//	Gamma(-Inf) = NaN
//	Gamma(NaN) = NaN
func Gamma(x float64) float64 {
	const Euler = 0.57721566490153286060651209008240243104215933593992 // A001620
	// special cases
	switch {
	case isNegInt(x) || IsInf(x, -1) || IsNaN(x):
		return NaN()
	case IsInf(x, 1):
		return Inf(1)
	case x == 0:
		if Signbit(x) {
			return Inf(-1)
		}
		return Inf(1)
	}
	q := Abs(x)
	p := Floor(q)