Questions on the factorial function, $n!=n\cdot(n-1)\cdot...\cdot1$. Consider using the tag (gamma-function) if dealing with noninteger arguments.

The factorial is defined as the product of all positive integers less than or equal to some integer $n$, written $n!$. Multiple $!$'s skip integers, so for example $10!!!=10\cdot7\cdot4\cdot1$.

This function is only defined over integers, but the gamma-function extends it to all complex numbers that are not non-positive integers.