public static string ComplexArgument(ComplexNum a) {
      Fraction pi = new Fraction(22, 7);
      Fraction minusPi = new Fraction(-22, 7);
      Fraction fi = new Fraction(1, 1);
      var str = "";
      if (a.r.CompareTo(0) > 0) {
        fi = a.i / a.r;
      }
      if (a.r.CompareTo(0) < 0 && a.i.CompareTo(0) > 0) {
        fi = a.i / a.r;
        str = pi.ToString() + "ARCTG" + fi.ToString();
      }
      if (a.r.CompareTo(0) < 0 && a.i.CompareTo(0) < 0) {
        fi = a.i / a.r;
        str = minusPi.ToString() + "ARCTG" + fi.ToString();
      }
      return str;
    }