code: public double normF () {
        double f = 0;
        for (int i = 0; i < m; i++) {
           for (int j = 0; j < n; j++) {
              f = Maths.hypot(f,A[i][j]);
           }
        }
        return f;
     }
docs: Frobenius norm
  @return    sqrt of sum of squares of all elements.