# FactorialPower and Factorial

After some computation, I have obtained a function FactorialPower[1, n, -1]. Clearly, FactorialPower[1, n, -1] equals Factorial[n] for all integer values of n. Is there a way to get this clearer representation?

Surprisingly, FullSimplify[FactorialPower[1, n, -1]] returns ComplexInfinity while FullSimplify[Factorial[n]] returns n!.

The aforementioned function is one of the solutions of a recursive equation and it would be nice if I could print out just n! instead of FactorialPower[1, n, -1].

Since you are interested in integer n, give that information to FullSimplify.
FullSimplify[FactorialPower[1, n, -1], Assumptions -> {n ∈ Integers, n >= 1}]

This reduces to n! as you expect. The problem arises because FactorialPower accepts other than integer input.
• Why is the assumption n >= 1 better than the assumption n >= 0? A call FactorialPower[1, 0, -1] yields 1, which equals 0!. On the other hand, calling FullSimplify[FactorialPower[1, n, -1], Assumptions -> {n ∈ Integers, n >= 0}], I get ComplexInfinity. – Antoine Feb 2 '15 at 9:44