Exception when trying to compute eigenpairs of Identity matrix

[jkflying]

SymEigsSolver<double, LARGEST_ALGE, SparseSymMatProd<double, Eigen::Lower>>

is throwing an exception when computing eigenpairs of an Identity matrix, with the message

TridiagEigen: eigen decomposition failed.

[yixuan]

Thanks. Could you also post a concrete example to reproduce this so that it is easier for me to debug?

[jkflying]

Sure, here is an example:

#include <spectra/SymEigsSolver.h>
#include <spectra/MatOp/SparseSymMatProd.h>

void findEigsOfIdentity()
{
    using namespace Eigen;
    Index N = 20;
    SparseMatrix<double> mat(N, N);
    mat.setIdentity();

    using namespace Spectra;
    SparseSymMatProd<double, Lower> op(mat);
    SymEigsSolver<double, LARGEST_ALGE, SparseSymMatProd<double, Lower>> eigs(&op, N-1, N-1);
    eigs.init();
    eigs.compute(); // this throws exception
}

[yixuan]

Hi @jkflying , ncv needs to be larger than nev, so if you change the code to eigs(&op, N - 1, N) it works fine. This is one of the reasons why Spectra does not compute all the eigenpairs (#22).