Function ln_gamma 

pub fn ln_gamma(x: f64) -> f64

Logarithm Gamma function

Description

Use Lanczos approximation to implement Gamma function ($g=5, n=7$)

References

• Robert Munafo, Coefficients for the Lanczos Approximation to the Gamma Function
• Paul Godfrey, A note on the computation of the convergent Lanczos complex Gamma approximation (web page), 2001.