Zero factorial is an often debated topic in mathematics. Whether 0! ought to be 0 or 1. Many argue that 0! ought to be 0. I can't remember the argument for that. However, math teachers say that 0! must be 1 to make probability work out correctly. There is a Calculus proof that 0! is 1 floating out somewhere. Update: the Euler gamma function (n) is used, int(x^(n-1)*e^(-x),x,x=0..INFIN);, say gamma(n)=(n-1)!, gamma(1)=1=0!. BTW, the int(fnc,x,bounds) notation is used in Maple and probably Mathematica.