xref: /petsc/src/ts/impls/implicit/theta/theta.c (revision 163d334ea1df412efbe58c2e8fce360a2afeaf87)

 /*
   Code for timestepping with implicit Theta method
 */
 #include <petsc/private/tsimpl.h>                /*I   "petscts.h"   I*/
 #include <petscsnes.h>
 #include <petscdm.h>
 #include <petscmat.h>

 typedef struct {
   PetscReal    stage_time;
   Vec          X0,X,Xdot;                /* Storage for stages and time derivative */
   Vec          affine;                   /* Affine vector needed for residual at beginning of step in endpoint formulation */

   PetscReal    Theta;
   PetscInt     order;
   PetscBool    endpoint;
   PetscBool    extrapolate;

   Vec          *VecsDeltaLam;            /* Increment of the adjoint sensitivity w.r.t IC at stage */
   Vec          *VecsDeltaMu;             /* Increment of the adjoint sensitivity w.r.t P at stage */
   Vec          *VecsSensiTemp;           /* Vector to be timed with Jacobian transpose */
   Vec          VecCostIntegral0;         /* Backup for roll-backs due to events */
   PetscReal    ptime;
   PetscReal    time_step;

   PetscBool    adapt;                    /* Use time-step adaptivity ? */
   PetscReal    time_step_prev;
   Vec          vec_sol_prev;
   Vec          vec_lte_work;

   TSStepStatus status;
 } TS_Theta;

 #undef __FUNCT__
 #define __FUNCT__ "TSThetaGetX0AndXdot"
 static PetscErrorCode TSThetaGetX0AndXdot(TS ts,DM dm,Vec *X0,Vec *Xdot)
 {
   TS_Theta       *th = (TS_Theta*)ts->data;
   PetscErrorCode ierr;

   PetscFunctionBegin;
   if (X0) {
     if (dm && dm != ts->dm) {
       ierr = DMGetNamedGlobalVector(dm,"TSTheta_X0",X0);CHKERRQ(ierr);
     } else *X0 = ts->vec_sol;
   }
   if (Xdot) {
     if (dm && dm != ts->dm) {
       ierr = DMGetNamedGlobalVector(dm,"TSTheta_Xdot",Xdot);CHKERRQ(ierr);
     } else *Xdot = th->Xdot;
   }
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "TSThetaRestoreX0AndXdot"
 static PetscErrorCode TSThetaRestoreX0AndXdot(TS ts,DM dm,Vec *X0,Vec *Xdot)
 {
   PetscErrorCode ierr;

   PetscFunctionBegin;
   if (X0) {
     if (dm && dm != ts->dm) {
       ierr = DMRestoreNamedGlobalVector(dm,"TSTheta_X0",X0);CHKERRQ(ierr);
     }
   }
   if (Xdot) {
     if (dm && dm != ts->dm) {
       ierr = DMRestoreNamedGlobalVector(dm,"TSTheta_Xdot",Xdot);CHKERRQ(ierr);
     }
   }
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "DMCoarsenHook_TSTheta"
 static PetscErrorCode DMCoarsenHook_TSTheta(DM fine,DM coarse,void *ctx)
 {

   PetscFunctionBegin;
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "DMRestrictHook_TSTheta"
 static PetscErrorCode DMRestrictHook_TSTheta(DM fine,Mat restrct,Vec rscale,Mat inject,DM coarse,void *ctx)
 {
   TS             ts = (TS)ctx;
   PetscErrorCode ierr;
   Vec            X0,Xdot,X0_c,Xdot_c;

   PetscFunctionBegin;
   ierr = TSThetaGetX0AndXdot(ts,fine,&X0,&Xdot);CHKERRQ(ierr);
   ierr = TSThetaGetX0AndXdot(ts,coarse,&X0_c,&Xdot_c);CHKERRQ(ierr);
   ierr = MatRestrict(restrct,X0,X0_c);CHKERRQ(ierr);
   ierr = MatRestrict(restrct,Xdot,Xdot_c);CHKERRQ(ierr);
   ierr = VecPointwiseMult(X0_c,rscale,X0_c);CHKERRQ(ierr);
   ierr = VecPointwiseMult(Xdot_c,rscale,Xdot_c);CHKERRQ(ierr);
   ierr = TSThetaRestoreX0AndXdot(ts,fine,&X0,&Xdot);CHKERRQ(ierr);
   ierr = TSThetaRestoreX0AndXdot(ts,coarse,&X0_c,&Xdot_c);CHKERRQ(ierr);
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "DMSubDomainHook_TSTheta"
 static PetscErrorCode DMSubDomainHook_TSTheta(DM dm,DM subdm,void *ctx)
 {

   PetscFunctionBegin;
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "DMSubDomainRestrictHook_TSTheta"
 static PetscErrorCode DMSubDomainRestrictHook_TSTheta(DM dm,VecScatter gscat,VecScatter lscat,DM subdm,void *ctx)
 {
   TS             ts = (TS)ctx;
   PetscErrorCode ierr;
   Vec            X0,Xdot,X0_sub,Xdot_sub;

   PetscFunctionBegin;
   ierr = TSThetaGetX0AndXdot(ts,dm,&X0,&Xdot);CHKERRQ(ierr);
   ierr = TSThetaGetX0AndXdot(ts,subdm,&X0_sub,&Xdot_sub);CHKERRQ(ierr);

   ierr = VecScatterBegin(gscat,X0,X0_sub,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
   ierr = VecScatterEnd(gscat,X0,X0_sub,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);

   ierr = VecScatterBegin(gscat,Xdot,Xdot_sub,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);
   ierr = VecScatterEnd(gscat,Xdot,Xdot_sub,INSERT_VALUES,SCATTER_FORWARD);CHKERRQ(ierr);

   ierr = TSThetaRestoreX0AndXdot(ts,dm,&X0,&Xdot);CHKERRQ(ierr);
   ierr = TSThetaRestoreX0AndXdot(ts,subdm,&X0_sub,&Xdot_sub);CHKERRQ(ierr);
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "TSForwardCostIntegral_Theta"
 static PetscErrorCode TSForwardCostIntegral_Theta(TS ts)
 {
   TS_Theta       *th = (TS_Theta*)ts->data;
   PetscErrorCode ierr;

   PetscFunctionBegin;
   /* backup cost integral */
   ierr = VecCopy(ts->vec_costintegral,th->VecCostIntegral0);CHKERRQ(ierr);
   if (th->endpoint) {
     ierr = TSAdjointComputeCostIntegrand(ts,th->ptime,th->X0,ts->vec_costintegrand);CHKERRQ(ierr);
     ierr = VecAXPY(ts->vec_costintegral,th->time_step*(1-th->Theta),ts->vec_costintegrand);CHKERRQ(ierr);
   }
   ierr = TSAdjointComputeCostIntegrand(ts,th->stage_time,th->X,ts->vec_costintegrand);CHKERRQ(ierr);
   if (th->endpoint) {
     ierr = VecAXPY(ts->vec_costintegral,th->time_step*th->Theta,ts->vec_costintegrand);CHKERRQ(ierr);
   } else {
     ierr = VecAXPY(ts->vec_costintegral,th->time_step,ts->vec_costintegrand);CHKERRQ(ierr);
   }
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "TSAdjointCostIntegral_Theta"
 static PetscErrorCode TSAdjointCostIntegral_Theta(TS ts)
 {
   TS_Theta       *th = (TS_Theta*)ts->data;
   PetscErrorCode ierr;

   PetscFunctionBegin;
   if (th->endpoint) {
     /* Evolve ts->vec_costintegral to compute integrals */
     ierr = TSAdjointComputeCostIntegrand(ts,ts->ptime,ts->vec_sol,ts->vec_costintegrand);CHKERRQ(ierr);
     ierr = VecAXPY(ts->vec_costintegral,-ts->time_step*th->Theta,ts->vec_costintegrand);CHKERRQ(ierr);
     if (th->Theta!=1) {
       ierr = TSAdjointComputeCostIntegrand(ts,th->ptime,th->X0,ts->vec_costintegrand);CHKERRQ(ierr);
       ierr = VecAXPY(ts->vec_costintegral,ts->time_step*(th->Theta-1),ts->vec_costintegrand);CHKERRQ(ierr);
     }
   }else {
     ierr = TSAdjointComputeCostIntegrand(ts,th->stage_time,th->X,ts->vec_costintegrand);CHKERRQ(ierr);
     ierr = VecAXPY(ts->vec_costintegral,-ts->time_step,ts->vec_costintegrand);CHKERRQ(ierr);
   }
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "TS_SNESSolve"
 static PetscErrorCode TS_SNESSolve(TS ts,Vec b,Vec x)
 {
   PetscInt       nits,lits;
   PetscErrorCode ierr;

   PetscFunctionBegin;
   ierr = SNESSolve(ts->snes,b,x);CHKERRQ(ierr);
   ierr = SNESGetIterationNumber(ts->snes,&nits);CHKERRQ(ierr);
   ierr = SNESGetLinearSolveIterations(ts->snes,&lits);CHKERRQ(ierr);
   ts->snes_its += nits; ts->ksp_its += lits;
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "TSStep_Theta"
 static PetscErrorCode TSStep_Theta(TS ts)
 {
   TS_Theta       *th = (TS_Theta*)ts->data;
   PetscInt       rejections     = 0;
   PetscBool      stageok,accept = PETSC_TRUE;
   PetscReal      next_time_step = ts->time_step;
   PetscErrorCode ierr;

   PetscFunctionBegin;

   if (!ts->steprollback) {
     if (th->adapt) { th->time_step_prev = ts->time_step_prev; }
     if (th->adapt) { ierr = VecCopy(th->X0,th->vec_sol_prev);CHKERRQ(ierr); }
     ierr = VecCopy(ts->vec_sol,th->X0);CHKERRQ(ierr);
   }

   th->status = TS_STEP_INCOMPLETE;
   while (!ts->reason && th->status != TS_STEP_COMPLETE) {

     PetscReal shift = 1/(th->Theta*ts->time_step);
     th->stage_time = ts->ptime + (th->endpoint ? (PetscReal)1 : th->Theta)*ts->time_step;
     ierr = TSPreStage(ts,th->stage_time);CHKERRQ(ierr);

     if (th->endpoint) { /* This formulation assumes linear time-independent mass matrix */
       if (!th->affine) {ierr = VecDuplicate(ts->vec_sol,&th->affine);CHKERRQ(ierr);}
       ierr = VecZeroEntries(th->Xdot);CHKERRQ(ierr);
       ierr = TSComputeIFunction(ts,ts->ptime,th->X0,th->Xdot,th->affine,PETSC_FALSE);CHKERRQ(ierr);
       ierr = VecScale(th->affine,(th->Theta-1)/th->Theta);CHKERRQ(ierr);
     } else if (th->affine) { /* Just in case th->endpoint is changed between calls to TSStep_Theta() */
       ierr = VecZeroEntries(th->affine);CHKERRQ(ierr);
     }
     if (th->extrapolate) {
       ierr = VecWAXPY(th->X,1/shift,th->Xdot,th->X0);CHKERRQ(ierr);
     } else {
       ierr = VecCopy(th->X0,th->X);CHKERRQ(ierr);
     }

     ierr = TS_SNESSolve(ts,th->affine,th->X);CHKERRQ(ierr);
     ierr = TSPostStage(ts,th->stage_time,0,&(th->X));CHKERRQ(ierr);
     ierr = TSAdaptCheckStage(ts->adapt,ts,th->stage_time,th->X,&stageok);CHKERRQ(ierr);
     if (!stageok) {accept = PETSC_FALSE; goto reject_step;}

     th->status = TS_STEP_PENDING;
     if (th->endpoint) {
       ierr = VecCopy(th->X,ts->vec_sol);CHKERRQ(ierr);
     } else {
       ierr = VecAXPBYPCZ(th->Xdot,-shift,shift,0,th->X0,th->X);CHKERRQ(ierr);
       ierr = VecAXPY(ts->vec_sol,ts->time_step,th->Xdot);CHKERRQ(ierr);
     }
     ierr = TSAdaptChoose(ts->adapt,ts,ts->time_step,NULL,&next_time_step,&accept);CHKERRQ(ierr);
     th->status = accept ? TS_STEP_COMPLETE : TS_STEP_INCOMPLETE;
     if (!accept) {
       ierr = VecCopy(th->X0,ts->vec_sol);CHKERRQ(ierr);
       ts->time_step = next_time_step; goto reject_step;
     }

     if (ts->costintegralfwd) { /* Save the info for the later use in cost integral evaluation */
       th->ptime     = ts->ptime;
       th->time_step = ts->time_step;
     }

     ts->ptime += ts->time_step;
     ts->time_step = next_time_step;
     ts->steps++;
     break;

 reject_step:
     ts->reject++;
     if (!ts->reason && ++rejections > ts->max_reject && ts->max_reject >= 0) {
       ts->reason = TS_DIVERGED_STEP_REJECTED;
       ierr = PetscInfo2(ts,"Step=%D, step rejections %D greater than current TS allowed, stopping solve\n",ts->steps,rejections);CHKERRQ(ierr);
     }
   }
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "TSAdjointStep_Theta"
 static PetscErrorCode TSAdjointStep_Theta(TS ts)
 {
   TS_Theta            *th = (TS_Theta*)ts->data;
   Vec                 *VecsDeltaLam = th->VecsDeltaLam,*VecsDeltaMu = th->VecsDeltaMu,*VecsSensiTemp = th->VecsSensiTemp;
   PetscInt            nadj;
   PetscErrorCode      ierr;
   Mat                 J,Jp;
   KSP                 ksp;
   PetscReal           shift;

   PetscFunctionBegin;

   th->status = TS_STEP_INCOMPLETE;
   ierr = SNESGetKSP(ts->snes,&ksp);CHKERRQ(ierr);
   ierr = TSGetIJacobian(ts,&J,&Jp,NULL,NULL);CHKERRQ(ierr);

   /* If endpoint=1, th->ptime and th->X0 will be used; if endpoint=0, th->stage_time and th->X will be used. */
   th->stage_time = ts->ptime + (th->endpoint ? ts->time_step : (1.-th->Theta)*ts->time_step); /* time_step is negative*/
   th->ptime      = ts->ptime + ts->time_step;

   /* Build RHS */
   if (ts->vec_costintegral) { /* Cost function has an integral term */
     if (th->endpoint) {
       ierr = TSAdjointComputeDRDYFunction(ts,ts->ptime,ts->vec_sol,ts->vecs_drdy);CHKERRQ(ierr);
     }else {
       ierr = TSAdjointComputeDRDYFunction(ts,th->stage_time,th->X,ts->vecs_drdy);CHKERRQ(ierr);
     }
   }
   for (nadj=0; nadj<ts->numcost; nadj++) {
     ierr = VecCopy(ts->vecs_sensi[nadj],VecsSensiTemp[nadj]);CHKERRQ(ierr);
     ierr = VecScale(VecsSensiTemp[nadj],-1./(th->Theta*ts->time_step));CHKERRQ(ierr);
     if (ts->vec_costintegral) {
       ierr = VecAXPY(VecsSensiTemp[nadj],1.,ts->vecs_drdy[nadj]);CHKERRQ(ierr);
     }
   }

   /* Build LHS */
   shift = -1./(th->Theta*ts->time_step);
   if (th->endpoint) {
     ierr = TSComputeIJacobian(ts,ts->ptime,ts->vec_sol,th->Xdot,shift,J,Jp,PETSC_FALSE);CHKERRQ(ierr);
   }else {
     ierr = TSComputeIJacobian(ts,th->stage_time,th->X,th->Xdot,shift,J,Jp,PETSC_FALSE);CHKERRQ(ierr);
   }
   ierr = KSPSetOperators(ksp,J,Jp);CHKERRQ(ierr);

   /* Solve LHS X = RHS */
   for (nadj=0; nadj<ts->numcost; nadj++) {
     ierr = KSPSolveTranspose(ksp,VecsSensiTemp[nadj],VecsDeltaLam[nadj]);CHKERRQ(ierr);
   }

   /* Update sensitivities, and evaluate integrals if there is any */
   if(th->endpoint) { /* two-stage case */
     if (th->Theta!=1.) {
       shift = -1./((th->Theta-1.)*ts->time_step);
       ierr  = TSComputeIJacobian(ts,th->ptime,th->X0,th->Xdot,shift,J,Jp,PETSC_FALSE);CHKERRQ(ierr);
       if (ts->vec_costintegral) {
         ierr = TSAdjointComputeDRDYFunction(ts,th->ptime,th->X0,ts->vecs_drdy);CHKERRQ(ierr);
       }
       for (nadj=0; nadj<ts->numcost; nadj++) {
         ierr = MatMultTranspose(J,VecsDeltaLam[nadj],ts->vecs_sensi[nadj]);CHKERRQ(ierr);
         if (ts->vec_costintegral) {
           ierr = VecAXPY(ts->vecs_sensi[nadj],-1.,ts->vecs_drdy[nadj]);CHKERRQ(ierr);
         }
         ierr = VecScale(ts->vecs_sensi[nadj],1./shift);CHKERRQ(ierr);
       }
     }else { /* backward Euler */
       shift = 0.0;
       ierr  = TSComputeIJacobian(ts,ts->ptime,ts->vec_sol,th->Xdot,shift,J,Jp,PETSC_FALSE);CHKERRQ(ierr); /* get -f_y */
       for (nadj=0; nadj<ts->numcost; nadj++) {
         ierr = MatMultTranspose(J,VecsDeltaLam[nadj],VecsSensiTemp[nadj]);CHKERRQ(ierr);
         ierr = VecAXPY(ts->vecs_sensi[nadj],ts->time_step,VecsSensiTemp[nadj]);CHKERRQ(ierr);
         if (ts->vec_costintegral) {
           ierr = VecAXPY(ts->vecs_sensi[nadj],-ts->time_step,ts->vecs_drdy[nadj]);CHKERRQ(ierr);
         }
       }
     }

     if (ts->vecs_sensip) { /* sensitivities wrt parameters */
       ierr = TSAdjointComputeRHSJacobian(ts,ts->ptime,ts->vec_sol,ts->Jacp);CHKERRQ(ierr);
       for (nadj=0; nadj<ts->numcost; nadj++) {
         ierr = MatMultTranspose(ts->Jacp,VecsDeltaLam[nadj],VecsDeltaMu[nadj]);CHKERRQ(ierr);
         ierr = VecAXPY(ts->vecs_sensip[nadj],-ts->time_step*th->Theta,VecsDeltaMu[nadj]);CHKERRQ(ierr);
       }
       if (th->Theta!=1.) {
         ierr = TSAdjointComputeRHSJacobian(ts,th->ptime,th->X0,ts->Jacp);CHKERRQ(ierr);
         for (nadj=0; nadj<ts->numcost; nadj++) {
           ierr = MatMultTranspose(ts->Jacp,VecsDeltaLam[nadj],VecsDeltaMu[nadj]);CHKERRQ(ierr);
           ierr = VecAXPY(ts->vecs_sensip[nadj],-ts->time_step*(1.-th->Theta),VecsDeltaMu[nadj]);CHKERRQ(ierr);
         }
       }
       if (ts->vec_costintegral) {
         ierr = TSAdjointComputeDRDPFunction(ts,ts->ptime,ts->vec_sol,ts->vecs_drdp);CHKERRQ(ierr);
         for (nadj=0; nadj<ts->numcost; nadj++) {
           ierr = VecAXPY(ts->vecs_sensip[nadj],-ts->time_step*th->Theta,ts->vecs_drdp[nadj]);CHKERRQ(ierr);
         }
         if (th->Theta!=1.) {
           ierr = TSAdjointComputeDRDPFunction(ts,th->ptime,th->X0,ts->vecs_drdp);CHKERRQ(ierr);
           for (nadj=0; nadj<ts->numcost; nadj++) {
             ierr = VecAXPY(ts->vecs_sensip[nadj],-ts->time_step*(1.-th->Theta),ts->vecs_drdp[nadj]);CHKERRQ(ierr);
           }
         }
       }
     }
   }else { /* one-stage case */
     shift = 0.0;
     ierr  = TSComputeIJacobian(ts,th->stage_time,th->X,th->Xdot,shift,J,Jp,PETSC_FALSE);CHKERRQ(ierr); /* get -f_y */
     if (ts->vec_costintegral) {
       ierr = TSAdjointComputeDRDYFunction(ts,th->stage_time,th->X,ts->vecs_drdy);CHKERRQ(ierr);
     }
     for (nadj=0; nadj<ts->numcost; nadj++) {
       ierr = MatMultTranspose(J,VecsDeltaLam[nadj],VecsSensiTemp[nadj]);CHKERRQ(ierr);
       ierr = VecAXPY(ts->vecs_sensi[nadj],ts->time_step,VecsSensiTemp[nadj]);CHKERRQ(ierr);
       if (ts->vec_costintegral) {
         ierr = VecAXPY(ts->vecs_sensi[nadj],-ts->time_step,ts->vecs_drdy[nadj]);CHKERRQ(ierr);
       }
     }
     if (ts->vecs_sensip) {
       ierr = TSAdjointComputeRHSJacobian(ts,th->stage_time,th->X,ts->Jacp);CHKERRQ(ierr);
       for (nadj=0; nadj<ts->numcost; nadj++) {
         ierr = MatMultTranspose(ts->Jacp,VecsDeltaLam[nadj],VecsDeltaMu[nadj]);CHKERRQ(ierr);
         ierr = VecAXPY(ts->vecs_sensip[nadj],-ts->time_step,VecsDeltaMu[nadj]);CHKERRQ(ierr);
       }
       if (ts->vec_costintegral) {
         ierr = TSAdjointComputeDRDPFunction(ts,th->stage_time,th->X,ts->vecs_drdp);CHKERRQ(ierr);
         for (nadj=0; nadj<ts->numcost; nadj++) {
           ierr = VecAXPY(ts->vecs_sensip[nadj],-ts->time_step,ts->vecs_drdp[nadj]);CHKERRQ(ierr);
         }
       }
     }
   }

   ts->steps++;
   th->status = TS_STEP_COMPLETE;
   PetscFunctionReturn(0);
 }

 #undef __FUNCT__
 #define __FUNCT__ "TSInterpolate_Theta"
 static PetscErrorCode TSInterpolate_Theta(TS ts,PetscReal t,Vec X)
 {
   TS_Theta       *th   = (TS_Theta*)ts->data;
   PetscReal      alpha = t - ts->ptime;
   PetscErrorCode ierr;

   PetscFunctionBegin;
   ierr = VecCopy(ts->vec_sol,th->X);CHKERRQ(ierr);
   if (th->endpoint) alpha *= th->Theta;
   ierr = VecWAXPY(X,alpha,th->Xdot,th->X);CHKERRQ(ierr);
   PetscFunctionReturn(0);
 }

#define TSEvent_Status(ts) (ts->event ? ts->event->status : TSEVENT_NONE)

#undef __FUNCT__
#define __FUNCT__ "TSEvaluateWLTE_Theta"
static PetscErrorCode TSEvaluateWLTE_Theta(TS ts,NormType wnormtype,PetscInt *order,PetscReal *wlte)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  Vec            X = ts->vec_sol;      /* X = solution */
  Vec            Y = th->vec_lte_work; /* Y = X + LTE  */
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (!ts->steps || TSEvent_Status(ts) == TSEVENT_RESET_NEXTSTEP) {
    /* Cannot compute LTE in first step or in restart after event */
    *wlte = -1;
  } else {
    /* Compute LTE using backward differences with non-constant time step */
    PetscReal   a = 1 + th->time_step_prev/ts->time_step;
    PetscScalar scal[3]; Vec vecs[3];
    scal[0] = +1/a; scal[1] = -1/(a-1); scal[2] = +1/(a*(a-1));
    vecs[0] = X;    vecs[1] = th->X0;   vecs[2] = th->vec_sol_prev;
    ierr = VecCopy(X,Y);CHKERRQ(ierr);
    ierr = VecMAXPY(Y,3,scal,vecs);CHKERRQ(ierr);
    ierr = TSErrorWeightedNorm(ts,X,Y,wnormtype,wlte);CHKERRQ(ierr);
  }
  if (order) *order = 2;
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSRollBack_Theta"
static PetscErrorCode TSRollBack_Theta(TS ts)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = VecCopy(th->X0,ts->vec_sol);CHKERRQ(ierr);
  if (ts->vec_costintegral && ts->costintegralfwd) {
    ierr = VecCopy(th->VecCostIntegral0,ts->vec_costintegral);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

/*------------------------------------------------------------*/
#undef __FUNCT__
#define __FUNCT__ "TSReset_Theta"
static PetscErrorCode TSReset_Theta(TS ts)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = VecDestroy(&th->X);CHKERRQ(ierr);
  ierr = VecDestroy(&th->Xdot);CHKERRQ(ierr);
  ierr = VecDestroy(&th->X0);CHKERRQ(ierr);
  ierr = VecDestroy(&th->affine);CHKERRQ(ierr);

  ierr = VecDestroy(&th->vec_sol_prev);CHKERRQ(ierr);
  ierr = VecDestroy(&th->vec_lte_work);CHKERRQ(ierr);

  ierr = VecDestroy(&th->VecCostIntegral0);CHKERRQ(ierr);
  ierr = VecDestroyVecs(ts->numcost,&th->VecsDeltaLam);CHKERRQ(ierr);
  ierr = VecDestroyVecs(ts->numcost,&th->VecsDeltaMu);CHKERRQ(ierr);
  ierr = VecDestroyVecs(ts->numcost,&th->VecsSensiTemp);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSDestroy_Theta"
static PetscErrorCode TSDestroy_Theta(TS ts)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = TSReset_Theta(ts);CHKERRQ(ierr);
  ierr = PetscFree(ts->data);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)ts,"TSThetaGetTheta_C",NULL);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)ts,"TSThetaSetTheta_C",NULL);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)ts,"TSThetaGetEndpoint_C",NULL);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)ts,"TSThetaSetEndpoint_C",NULL);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*
  This defines the nonlinear equation that is to be solved with SNES
  G(U) = F[t0+Theta*dt, U, (U-U0)*shift] = 0
*/
#undef __FUNCT__
#define __FUNCT__ "SNESTSFormFunction_Theta"
static PetscErrorCode SNESTSFormFunction_Theta(SNES snes,Vec x,Vec y,TS ts)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  PetscErrorCode ierr;
  Vec            X0,Xdot;
  DM             dm,dmsave;
  PetscReal      shift = 1/(th->Theta*ts->time_step);

  PetscFunctionBegin;
  ierr = SNESGetDM(snes,&dm);CHKERRQ(ierr);
  /* When using the endpoint variant, this is actually 1/Theta * Xdot */
  ierr = TSThetaGetX0AndXdot(ts,dm,&X0,&Xdot);CHKERRQ(ierr);
  ierr = VecAXPBYPCZ(Xdot,-shift,shift,0,X0,x);CHKERRQ(ierr);

  /* DM monkey-business allows user code to call TSGetDM() inside of functions evaluated on levels of FAS */
  dmsave = ts->dm;
  ts->dm = dm;
  ierr   = TSComputeIFunction(ts,th->stage_time,x,Xdot,y,PETSC_FALSE);CHKERRQ(ierr);
  ts->dm = dmsave;
  ierr   = TSThetaRestoreX0AndXdot(ts,dm,&X0,&Xdot);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "SNESTSFormJacobian_Theta"
static PetscErrorCode SNESTSFormJacobian_Theta(SNES snes,Vec x,Mat A,Mat B,TS ts)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  PetscErrorCode ierr;
  Vec            Xdot;
  DM             dm,dmsave;
  PetscReal      shift = 1/(th->Theta*ts->time_step);

  PetscFunctionBegin;
  ierr = SNESGetDM(snes,&dm);CHKERRQ(ierr);

  /* th->Xdot has already been computed in SNESTSFormFunction_Theta (SNES guarantees this) */
  ierr = TSThetaGetX0AndXdot(ts,dm,NULL,&Xdot);CHKERRQ(ierr);

  dmsave = ts->dm;
  ts->dm = dm;
  ierr   = TSComputeIJacobian(ts,th->stage_time,x,Xdot,shift,A,B,PETSC_FALSE);CHKERRQ(ierr);
  ts->dm = dmsave;
  ierr   = TSThetaRestoreX0AndXdot(ts,dm,NULL,&Xdot);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSSetUp_Theta"
static PetscErrorCode TSSetUp_Theta(TS ts)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (!th->VecCostIntegral0 && ts->vec_costintegral && ts->costintegralfwd) { /* back up cost integral */
    ierr = VecDuplicate(ts->vec_costintegral,&th->VecCostIntegral0);CHKERRQ(ierr);
  }
  if (!th->X) {
    ierr = VecDuplicate(ts->vec_sol,&th->X);CHKERRQ(ierr);
  }
  if (!th->Xdot) {
    ierr = VecDuplicate(ts->vec_sol,&th->Xdot);CHKERRQ(ierr);
  }
  if (!th->X0) {
    ierr = VecDuplicate(ts->vec_sol,&th->X0);CHKERRQ(ierr);
  }
  if (th->endpoint) {
    ierr = VecDuplicate(ts->vec_sol,&th->affine);CHKERRQ(ierr);
  }

  th->order = (th->Theta == 0.5) ? 2 : 1;

  ierr = TSGetDM(ts,&ts->dm);CHKERRQ(ierr);
  ierr = DMCoarsenHookAdd(ts->dm,DMCoarsenHook_TSTheta,DMRestrictHook_TSTheta,ts);CHKERRQ(ierr);
  ierr = DMSubDomainHookAdd(ts->dm,DMSubDomainHook_TSTheta,DMSubDomainRestrictHook_TSTheta,ts);CHKERRQ(ierr);

  ierr = TSGetAdapt(ts,&ts->adapt);CHKERRQ(ierr);
  ierr = TSAdaptCandidatesClear(ts->adapt);CHKERRQ(ierr);
  if (!th->adapt) {
    ierr = TSAdaptSetType(ts->adapt,TSADAPTNONE);CHKERRQ(ierr);
  } else {
    ierr = VecDuplicate(ts->vec_sol,&th->vec_sol_prev);CHKERRQ(ierr);
    ierr = VecDuplicate(ts->vec_sol,&th->vec_lte_work);CHKERRQ(ierr);
    if (ts->exact_final_time == TS_EXACTFINALTIME_UNSPECIFIED)
      ts->exact_final_time = TS_EXACTFINALTIME_MATCHSTEP;
  }

  ierr = TSGetSNES(ts,&ts->snes);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*------------------------------------------------------------*/

#undef __FUNCT__
#define __FUNCT__ "TSAdjointSetUp_Theta"
static PetscErrorCode TSAdjointSetUp_Theta(TS ts)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = VecDuplicateVecs(ts->vecs_sensi[0],ts->numcost,&th->VecsDeltaLam);CHKERRQ(ierr);
  if(ts->vecs_sensip) {
    ierr = VecDuplicateVecs(ts->vecs_sensip[0],ts->numcost,&th->VecsDeltaMu);CHKERRQ(ierr);
  }
  ierr = VecDuplicateVecs(ts->vecs_sensi[0],ts->numcost,&th->VecsSensiTemp);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}
/*------------------------------------------------------------*/

#undef __FUNCT__
#define __FUNCT__ "TSSetFromOptions_Theta"
static PetscErrorCode TSSetFromOptions_Theta(PetscOptionItems *PetscOptionsObject,TS ts)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = PetscOptionsHead(PetscOptionsObject,"Theta ODE solver options");CHKERRQ(ierr);
  {
    ierr = PetscOptionsReal("-ts_theta_theta","Location of stage (0<Theta<=1)","TSThetaSetTheta",th->Theta,&th->Theta,NULL);CHKERRQ(ierr);
    ierr = PetscOptionsBool("-ts_theta_extrapolate","Extrapolate stage solution from previous solution (sometimes unstable)","TSThetaSetExtrapolate",th->extrapolate,&th->extrapolate,NULL);CHKERRQ(ierr);
    ierr = PetscOptionsBool("-ts_theta_endpoint","Use the endpoint instead of midpoint form of the Theta method","TSThetaSetEndpoint",th->endpoint,&th->endpoint,NULL);CHKERRQ(ierr);
    ierr = PetscOptionsBool("-ts_theta_adapt","Use time-step adaptivity with the Theta method","",th->adapt,&th->adapt,NULL);CHKERRQ(ierr);
  }
  ierr = PetscOptionsTail();CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSView_Theta"
static PetscErrorCode TSView_Theta(TS ts,PetscViewer viewer)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  PetscBool      iascii;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = PetscObjectTypeCompare((PetscObject)viewer,PETSCVIEWERASCII,&iascii);CHKERRQ(ierr);
  if (iascii) {
    ierr = PetscViewerASCIIPrintf(viewer,"  Theta=%g\n",(double)th->Theta);CHKERRQ(ierr);
    ierr = PetscViewerASCIIPrintf(viewer,"  Extrapolation=%s\n",th->extrapolate ? "yes" : "no");CHKERRQ(ierr);
  }
  if (ts->snes) {ierr = SNESView(ts->snes,viewer);CHKERRQ(ierr);}
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSThetaGetTheta_Theta"
PetscErrorCode  TSThetaGetTheta_Theta(TS ts,PetscReal *theta)
{
  TS_Theta *th = (TS_Theta*)ts->data;

  PetscFunctionBegin;
  *theta = th->Theta;
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSThetaSetTheta_Theta"
PetscErrorCode  TSThetaSetTheta_Theta(TS ts,PetscReal theta)
{
  TS_Theta *th = (TS_Theta*)ts->data;

  PetscFunctionBegin;
  if (theta <= 0 || 1 < theta) SETERRQ1(PetscObjectComm((PetscObject)ts),PETSC_ERR_ARG_OUTOFRANGE,"Theta %g not in range (0,1]",(double)theta);
  th->Theta = theta;
  th->order = (th->Theta == 0.5) ? 2 : 1;
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSThetaGetEndpoint_Theta"
PetscErrorCode  TSThetaGetEndpoint_Theta(TS ts,PetscBool *endpoint)
{
  TS_Theta *th = (TS_Theta*)ts->data;

  PetscFunctionBegin;
  *endpoint = th->endpoint;
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSThetaSetEndpoint_Theta"
PetscErrorCode  TSThetaSetEndpoint_Theta(TS ts,PetscBool flg)
{
  TS_Theta *th = (TS_Theta*)ts->data;

  PetscFunctionBegin;
  th->endpoint = flg;
  PetscFunctionReturn(0);
}

#if defined(PETSC_HAVE_COMPLEX)
#undef __FUNCT__
#define __FUNCT__ "TSComputeLinearStability_Theta"
static PetscErrorCode TSComputeLinearStability_Theta(TS ts,PetscReal xr,PetscReal xi,PetscReal *yr,PetscReal *yi)
{
  PetscComplex   z   = xr + xi*PETSC_i,f;
  TS_Theta       *th = (TS_Theta*)ts->data;
  const PetscReal one = 1.0;

  PetscFunctionBegin;
  f   = (one + (one - th->Theta)*z)/(one - th->Theta*z);
  *yr = PetscRealPartComplex(f);
  *yi = PetscImaginaryPartComplex(f);
  PetscFunctionReturn(0);
}
#endif

#undef __FUNCT__
#define __FUNCT__ "TSGetStages_Theta"
static PetscErrorCode  TSGetStages_Theta(TS ts,PetscInt *ns,Vec **Y)
{
  TS_Theta     *th = (TS_Theta*)ts->data;

  PetscFunctionBegin;
  if (ns) *ns = 1;
  if (Y)  *Y  = th->endpoint ? &(th->X0) : &(th->X);
  PetscFunctionReturn(0);
}

/* ------------------------------------------------------------ */
/*MC
      TSTHETA - DAE solver using the implicit Theta method

   Level: beginner

   Options Database:
+  -ts_theta_theta <Theta> - Location of stage (0<Theta<=1)
.  -ts_theta_extrapolate <flg> - Extrapolate stage solution from previous solution (sometimes unstable)
.  -ts_theta_endpoint <flag> - Use the endpoint (like Crank-Nicholson) instead of midpoint form of the Theta method
-  -ts_theta_adapt <flg> - Use time-step adaptivity with the Theta method

   Notes:
$  -ts_type theta -ts_theta_theta 1.0 corresponds to backward Euler (TSBEULER)
$  -ts_type theta -ts_theta_theta 0.5 corresponds to the implicit midpoint rule
$  -ts_type theta -ts_theta_theta 0.5 -ts_theta_endpoint corresponds to Crank-Nicholson (TSCN)

   This method can be applied to DAE.

   This method is cast as a 1-stage implicit Runge-Kutta method.

.vb
  Theta | Theta
  -------------
        |  1
.ve

   For the default Theta=0.5, this is also known as the implicit midpoint rule.

   When the endpoint variant is chosen, the method becomes a 2-stage method with first stage explicit:

.vb
  0 | 0         0
  1 | 1-Theta   Theta
  -------------------
    | 1-Theta   Theta
.ve

   For the default Theta=0.5, this is the trapezoid rule (also known as Crank-Nicolson, see TSCN).

   To apply a diagonally implicit RK method to DAE, the stage formula

$  Y_i = X + h sum_j a_ij Y'_j

   is interpreted as a formula for Y'_i in terms of Y_i and known values (Y'_j, j<i)

.seealso:  TSCreate(), TS, TSSetType(), TSCN, TSBEULER, TSThetaSetTheta(), TSThetaSetEndpoint()

M*/
#undef __FUNCT__
#define __FUNCT__ "TSCreate_Theta"
PETSC_EXTERN PetscErrorCode TSCreate_Theta(TS ts)
{
  TS_Theta       *th;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ts->ops->reset           = TSReset_Theta;
  ts->ops->destroy         = TSDestroy_Theta;
  ts->ops->view            = TSView_Theta;
  ts->ops->setup           = TSSetUp_Theta;
  ts->ops->adjointsetup    = TSAdjointSetUp_Theta;
  ts->ops->step            = TSStep_Theta;
  ts->ops->interpolate     = TSInterpolate_Theta;
  ts->ops->evaluatewlte    = TSEvaluateWLTE_Theta;
  ts->ops->rollback        = TSRollBack_Theta;
  ts->ops->setfromoptions  = TSSetFromOptions_Theta;
  ts->ops->snesfunction    = SNESTSFormFunction_Theta;
  ts->ops->snesjacobian    = SNESTSFormJacobian_Theta;
#if defined(PETSC_HAVE_COMPLEX)
  ts->ops->linearstability = TSComputeLinearStability_Theta;
#endif
  ts->ops->getstages       = TSGetStages_Theta;
  ts->ops->adjointstep     = TSAdjointStep_Theta;
  ts->ops->adjointintegral = TSAdjointCostIntegral_Theta;
  ts->ops->forwardintegral = TSForwardCostIntegral_Theta;

  ierr = PetscNewLog(ts,&th);CHKERRQ(ierr);
  ts->data = (void*)th;

  th->extrapolate = PETSC_FALSE;
  th->Theta       = 0.5;
  th->order       = 2;
  th->adapt       = PETSC_FALSE;
  ierr = PetscObjectComposeFunction((PetscObject)ts,"TSThetaGetTheta_C",TSThetaGetTheta_Theta);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)ts,"TSThetaSetTheta_C",TSThetaSetTheta_Theta);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)ts,"TSThetaGetEndpoint_C",TSThetaGetEndpoint_Theta);CHKERRQ(ierr);
  ierr = PetscObjectComposeFunction((PetscObject)ts,"TSThetaSetEndpoint_C",TSThetaSetEndpoint_Theta);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSThetaGetTheta"
/*@
  TSThetaGetTheta - Get the abscissa of the stage in (0,1].

  Not Collective

  Input Parameter:
.  ts - timestepping context

  Output Parameter:
.  theta - stage abscissa

  Note:
  Use of this function is normally only required to hack TSTHETA to use a modified integration scheme.

  Level: Advanced

.seealso: TSThetaSetTheta()
@*/
PetscErrorCode  TSThetaGetTheta(TS ts,PetscReal *theta)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(ts,TS_CLASSID,1);
  PetscValidPointer(theta,2);
  ierr = PetscUseMethod(ts,"TSThetaGetTheta_C",(TS,PetscReal*),(ts,theta));CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSThetaSetTheta"
/*@
  TSThetaSetTheta - Set the abscissa of the stage in (0,1].

  Not Collective

  Input Parameter:
+  ts - timestepping context
-  theta - stage abscissa

  Options Database:
.  -ts_theta_theta <theta>

  Level: Intermediate

.seealso: TSThetaGetTheta()
@*/
PetscErrorCode  TSThetaSetTheta(TS ts,PetscReal theta)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(ts,TS_CLASSID,1);
  ierr = PetscTryMethod(ts,"TSThetaSetTheta_C",(TS,PetscReal),(ts,theta));CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSThetaGetEndpoint"
/*@
  TSThetaGetEndpoint - Gets whether to use the endpoint variant of the method (e.g. trapezoid/Crank-Nicolson instead of midpoint rule).

  Not Collective

  Input Parameter:
.  ts - timestepping context

  Output Parameter:
.  endpoint - PETSC_TRUE when using the endpoint variant

  Level: Advanced

.seealso: TSThetaSetEndpoint(), TSTHETA, TSCN
@*/
PetscErrorCode TSThetaGetEndpoint(TS ts,PetscBool *endpoint)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(ts,TS_CLASSID,1);
  PetscValidPointer(endpoint,2);
  ierr = PetscUseMethod(ts,"TSThetaGetEndpoint_C",(TS,PetscBool*),(ts,endpoint));CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSThetaSetEndpoint"
/*@
  TSThetaSetEndpoint - Sets whether to use the endpoint variant of the method (e.g. trapezoid/Crank-Nicolson instead of midpoint rule).

  Not Collective

  Input Parameter:
+  ts - timestepping context
-  flg - PETSC_TRUE to use the endpoint variant

  Options Database:
.  -ts_theta_endpoint <flg>

  Level: Intermediate

.seealso: TSTHETA, TSCN
@*/
PetscErrorCode TSThetaSetEndpoint(TS ts,PetscBool flg)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(ts,TS_CLASSID,1);
  ierr = PetscTryMethod(ts,"TSThetaSetEndpoint_C",(TS,PetscBool),(ts,flg));CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*
 * TSBEULER and TSCN are straightforward specializations of TSTHETA.
 * The creation functions for these specializations are below.
 */

#undef __FUNCT__
#define __FUNCT__ "TSSetUp_BEuler"
static PetscErrorCode TSSetUp_BEuler(TS ts)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (th->Theta != 1.0) SETERRQ(PetscObjectComm((PetscObject)ts),PETSC_ERR_OPT_OVERWRITE,"Can not change the default value (1) of theta when using backward Euler\n");
  if (th->endpoint) SETERRQ(PetscObjectComm((PetscObject)ts),PETSC_ERR_OPT_OVERWRITE,"Can not change to the endpoint form of the Theta methods when using backward Euler\n");
  ierr = TSSetUp_Theta(ts);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSView_BEuler"
static PetscErrorCode TSView_BEuler(TS ts,PetscViewer viewer)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (ts->snes) {ierr = SNESView(ts->snes,viewer);CHKERRQ(ierr);}
  PetscFunctionReturn(0);
}

/*MC
      TSBEULER - ODE solver using the implicit backward Euler method

  Level: beginner

  Notes:
  TSBEULER is equivalent to TSTHETA with Theta=1.0

$  -ts_type theta -ts_theta_theta 1.0

.seealso:  TSCreate(), TS, TSSetType(), TSEULER, TSCN, TSTHETA

M*/
#undef __FUNCT__
#define __FUNCT__ "TSCreate_BEuler"
PETSC_EXTERN PetscErrorCode TSCreate_BEuler(TS ts)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = TSCreate_Theta(ts);CHKERRQ(ierr);
  ierr = TSThetaSetTheta(ts,1.0);CHKERRQ(ierr);
  ierr = TSThetaSetEndpoint(ts,PETSC_FALSE);CHKERRQ(ierr);
  ts->ops->setup = TSSetUp_BEuler;
  ts->ops->view  = TSView_BEuler;
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSSetUp_CN"
static PetscErrorCode TSSetUp_CN(TS ts)
{
  TS_Theta       *th = (TS_Theta*)ts->data;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (th->Theta != 0.5) SETERRQ(PetscObjectComm((PetscObject)ts),PETSC_ERR_OPT_OVERWRITE,"Can not change the default value (0.5) of theta when using Crank-Nicolson\n");
  if (!th->endpoint) SETERRQ(PetscObjectComm((PetscObject)ts),PETSC_ERR_OPT_OVERWRITE,"Can not change to the midpoint form of the Theta methods when using Crank-Nicolson\n");
  ierr = TSSetUp_Theta(ts);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNCT__
#define __FUNCT__ "TSView_CN"
static PetscErrorCode TSView_CN(TS ts,PetscViewer viewer)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  if (ts->snes) {ierr = SNESView(ts->snes,viewer);CHKERRQ(ierr);}
  PetscFunctionReturn(0);
}

/*MC
      TSCN - ODE solver using the implicit Crank-Nicolson method.

  Level: beginner

  Notes:
  TSCN is equivalent to TSTHETA with Theta=0.5 and the "endpoint" option set. I.e.

$  -ts_type theta -ts_theta_theta 0.5 -ts_theta_endpoint

.seealso:  TSCreate(), TS, TSSetType(), TSBEULER, TSTHETA

M*/
#undef __FUNCT__
#define __FUNCT__ "TSCreate_CN"
PETSC_EXTERN PetscErrorCode TSCreate_CN(TS ts)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = TSCreate_Theta(ts);CHKERRQ(ierr);
  ierr = TSThetaSetTheta(ts,0.5);CHKERRQ(ierr);
  ierr = TSThetaSetEndpoint(ts,PETSC_TRUE);CHKERRQ(ierr);
  ts->ops->setup = TSSetUp_CN;
  ts->ops->view  = TSView_CN;
  PetscFunctionReturn(0);
}