29 Numerics library [numerics]

29.9 Mathematical functions for floating-point types [c.math]

29.9.5 Mathematical special functions [sf.cmath] Beta function [sf.cmath.beta]

double beta(double x, double y); float betaf(float x, float y); long double betal(long double x, long double y);
Effects: These functions compute the beta function of their respective arguments x and y.

where x is x and y is y.