TITLE: Factorial of a non-integer number
QUESTION [13 upvotes]: My TI-83 calculator doesnt allow me to do this, but using Windows calculator, I can compute the factorial of say 5.8. What does this mean and how does it work?

REPLY [19 votes]: Perhaps you're looking for the Gamma function. It restricts to the usual factorial on natural numbers.
Wikipedia's article on Gamma function