— Function: double gsl_sf_beta_inc (double a, double b, double x)
— Function: int gsl_sf_beta_inc_e (double a, double b, double x, gsl_sf_result * result)

These routines compute the normalized incomplete Beta function B_x(a,b)/B(a,b) where B_x(a,b) = \int_0^x t^{a-1} (1-t)^{b-1} dt for a > 0, b > 0, and 0 <= x <= 1.