Accord.Math.Gamma.Function C# (CSharp) Method

        public static double Function(double x)
        {
            double p, z;

            double q = System.Math.Abs(x);

            if (q > 33.0)
            {
                if (x < 0.0)
                {
                    p = System.Math.Floor(q);

                    if (p == q)
                        throw new OverflowException();

                    z = q - p;
                    if (z > 0.5)
                    {
                        p += 1.0;
                        z = q - p;
                    }
                    z = q * System.Math.Sin(System.Math.PI * z);

                    if (z == 0.0)
                        throw new OverflowException();

                    z = System.Math.Abs(z);
                    z = System.Math.PI / (z * Stirling(q));

                    return -z;
                }
                else
                {
                    return Stirling(x);
                }
            }

            z = 1.0;
            while (x >= 3.0)
            {
                x -= 1.0;
                z *= x;
            }

            while (x < 0.0)
            {
                if (x == 0.0)
                {
                    throw new ArithmeticException();
                }
                else if (x > -1.0E-9)
                {
                    return (z / ((1.0 + 0.5772156649015329 * x) * x));
                }
                z /= x;
                x += 1.0;
            }

            while (x < 2.0)
            {
                if (x == 0.0)
                {
                    throw new ArithmeticException();
                }
                else if (x < 1.0E-9)
                {
                    return (z / ((1.0 + 0.5772156649015329 * x) * x));
                }

                z /= x;
                x += 1.0;
            }

            if ((x == 2.0) || (x == 3.0))
                return z;

            x -= 2.0;
            p = Special.Polevl(x, gamma_P, 6);
            q = Special.Polevl(x, gamma_Q, 7);
            return z * p / q;

        }

        /// <summary>
        ///   Multinomial Beta function.
        /// </summary>
        ///
        /// <example>
        ///   Please see <see cref="Beta"/>
        /// </example>
        ///
        public static double Multinomial(params double[] x)
        {
            double sum = 0;
            double prd = 1;

            for (int i = 0; i < x.Length; i++)
            {
                sum += x[i];
                prd *= Gamma.Function(x[i]);
            }

            return(prd / Gamma.Function(sum));
        }