B(a, b) = (0, 1) ua - 1(1 - u)b-1 du.
B(a, b) = gam(a) gam(b) / gam(a + b).
B(j, k) = (j - 1)!(k - 1)! / (j + k -1)!
f(u) = ua - 1 (1 - u)b - 1 / B(a, b), 0 < u < 1
Fp(k - 1) = Gk(1 - p)
E(Uk) = B(a + k, b) / B(a, b).
U = (m / n)X / [1 + (m / n)X]