y = beta(z,w).
y = integral from 0 to 1 of t.^(z-1) .* (1-t).^(w-1) dt.
z and w must be compatible for addition.
Incomplete beta function.
y = beta(x,a,b).
The elements of x must be in the interval [0,1].
a and b must be scalars.
See also BETAINC , BETALN .