Date: 2024-12-02T08:57:15+00:00

Actual source code: ex32.c
slepc-3.22.2 2024-12-02
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      1: /*
      2:    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
      3:    SLEPc - Scalable Library for Eigenvalue Problem Computations
      4:    Copyright (c) 2002-, Universitat Politecnica de Valencia, Spain

      6:    This file is part of SLEPc.
      7:    SLEPc is distributed under a 2-clause BSD license (see LICENSE).
      8:    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
      9: */

     11: static char help[] = "Solves a Lypunov equation with the shifted 2-D Laplacian.\n\n"
     12:   "The command line options are:\n"
     13:   "  -n <n>, where <n> = number of grid subdivisions in x dimension.\n"
     14:   "  -m <m>, where <m> = number of grid subdivisions in y dimension.\n\n";

     16: #include <slepclme.h>

     18: int main(int argc,char **argv)
     19: {
     20:   Mat                A;           /* problem matrix */
     21:   Mat                C,C1;        /* right-hand side */
     22:   Mat                X,X1;        /* solution */
     23:   LME                lme;
     24:   PetscReal          tol,errest,error;
     25:   PetscScalar        *u,sigma=0.0;
     26:   PetscInt           N,n=10,m,Istart,Iend,II,maxit,its,ncv,i,j,rank=0;
     27:   PetscBool          flag;
     28:   LMEConvergedReason reason;

     30:   PetscFunctionBeginUser;
     31:   PetscCall(SlepcInitialize(&argc,&argv,NULL,help));

     33:   PetscCall(PetscOptionsGetInt(NULL,NULL,"-n",&n,NULL));
     34:   PetscCall(PetscOptionsGetInt(NULL,NULL,"-m",&m,&flag));
     35:   if (!flag) m=n;
     36:   N = n*m;
     37:   PetscCall(PetscOptionsGetScalar(NULL,NULL,"-sigma",&sigma,NULL));
     38:   PetscCall(PetscOptionsGetInt(NULL,NULL,"-rank",&rank,NULL));
     39:   PetscCall(PetscPrintf(PETSC_COMM_WORLD,"\nLyapunov equation, N=%" PetscInt_FMT " (%" PetscInt_FMT "x%" PetscInt_FMT " grid)\n\n",N,n,m));

     41:   /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     42:                        Create the 2-D Laplacian, A
     43:      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

     45:   PetscCall(MatCreate(PETSC_COMM_WORLD,&A));
     46:   PetscCall(MatSetSizes(A,PETSC_DECIDE,PETSC_DECIDE,N,N));
     47:   PetscCall(MatSetFromOptions(A));
     48:   PetscCall(MatGetOwnershipRange(A,&Istart,&Iend));
     49:   for (II=Istart;II<Iend;II++) {
     50:     i = II/n; j = II-i*n;
     51:     if (i>0) PetscCall(MatSetValue(A,II,II-n,1.0,INSERT_VALUES));
     52:     if (i<m-1) PetscCall(MatSetValue(A,II,II+n,1.0,INSERT_VALUES));
     53:     if (j>0) PetscCall(MatSetValue(A,II,II-1,1.0,INSERT_VALUES));
     54:     if (j<n-1) PetscCall(MatSetValue(A,II,II+1,1.0,INSERT_VALUES));
     55:     PetscCall(MatSetValue(A,II,II,-4.0-sigma,INSERT_VALUES));
     56:   }
     57:   PetscCall(MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY));
     58:   PetscCall(MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY));

     60:   /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     61:        Create a low-rank Mat to store the right-hand side C = C1*C1'
     62:      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

     64:   PetscCall(MatCreate(PETSC_COMM_WORLD,&C1));
     65:   PetscCall(MatSetSizes(C1,PETSC_DECIDE,PETSC_DECIDE,N,2));
     66:   PetscCall(MatSetType(C1,MATDENSE));
     67:   PetscCall(MatGetOwnershipRange(C1,&Istart,&Iend));
     68:   PetscCall(MatDenseGetArray(C1,&u));
     69:   for (i=Istart;i<Iend;i++) {
     70:     if (i<N/2) u[i-Istart] = 1.0;
     71:     if (i==0) u[i+Iend-2*Istart] = -2.0;
     72:     if (i==1) u[i+Iend-2*Istart] = -1.0;
     73:     if (i==2) u[i+Iend-2*Istart] = -1.0;
     74:   }
     75:   PetscCall(MatDenseRestoreArray(C1,&u));
     76:   PetscCall(MatAssemblyBegin(C1,MAT_FINAL_ASSEMBLY));
     77:   PetscCall(MatAssemblyEnd(C1,MAT_FINAL_ASSEMBLY));
     78:   PetscCall(MatCreateLRC(NULL,C1,NULL,NULL,&C));
     79:   PetscCall(MatDestroy(&C1));

     81:   /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     82:                 Create the solver and set various options
     83:      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
     84:   /*
     85:      Create the matrix equation solver context
     86:   */
     87:   PetscCall(LMECreate(PETSC_COMM_WORLD,&lme));

     89:   /*
     90:      Set the type of equation
     91:   */
     92:   PetscCall(LMESetProblemType(lme,LME_LYAPUNOV));

     94:   /*
     95:      Set the matrix coefficients, the right-hand side, and the solution.
     96:      In this case, it is a Lyapunov equation A*X+X*A'=-C where both
     97:      C and X are symmetric and low-rank, C=C1*C1', X=X1*X1'
     98:   */
     99:   PetscCall(LMESetCoefficients(lme,A,NULL,NULL,NULL));
    100:   PetscCall(LMESetRHS(lme,C));

    102:   if (rank) {  /* Create X only if the user has specified a nonzero value of rank */
    103:     PetscCall(PetscPrintf(PETSC_COMM_WORLD," Computing a solution with prescribed rank=%" PetscInt_FMT "\n",rank));
    104:     PetscCall(MatCreate(PETSC_COMM_WORLD,&X1));
    105:     PetscCall(MatSetSizes(X1,PETSC_DECIDE,PETSC_DECIDE,N,rank));
    106:     PetscCall(MatSetType(X1,MATDENSE));
    107:     PetscCall(MatAssemblyBegin(X1,MAT_FINAL_ASSEMBLY));
    108:     PetscCall(MatAssemblyEnd(X1,MAT_FINAL_ASSEMBLY));
    109:     PetscCall(MatCreateLRC(NULL,X1,NULL,NULL,&X));
    110:     PetscCall(MatDestroy(&X1));
    111:     PetscCall(LMESetSolution(lme,X));
    112:     PetscCall(MatDestroy(&X));
    113:   }

    115:   /*
    116:      (Optional) Set other solver options
    117:   */
    118:   PetscCall(LMESetTolerances(lme,1e-07,PETSC_CURRENT));

    120:   /*
    121:      Set solver parameters at runtime
    122:   */
    123:   PetscCall(LMESetFromOptions(lme));

    125:   /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    126:                    Solve the matrix equation, A*X+X*A'=-C
    127:      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

    129:   PetscCall(LMESolve(lme));
    130:   PetscCall(LMEGetConvergedReason(lme,&reason));
    131:   PetscCheck(reason>=0,PETSC_COMM_WORLD,PETSC_ERR_CONV_FAILED,"Solver did not converge");

    133:   if (!rank) {  /* X1 was created by the solver, so extract it and see how many columns it has */
    134:     PetscCall(LMEGetSolution(lme,&X));
    135:     PetscCall(MatLRCGetMats(X,NULL,&X1,NULL,NULL));
    136:     PetscCall(MatGetSize(X1,NULL,&rank));
    137:     PetscCall(PetscPrintf(PETSC_COMM_WORLD," The solver has computed a solution with rank=%" PetscInt_FMT "\n",rank));
    138:   }

    140:   /*
    141:      Optional: Get some information from the solver and display it
    142:   */
    143:   PetscCall(LMEGetIterationNumber(lme,&its));
    144:   PetscCall(PetscPrintf(PETSC_COMM_WORLD," Number of iterations of the method: %" PetscInt_FMT "\n",its));
    145:   PetscCall(LMEGetDimensions(lme,&ncv));
    146:   PetscCall(PetscPrintf(PETSC_COMM_WORLD," Subspace dimension: %" PetscInt_FMT "\n",ncv));
    147:   PetscCall(LMEGetTolerances(lme,&tol,&maxit));
    148:   PetscCall(PetscPrintf(PETSC_COMM_WORLD," Stopping condition: tol=%.4g, maxit=%" PetscInt_FMT "\n",(double)tol,maxit));

    150:   /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    151:                         Compute residual error
    152:      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

    154:   PetscCall(LMEGetErrorEstimate(lme,&errest));
    155:   PetscCall(PetscPrintf(PETSC_COMM_WORLD," Error estimate reported by the solver: %.4g\n",(double)errest));
    156:   if (n<=150) {
    157:     PetscCall(LMEComputeError(lme,&error));
    158:     PetscCall(PetscPrintf(PETSC_COMM_WORLD," Computed residual norm: %.4g\n\n",(double)error));
    159:   } else PetscCall(PetscPrintf(PETSC_COMM_WORLD," Matrix too large to compute residual norm\n\n"));

    161:   /*
    162:      Free work space
    163:   */
    164:   PetscCall(LMEDestroy(&lme));
    165:   PetscCall(MatDestroy(&A));
    166:   PetscCall(MatDestroy(&C));
    167:   PetscCall(SlepcFinalize());
    168:   return 0;
    169: }

    171: /*TEST

    173:    test:
    174:       suffix: 1
    175:       requires: !single

    177:    test:
    178:       suffix: 2
    179:       args: -rank 40
    180:       requires: !single

    182: TEST*/