Sum over k of Unsigned Stirling Numbers of First Kind by x^k

Theorem

 * $\ds \sum_k {n \brack k} x^k = x^{\overline n}$

where:
 * $\ds {n \brack k}$ denotes an unsigned Stirling number of the first kind
 * $x^{\overline n}$ denotes $x$ to the $n$ rising.