I know that n! for 6 is 6*5*4*3*2*1 but what is 6.1! ? My calculator says 868.957. How do they come up with this? I'm trying to write a computer program that mimics the calculator program that comes with windows. I know that using my above formula that I have to make an exception for 0!=1, and Invalid Input for Function for a negative number. Anything else I should know about n! ??