xref: /petsc/src/snes/tutorials/ex59.c (revision 327415f76d85372a4417cf1aaa14db707d4d6c04)

static const char help[] = "Tries to solve u`` + u^{2} = f for an easy case and an impossible case.\n\n";

/*
       This example was contributed by Peter Graf to show how SNES fails when given a nonlinear problem with no solution.

       Run with -n 14 to see fail to converge and -n 15 to see convergence

       The option -second_order uses a different discretization of the Neumann boundary condition and always converges

*/

#include <petscsnes.h>

PetscBool second_order = PETSC_FALSE;
#define X0DOT      -2.0
#define X1          5.0
#define KPOW        2.0
const PetscScalar sperturb = 1.1;

/*
   User-defined routines
*/
PetscErrorCode FormJacobian(SNES,Vec,Mat,Mat,void*);
PetscErrorCode FormFunction(SNES,Vec,Vec,void*);

int main(int argc,char **argv)
{
  SNES              snes;                /* SNES context */
  Vec               x,r,F;               /* vectors */
  Mat               J;                   /* Jacobian */
  PetscInt          it,n = 11,i;
  PetscReal         h,xp = 0.0;
  PetscScalar       v;
  const PetscScalar a = X0DOT;
  const PetscScalar b = X1;
  const PetscScalar k = KPOW;
  PetscScalar       v2;
  PetscScalar       *xx;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc,&argv,(char*)0,help));
  PetscCall(PetscOptionsGetInt(NULL,NULL,"-n",&n,NULL));
  PetscCall(PetscOptionsGetBool(NULL,NULL,"-second_order",&second_order,NULL));
  h    = 1.0/(n-1);

  /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Create nonlinear solver context
     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

  PetscCall(SNESCreate(PETSC_COMM_WORLD,&snes));

  /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Create vector data structures; set function evaluation routine
     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

  PetscCall(VecCreate(PETSC_COMM_SELF,&x));
  PetscCall(VecSetSizes(x,PETSC_DECIDE,n));
  PetscCall(VecSetFromOptions(x));
  PetscCall(VecDuplicate(x,&r));
  PetscCall(VecDuplicate(x,&F));

  PetscCall(SNESSetFunction(snes,r,FormFunction,(void*)F));

  /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Create matrix data structures; set Jacobian evaluation routine
     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

  PetscCall(MatCreateSeqAIJ(PETSC_COMM_SELF,n,n,3,NULL,&J));

  /*
     Note that in this case we create separate matrices for the Jacobian
     and preconditioner matrix.  Both of these are computed in the
     routine FormJacobian()
  */
  /*  PetscCall(SNESSetJacobian(snes,NULL,JPrec,FormJacobian,0)); */
  PetscCall(SNESSetJacobian(snes,J,J,FormJacobian,0));
  /*  PetscCall(SNESSetJacobian(snes,J,JPrec,FormJacobian,0)); */

  /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Customize nonlinear solver; set runtime options
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

  PetscCall(SNESSetFromOptions(snes));

  /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Initialize application:
     Store right-hand-side of PDE and exact solution
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

  /* set right hand side and initial guess to be exact solution of continuim problem */
#define SQR(x) ((x)*(x))
  xp = 0.0;
  for (i=0; i<n; i++)
  {
    v    = k*(k-1.)*(b-a)*PetscPowScalar(xp,k-2.) + SQR(a*xp) + SQR(b-a)*PetscPowScalar(xp,2.*k) + 2.*a*(b-a)*PetscPowScalar(xp,k+1.);
    PetscCall(VecSetValues(F,1,&i,&v,INSERT_VALUES));
    v2   = a*xp + (b-a)*PetscPowScalar(xp,k);
    PetscCall(VecSetValues(x,1,&i,&v2,INSERT_VALUES));
    xp  += h;
  }

  /* perturb initial guess */
  PetscCall(VecGetArray(x,&xx));
  for (i=0; i<n; i++) {
    v2   = xx[i]*sperturb;
    PetscCall(VecSetValues(x,1,&i,&v2,INSERT_VALUES));
  }
  PetscCall(VecRestoreArray(x,&xx));

  PetscCall(SNESSolve(snes,NULL,x));
  PetscCall(SNESGetIterationNumber(snes,&it));
  PetscCall(PetscPrintf(PETSC_COMM_SELF,"SNES iterations = %" PetscInt_FMT "\n\n",it));

  /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Free work space.  All PETSc objects should be destroyed when they
     are no longer needed.
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

  PetscCall(VecDestroy(&x));     PetscCall(VecDestroy(&r));
  PetscCall(VecDestroy(&F));     PetscCall(MatDestroy(&J));
  PetscCall(SNESDestroy(&snes));
  PetscCall(PetscFinalize());
  return 0;
}

PetscErrorCode FormFunction(SNES snes,Vec x,Vec f,void *dummy)
{
  const PetscScalar *xx;
  PetscScalar       *ff,*FF,d,d2;
  PetscInt          i,n;

  PetscCall(VecGetArrayRead(x,&xx));
  PetscCall(VecGetArray(f,&ff));
  PetscCall(VecGetArray((Vec)dummy,&FF));
  PetscCall(VecGetSize(x,&n));
  d    = (PetscReal)(n - 1); d2 = d*d;

  if (second_order) ff[0] = d*(0.5*d*(-xx[2] + 4.*xx[1] - 3.*xx[0]) - X0DOT);
  else ff[0] = d*(d*(xx[1] - xx[0]) - X0DOT);

  for (i=1; i<n-1; i++) ff[i] = d2*(xx[i-1] - 2.*xx[i] + xx[i+1]) + xx[i]*xx[i] - FF[i];

  ff[n-1] = d*d*(xx[n-1] - X1);
  PetscCall(VecRestoreArrayRead(x,&xx));
  PetscCall(VecRestoreArray(f,&ff));
  PetscCall(VecRestoreArray((Vec)dummy,&FF));
  return 0;
}

PetscErrorCode FormJacobian(SNES snes,Vec x,Mat jac,Mat prejac,void *dummy)
{
  const PetscScalar *xx;
  PetscScalar       A[3],d,d2;
  PetscInt          i,n,j[3];

  PetscCall(VecGetSize(x,&n));
  PetscCall(VecGetArrayRead(x,&xx));
  d    = (PetscReal)(n - 1); d2 = d*d;

  i = 0;
  if (second_order) {
    j[0] = 0; j[1] = 1; j[2] = 2;
    A[0] = -3.*d*d*0.5; A[1] = 4.*d*d*0.5;  A[2] = -1.*d*d*0.5;
    PetscCall(MatSetValues(prejac,1,&i,3,j,A,INSERT_VALUES));
  } else {
    j[0] = 0; j[1] = 1;
    A[0] = -d*d; A[1] = d*d;
    PetscCall(MatSetValues(prejac,1,&i,2,j,A,INSERT_VALUES));
  }
  for (i=1; i<n-1; i++) {
    j[0] = i - 1; j[1] = i;                   j[2] = i + 1;
    A[0] = d2;    A[1] = -2.*d2 + 2.*xx[i];  A[2] = d2;
    PetscCall(MatSetValues(prejac,1,&i,3,j,A,INSERT_VALUES));
  }

  i    = n-1;
  A[0] = d*d;
  PetscCall(MatSetValues(prejac,1,&i,1,&i,&A[0],INSERT_VALUES));

  PetscCall(MatAssemblyBegin(jac,MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(jac,MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyBegin(prejac,MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(prejac,MAT_FINAL_ASSEMBLY));

  PetscCall(VecRestoreArrayRead(x,&xx));
  return 0;
}

/*TEST

   test:
      args: -n 14 -snes_monitor_short -snes_converged_reason
      requires: !single

   test:
      suffix: 2
      args: -n 15 -snes_monitor_short -snes_converged_reason
      requires: !single

   test:
      suffix: 3
      args: -n 14 -second_order -snes_monitor_short -snes_converged_reason
      requires: !single

TEST*/