static double[][] plusMatrix(double[][] first, double[][] second)
        {
            //императивный вариант
            int n = first.Length;
            double[][] R = new double[n][];
            for (int i = 0; i < n; i++)
            {
                R[i] = new double[n];
                for (int j = 0; j < n; j++)
                    R[i][j] = first[i][j] + second[i][j];
            }
            return R;
            //функциональный вариант
            return first.Zip(second, (a, b) => a.Zip(b, (x, y) => x + y).ToArray()).ToArray();
        }