private static Complex Muavr(Complex A) {
      double fi_cos = Math.Acos(A.Real / Complex.Abs(A));
      double fi_sin = Math.Asin(A.Imaginary / Complex.Abs(A));

      Complex FirtsRoot = new Complex(Math.Pow(Complex.Abs(A), 1.0 / 3.0) * Math.Cos(1.0 / 3.0 * fi_cos),
                                      Math.Pow(Complex.Abs(A), 1.0 / 3.0) * Math.Sin(1.0 / 3.0 * fi_sin));
      return FirtsRoot;
    }