pub fn binom_dk<F: Float>(n: F, k: F) -> FExpand description
∂C(n, k)/∂k, the binomial coefficient’s slope in k: C(n, k)(ψ(n - k + 1) - ψ(k + 1)). At a whole n from 0 below a whole k, it is the negative of binom_dn. Other poles of ψ, as at a negative whole k, give NaN. For a whole k from 0 to 64, its error is within 2k ulps of C(n, k) times ψ(n - k + 1) - ψ(k + 1), plus the error of that difference, so near n = 2k, where the two ψ cancel, its relative error has no bound.