Function digamma 

pub fn digamma<F>(x: F) -> F
where
    F: Float,

ψ(x), the digamma function: the derivative of ln Γ(x), Γ′(x)/Γ(x). ψ(+0) is -∞, ψ(-0) is ∞, a negative whole x and -∞ give NaN, and ψ(∞) is ∞. Near its zeros its error is within 4 ulps of 1, not of the result, and below -20 within 4 ulps of ln(1 - x).