After I had a conversation with a friend on a very long bus ride about inverse factorial a stork came and put this page on the web.

Before we look at inverse factorial lets look at factorial: ~The most commonly used definition of
factorial is only defined for non negative integers. This definition of
factorial is: Note: 0!=1 ~However as the above definition is
not defined for non integers or negative values I will use: where gamma is defined as: Or in words: the area under the curve of
with
values of t ranging from 0 to infinity. ~For more on the gamma function go to: ~Looking at Inverse factorial: ~Inverse factorial is just the inverse function of factorial ~I would use Γ ~Now Lets look at what the factorial function looks like: ~ x {0,3} ~ x {0,10}
~And my personal favoritex: {0,100} ~And here are the graphs of Inverse Factorial: ~ y {0, 3}
~ y {0, 10}
~ y {0, 150} ~A note: be careful inverse factorial is not a function so there are multiple values for any one x (unless you limit the domain to x>/=1) |

.: Enjoy :.