Function debye_d1 

pub fn debye_d1(x: f64) -> f64

Debye function of order 1, D1(x) = (1/x) int_0^x t/(e^t - 1) dt, for x > 0. D1(0) = 1, D1(x) -> pi^2/(6x) as x -> infinity.

For x <= 3 the integrand’s Taylor series t/(e^t-1) = sum b_n t^n (radius of convergence 2 pi) is integrated term by term; the b_n are generated from (t/(e^t-1)) * ((e^t-1)/t) = 1. For larger x the integral is pi^2/6 - int_x^inf, and the tail is the convergent series sum_k e^{-kx} (x/k + 1/k^2).