Date: 2026-03-31T05:46:13+00:00

Actual source code: cgeig.c

      1: /*
      2:    Code for calculating extreme eigenvalues via the Lanczos method
      3:    running with CG. Note this only works for symmetric real and Hermitian
      4:    matrices (not complex matrices that are symmetric).
      5: */
      6: #include <../src/ksp/ksp/impls/cg/cgimpl.h>
      7: #include <../include/petscblaslapack.h>

      9: PetscErrorCode KSPComputeEigenvalues_CG(KSP ksp, PetscInt nmax, PetscReal *r, PetscReal *c, PetscInt *neig)
     10: {
     11:   KSP_CG      *cgP = (KSP_CG *)ksp->data;
     12:   PetscScalar *d, *e;
     13:   PetscReal   *ee;
     14:   PetscInt     n = ksp->its;
     15:   PetscBLASInt bn, lierr = 0, ldz = 1;

     17:   PetscFunctionBegin;
     18:   PetscCheck(nmax >= n, PetscObjectComm((PetscObject)ksp), PETSC_ERR_ARG_SIZ, "Not enough room in work space r and c for eigenvalues");
     19:   *neig = n;

     21:   PetscCall(PetscArrayzero(c, nmax));
     22:   if (!n) PetscFunctionReturn(PETSC_SUCCESS);
     23:   d  = cgP->d;
     24:   e  = cgP->e;
     25:   ee = cgP->ee;

     27:   /* copy tridiagonal matrix to work space */
     28:   for (PetscInt j = 0; j < n; j++) {
     29:     r[j]  = PetscRealPart(d[j]);
     30:     ee[j] = PetscRealPart(e[j + 1]);
     31:   }

     33:   PetscCall(PetscBLASIntCast(n, &bn));
     34:   PetscCall(PetscFPTrapPush(PETSC_FP_TRAP_OFF));
     35:   PetscCallBLAS("LAPACKREALstev", LAPACKREALstev_("N", &bn, r, ee, NULL, &ldz, NULL, &lierr));
     36:   PetscCheck(!lierr, PETSC_COMM_SELF, PETSC_ERR_PLIB, "xSTEV error");
     37:   PetscCall(PetscFPTrapPop());
     38:   PetscCall(PetscSortReal(n, r));
     39:   PetscFunctionReturn(PETSC_SUCCESS);
     40: }

     42: PetscErrorCode KSPComputeExtremeSingularValues_CG(KSP ksp, PetscReal *emax, PetscReal *emin)
     43: {
     44:   KSP_CG      *cgP = (KSP_CG *)ksp->data;
     45:   PetscScalar *d, *e;
     46:   PetscReal   *dd, *ee;
     47:   PetscInt     n = ksp->its;
     48:   PetscBLASInt bn, lierr = 0, ldz = 1;

     50:   PetscFunctionBegin;
     51:   if (!n) {
     52:     *emax = *emin = 1.0;
     53:     PetscFunctionReturn(PETSC_SUCCESS);
     54:   }
     55:   d  = cgP->d;
     56:   e  = cgP->e;
     57:   dd = cgP->dd;
     58:   ee = cgP->ee;

     60:   /* copy tridiagonal matrix to work space */
     61:   for (PetscInt j = 0; j < n; j++) {
     62:     dd[j] = PetscRealPart(d[j]);
     63:     ee[j] = PetscRealPart(e[j + 1]);
     64:   }

     66:   PetscCall(PetscBLASIntCast(n, &bn));
     67:   PetscCall(PetscFPTrapPush(PETSC_FP_TRAP_OFF));
     68:   PetscCallBLAS("LAPACKREALstev", LAPACKREALstev_("N", &bn, dd, ee, NULL, &ldz, NULL, &lierr));
     69:   PetscCheck(!lierr, PETSC_COMM_SELF, PETSC_ERR_PLIB, "xSTEV error");
     70:   PetscCall(PetscFPTrapPop());
     71:   *emin = dd[0];
     72:   *emax = dd[n - 1];
     73:   PetscFunctionReturn(PETSC_SUCCESS);
     74: }