Function upper_gamma 

pub(crate) fn upper_gamma(n: f64, z: Complex) -> Complex

The upper incomplete gamma function Gamma(n, z) of integer or half-integer n.

It takes the continued fraction where Re z > 4 and |z| > |n| + 2; the power series or the continued fraction where a downward recurrence would lose accuracy; otherwise the degree recurrence from E1 or erfc, or the Kummer sum or the power series where their error bounds are smaller.