code: public static EVD eigen(Matrix A, int k, String which) {
          return eigen(A, k, which, 1E-8, 10 * A.nrows());
      }
docs: 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 which Specify which of the Ritz values to compute.