Function rgsl::exponential::exprel_n

pub fn exprel_n(n: i32, x: f64) -> f64

This routine computes the N-relative exponential, which is the n-th generalization of the functions gsl_sf_exprel and gsl_sf_exprel_2. The N-relative exponential is given by:

exprel_N(x) = N!/x^N (\exp(x) - \sum_{k=0}^{N-1} x^k/k!)

            = 1 + x/(N+1) + x^2/((N+1)(N+2)) + ...

            = 1F1 (1,1+N,x)