fn kambe_series<const K: usize>(
orders: [i32; K],
z: Complex,
eta: Complex,
) -> [(Complex, f64); K]Expand description
Kambe integrals by expanding exp(1/(2t^2)): with a = (z eta)^2 / 2,
c = 1 / (2 eta^2) and R(d) = Gamma(d, a) / a^d (-1/d as a -> 0),
I_n = eta^(n+1)/2 sum_j c^j/j! R((n+1)/2 - j), whose terms cancel by up to
e^(|c| - Re c) for complex eta. Orders of one parity share one ladder of R, each
with the magnitude of its terms as its error bound. Even orders take the branch
sqrt(a) = z eta / sqrt 2 with Re z >= 0.