static char help[] = "Tests repeated solving linear system on 2 by 2 matrix provided by MUMPS developer, Dec 17, 2012.\n\n";
/*
We have investigated the problem further, and we have
been able to reproduce it and obtain an erroneous
solution with an even smaller, 2x2, matrix:
    [1 2]
    [2 3]
and a right-hand side vector with all ones (1,1)
The correct solution is the vector (-1,1), in both solves.

mpiexec -n 2 ./ex2 -ksp_type preonly -pc_type lu  -pc_factor_mat_solver_type mumps  -mat_mumps_icntl_7 6 -mat_mumps_cntl_1 0.99

With this combination of options, I get off-diagonal pivots during the
factorization, which is the cause of the problem (different isol_loc
returned in the second solve, whereas, as I understand it, Petsc expects
isol_loc not to change between successive solves).
*/

#include <petscksp.h>

int main(int argc, char **args)
{
  Mat         C;
  PetscInt    N = 2, rowidx, colidx;
  Vec         u, b, r;
  KSP         ksp;
  PetscReal   norm;
  PetscMPIInt rank, size;
  PetscScalar v;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &args, NULL, help));
  PetscCallMPI(MPI_Comm_rank(PETSC_COMM_WORLD, &rank));
  PetscCallMPI(MPI_Comm_size(PETSC_COMM_WORLD, &size));

  /* create stiffness matrix C = [1 2; 2 3] */
  PetscCall(MatCreate(PETSC_COMM_WORLD, &C));
  PetscCall(MatSetSizes(C, PETSC_DECIDE, PETSC_DECIDE, N, N));
  PetscCall(MatSetFromOptions(C));
  PetscCall(MatSetUp(C));
  if (rank == 0) {
    rowidx = 0;
    colidx = 0;
    v      = 1.0;
    PetscCall(MatSetValues(C, 1, &rowidx, 1, &colidx, &v, INSERT_VALUES));
    rowidx = 0;
    colidx = 1;
    v      = 2.0;
    PetscCall(MatSetValues(C, 1, &rowidx, 1, &colidx, &v, INSERT_VALUES));

    rowidx = 1;
    colidx = 0;
    v      = 2.0;
    PetscCall(MatSetValues(C, 1, &rowidx, 1, &colidx, &v, INSERT_VALUES));
    rowidx = 1;
    colidx = 1;
    v      = 3.0;
    PetscCall(MatSetValues(C, 1, &rowidx, 1, &colidx, &v, INSERT_VALUES));
  }
  PetscCall(MatAssemblyBegin(C, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(C, MAT_FINAL_ASSEMBLY));

  /* create right-hand side and solution */
  PetscCall(VecCreate(PETSC_COMM_WORLD, &u));
  PetscCall(VecSetSizes(u, PETSC_DECIDE, N));
  PetscCall(VecSetFromOptions(u));
  PetscCall(VecDuplicate(u, &b));
  PetscCall(VecDuplicate(u, &r));
  PetscCall(VecSet(u, 0.0));
  PetscCall(VecSet(b, 1.0));

  /* solve linear system C*u = b */
  PetscCall(KSPCreate(PETSC_COMM_WORLD, &ksp));
  PetscCall(KSPSetOperators(ksp, C, C));
  PetscCall(KSPSetFromOptions(ksp));
  PetscCall(KSPSolve(ksp, b, u));

  /* check residual r = C*u - b */
  PetscCall(MatMult(C, u, r));
  PetscCall(VecAXPY(r, -1.0, b));
  PetscCall(VecNorm(r, NORM_2, &norm));
  PetscCall(PetscPrintf(PETSC_COMM_WORLD, "|| C*u - b|| = %g\n", (double)norm));

  /* solve C^T*u = b twice */
  PetscCall(KSPSolveTranspose(ksp, b, u));
  /* check residual r = C^T*u - b */
  PetscCall(MatMultTranspose(C, u, r));
  PetscCall(VecAXPY(r, -1.0, b));
  PetscCall(VecNorm(r, NORM_2, &norm));
  PetscCall(PetscPrintf(PETSC_COMM_WORLD, "|| C^T*u - b|| =  %g\n", (double)norm));

  PetscCall(KSPSolveTranspose(ksp, b, u));
  PetscCall(MatMultTranspose(C, u, r));
  PetscCall(VecAXPY(r, -1.0, b));
  PetscCall(VecNorm(r, NORM_2, &norm));
  PetscCall(PetscPrintf(PETSC_COMM_WORLD, "|| C^T*u - b|| =  %g\n", (double)norm));

  /* solve C*u = b again */
  PetscCall(KSPSolve(ksp, b, u));
  PetscCall(MatMult(C, u, r));
  PetscCall(VecAXPY(r, -1.0, b));
  PetscCall(VecNorm(r, NORM_2, &norm));
  PetscCall(PetscPrintf(PETSC_COMM_WORLD, "|| C*u - b|| = %g\n", (double)norm));

  PetscCall(KSPDestroy(&ksp));
  PetscCall(VecDestroy(&u));
  PetscCall(VecDestroy(&r));
  PetscCall(VecDestroy(&b));
  PetscCall(MatDestroy(&C));
  PetscCall(PetscFinalize());
  return 0;
}
