Gamma function

GammaFunction

double Gamma(double x)
{
          
    if (x < 0)
    {
        return double.PositiveInfinity;
    }

    // Gamma(x)  = (x-1)! so add 1, to make user gets 
    // expected result, I really don't want use increamental operator here.
    x += 1;

    double p1 = Math.Sqrt(2 * Math.PI / x);
    double p2 = x + 1.0 / (12 * x - 1.0 / (10 * x));
    p2 = Math.Pow(p2 / Math.E, x);

    return Math.Round(p1 * p2);
}