Hi Everyone,

Let's say I have a sparse matrix M and a dense vector v. The following works:

VectorXd p = M*v;

If I have a a sparse matrix M and a sparse matrix G, the following fails:

MatrixXd Q = G.transpose()*M*G;

The following, however, works:

SparseMatrix<double> Qsparse = G.transpose()*M*G;
MatrixXd Qdense = Qsparse;


Any thoughts as to what I'm doing incorrectly?

Thanks!

hi, the product of a sparse and dense matrix is dense while the product of two sparse matrices is unlikely to be dense, that's why it is implemented to be evaluated into a sparse matrix.