pub fn betai(a: f64, b: f64, x: f64) -> f64
Regularized incomplete beta I_x(a, b), 0 ≤ x ≤ 1, a,b > 0. Continued fraction (Lentz) with the symmetry switch for fast convergence.
I_x(a, b)
0 ≤ x ≤ 1
a,b > 0