fn gamma_kummer(n: f64, z: Complex) -> (Complex, f64)Expand description
The Kummer sum of DLMF 8.7.1 with 8.2.7,
Gamma(a, z) = Gamma(a) - z^a e^-z sum_k z^k / (a (a+1) ... (a+k)), whose terms fall
at any phase of z for 0 < |z| < a, with the magnitude of the terms as the
absolute error bound in units of epsilon.