Function dense_matvec 

pub fn dense_matvec(a: &SparseColMat<usize, f64>, x: &Col<f64>) -> Col<f64>

Compute dense matrix-vector product A * x.

Converts A to dense and uses BLAS-3 matmul. Intended for validation of small-to-medium matrices in tests, not for production solves.

Future optimization

For large matrices, use faer::sparse::linalg::matmul::sparse_dense_matmul to avoid the O(n^2) dense conversion.