------------------------------------*/
PetscErrorCode femBg(PetscScalar btri[][3], PetscScalar *f, PetscInt nz, PetscScalar *z, PetscReal t)
{
  PetscInt    i, j, jj, il, ip, ipp, ipq, iq, iquad, iqq;
  PetscInt    nli[num_z][2], indx[num_z];
  PetscScalar dd, dl, zip, zipq, zz, b_z, bb_z, bij;
  PetscScalar zquad[num_z][3], dlen[num_z], qdwt[3];

  PetscFunctionBeginUser;
  /*  initializing everything - btri and f are initialized in rhs.c  */
  for (i = 0; i < nz; i++) {
    nli[i][0]   = 0;
    nli[i][1]   = 0;
    indx[i]     = 0;
    zquad[i][0] = 0.0;
    zquad[i][1] = 0.0;
    zquad[i][2] = 0.0;
    dlen[i]     = 0.0;
  } /*end for (i)*/

  /*  quadrature weights  */
  qdwt[0] = 1.0 / 6.0;
  qdwt[1] = 4.0 / 6.0;
  qdwt[2] = 1.0 / 6.0;

  /* 1st and last nodes have Dirichlet boundary condition -
     set indices there to -1 */

  for (i = 0; i < nz - 1; i++) indx[i] = i - 1;
  indx[nz - 1] = -1;

  ipq = 0;
  for (il = 0; il < nz - 1; il++) {
    ip           = ipq;
    ipq          = ip + 1;
    zip          = z[ip];
    zipq         = z[ipq];
    dl           = zipq - zip;
    zquad[il][0] = zip;
    zquad[il][1] = (0.5) * (zip + zipq);
    zquad[il][2] = zipq;
    dlen[il]     = PetscAbsScalar(dl);
    nli[il][0]   = ip;
    nli[il][1]   = ipq;
  }

  for (il = 0; il < nz - 1; il++) {
    for (iquad = 0; iquad < 3; iquad++) {
      dd = (dlen[il]) * (qdwt[iquad]);
      zz = zquad[il][iquad];

      for (iq = 0; iq < 2; iq++) {
        ip  = nli[il][iq];
        b_z = bspl(z, zz, il, iq, nli, 2);
        i   = indx[ip];

        if (i > -1) {
          for (iqq = 0; iqq < 2; iqq++) {
            ipp  = nli[il][iqq];
            bb_z = bspl(z, zz, il, iqq, nli, 2);
            j    = indx[ipp];
            bij  = -b_z * bb_z;

            if (j > -1) {
              jj = 1 + j - i;
              btri[i][jj] += bij * dd;
            } else {
              f[i] += bij * dd * exact(z[ipp], t);
              /* f[i] += 0.0; */
              /* if (il==0 && j==-1) { */
              /* f[i] += bij*dd*exact(zz,t); */
              /* }*/ /*end if*/
            } /*end else*/
          } /*end for (iqq)*/
        } /*end if (i>0)*/
      } /*end for (iq)*/
    } /*end for (iquad)*/
  } /*end for (il)*/
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode femA(AppCtx *obj, PetscInt nz, PetscScalar *z)
{
  PetscInt    i, j, il, ip, ipp, ipq, iq, iquad, iqq;
  PetscInt    nli[num_z][2], indx[num_z];
  PetscScalar dd, dl, zip, zipq, zz, bb, bbb, aij;
  PetscScalar rquad[num_z][3], dlen[num_z], qdwt[3], add_term;

  PetscFunctionBeginUser;
  /*  initializing everything  */
  for (i = 0; i < nz; i++) {
    nli[i][0]   = 0;
    nli[i][1]   = 0;
    indx[i]     = 0;
    rquad[i][0] = 0.0;
    rquad[i][1] = 0.0;
    rquad[i][2] = 0.0;
    dlen[i]     = 0.0;
  } /*end for (i)*/

  /*  quadrature weights  */
  qdwt[0] = 1.0 / 6.0;
  qdwt[1] = 4.0 / 6.0;
  qdwt[2] = 1.0 / 6.0;

  /* 1st and last nodes have Dirichlet boundary condition -
     set indices there to -1 */

  for (i = 0; i < nz - 1; i++) indx[i] = i - 1;
  indx[nz - 1] = -1;

  ipq = 0;

  for (il = 0; il < nz - 1; il++) {
    ip           = ipq;
    ipq          = ip + 1;
    zip          = z[ip];
    zipq         = z[ipq];
    dl           = zipq - zip;
    rquad[il][0] = zip;
    rquad[il][1] = (0.5) * (zip + zipq);
    rquad[il][2] = zipq;
    dlen[il]     = PetscAbsScalar(dl);
    nli[il][0]   = ip;
    nli[il][1]   = ipq;
  } /*end for (il)*/

  for (il = 0; il < nz - 1; il++) {
    for (iquad = 0; iquad < 3; iquad++) {
      dd = (dlen[il]) * (qdwt[iquad]);
      zz = rquad[il][iquad];

      for (iq = 0; iq < 2; iq++) {
        ip = nli[il][iq];
        bb = bspl(z, zz, il, iq, nli, 1);
        i  = indx[ip];
        if (i > -1) {
          for (iqq = 0; iqq < 2; iqq++) {
            ipp = nli[il][iqq];
            bbb = bspl(z, zz, il, iqq, nli, 1);
            j   = indx[ipp];
            aij = bb * bbb;
            if (j > -1) {
              add_term = aij * dd;
              PetscCall(MatSetValue(obj->Amat, i, j, add_term, ADD_VALUES));
            } /*endif*/
          } /*end for (iqq)*/
        } /*end if (i>0)*/
      } /*end for (iq)*/
    } /*end for (iquad)*/
  } /*end for (il)*/
  PetscCall(MatAssemblyBegin(obj->Amat, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(obj->Amat, MAT_FINAL_ASSEMBLY));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*---------------------------------------------------------
        Function to fill the rhs vector with
        By + g values ****
---------------------------------------------------------*/
PetscErrorCode rhs(AppCtx *obj, PetscScalar *y, PetscInt nz, PetscScalar *z, PetscReal t)
{
  PetscInt    i, j, js, je, jj;
  PetscScalar val, g[num_z], btri[num_z][3], add_term;

  PetscFunctionBeginUser;
  for (i = 0; i < nz - 2; i++) {
    for (j = 0; j <= 2; j++) btri[i][j] = 0.0;
    g[i] = 0.0;
  }

  /*  call femBg to set the tri-diagonal b matrix and vector g  */
  PetscCall(femBg(btri, g, nz, z, t));

  /*  setting the entries of the right-hand side vector  */
  for (i = 0; i < nz - 2; i++) {
    val = 0.0;
    js  = 0;
    if (i == 0) js = 1;
    je = 2;
    if (i == nz - 2) je = 1;

    for (jj = js; jj <= je; jj++) {
      j = i + jj - 1;
      val += (btri[i][jj]) * (y[j]);
    }
    add_term = val + g[i];
    PetscCall(VecSetValue(obj->ksp_rhs, i, add_term, INSERT_VALUES));
  }
  PetscCall(VecAssemblyBegin(obj->ksp_rhs));
  PetscCall(VecAssemblyEnd(obj->ksp_rhs));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%   Function to form the right-hand side of the time-stepping problem.                       %%
%% -------------------------------------------------------------------------------------------%%
  if (useAlhs):
    globalout = By+g
  else if (!useAlhs):
    globalout = f(y,t)=Ainv(By+g),
      in which the ksp solver to transform the problem A*ydot=By+g
      to the problem ydot=f(y,t)=inv(A)*(By+g)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/

PetscErrorCode RHSfunction(TS ts, PetscReal t, Vec globalin, Vec globalout, PetscCtx ctx)
{
  AppCtx            *obj = (AppCtx *)ctx;
  PetscScalar        soln[num_z];
  const PetscScalar *soln_ptr;
  PetscInt           i, nz = obj->nz;
  PetscReal          time;

  PetscFunctionBeginUser;
  /* get the previous solution to compute updated system */
  PetscCall(VecGetArrayRead(globalin, &soln_ptr));
  for (i = 0; i < num_z - 2; i++) soln[i] = soln_ptr[i];
  PetscCall(VecRestoreArrayRead(globalin, &soln_ptr));
  soln[num_z - 1] = 0.0;
  soln[num_z - 2] = 0.0;

  /* clear out the matrix and rhs for ksp to keep things straight */
  PetscCall(VecSet(obj->ksp_rhs, (PetscScalar)0.0));

  time = t;
  /* get the updated system */
  PetscCall(rhs(obj, soln, nz, obj->z, time)); /* setup of the By+g rhs */

  /* do a ksp solve to get the rhs for the ts problem */
  if (obj->useAlhs) {
    /* ksp_sol = ksp_rhs */
    PetscCall(VecCopy(obj->ksp_rhs, globalout));
  } else {
    /* ksp_sol = inv(Amat)*ksp_rhs */
    PetscCall(Petsc_KSPSolve(obj));
    PetscCall(VecCopy(obj->ksp_sol, globalout));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*TEST

    build:
      requires: !complex

    test:
      suffix: euler
      output_file: output/ex3.out

    test:
      suffix: 2
      args: -useAlhs
      output_file: output/ex3.out
      TODO: Broken because SNESComputeJacobianDefault is incompatible with TSComputeIJacobianConstant

TEST*/