public static EVD eigen(Matrix A, int k, Ritz ritz, double kappa, int maxIter) {
        return eigen(A, k, ritz.name(), kappa, maxIter);
    }
public static EVD eigen ( Matrix A , int k , Ritz ritz , double kappa , int maxIter ) { return eigen ( A , k , ritz . name ( ) , kappa , maxIter ) ; }
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.
@param kappa Relative accuracy of ritz values acceptable as eigenvalues.
@param maxIter Maximum number of iterations.