# Resolvent operator with FF/PETSc (MatMatSolve?)

**URL:** <https://community.freefem.org/t/resolvent-operator-with-ff-petsc-matmatsolve/1603>\
**Category:** General Discussion\
**Created:** [March 16, 2022, 4:05pm UTC](https://community.freefem.org/t/resolvent-operator-with-ff-petsc-matmatsolve/1603 "2022-03-16T16:05:45Z")\
**Posts on this page:** 1\
**Showing post:** 13

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**Author:** ![cmd](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/cmd/32/67_2.png) [@cmd](https://community.freefem.org/u/cmd)\
**Post date:** [December 16, 2023, 12:07am UTC](https://community.freefem.org/t/resolvent-operator-with-ff-petsc-matmatsolve/1603/13 "2023-12-16T00:07:25Z")

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Hi Tea,

I will let you know when the repo goes public. For now, the relevant portion of the code is below.

Cheers!

```auto
// construct matrices
M = vMq(XMh, XMh); // Response Norm
Mf = vMf(Xh, Xh); // Forcing Norm
matrix<complex> LocPQ = vP(Xh, XMh); // Forcing/Response Correspondence
Mat<complex> PQ(M, Mf, LocPQ);

func complex[int] LHSop(complex[int]& inPETSc) {
  complex[int] temp(XMh.ndof), outPETSc(inPETSc.n);
  MatMult(PQ, inPETSc, outPETSc);
  KSPSolve(J, outPETSc, temp);
  MatMult(M, temp, outPETSc);
  KSPSolveHermitianTranspose(J, outPETSc, temp);
  MatMultHermitianTranspose(PQ, temp, outPETSc);
  return outPETSc;
}

Mat<complex> LHS(Mf, LHSop);

J = vJ(XMh, XMh, tgv = -1); //Linear operator
set(J, sparams = KSPparams);
int k = EPSSolve(LHS, Mf, vectors = fvec, values = val,
                 sparams = "-eps_type krylovschur -eps_largest_real -eps_monitor_conv -options_left no -eps_gen_hermitian");

```

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