If you use Frobenius norm, I think you forgot to take a square root of sum of all squared elements of matrix.

[Biunovich]

I think here should be return std::sqrt(r2);

lambda-lanczos/include/lambda_lanczos/lambda_lanczos.hpp

Line 310 in 0858d40

return r2;

[mrcdr]

Thank you for your comment and your indication is right.
The routine may cause trouble in the situation r2 < 1. I've forgotten that!
I'll fix it to use sqrt or 1-norm.
Thank you again.

[mrcdr]

The issue fixed at aa3d260.