public static EVD eigen(Matrix A, int k, Ritz ritz) {
        return eigen(A, k, ritz, 1E-8, 10 * A.nrows());
    }
public static EVD eigen ( Matrix A , int k , Ritz ritz ) { return eigen ( A , k , ritz , 1E-8 , 10 * A . nrows ( ) ) ; }
Find k approximate eigen pairs of a symmetric matrix by the
Lanczos algorithm.

@param k Number of eigenvalues of OP to be computed. 0 < k < N.
@param ritz Specify which of the Ritz values to compute.