Let $X$ be a D17: Finite set such that

(i)	\begin{equation} |X| = N \in \mathbb{N} \end{equation}
(ii)	$\text{Part} X$ be the D2910: Set of proper set partitions for $X$

Then \begin{equation} | \text{Part} X | = \sum_{n = 0}^N {N \brace n} \end{equation}