Definition:Bell Number

Definition
The Bell Numbers $B_n$ are a sequence of natural numbers defined as the number of ways a set with $n$ elements can be partitioned.

The values of the first Bell numbers are:


 * $(1), 1, 2, 5, 15, 52, 203, 877, 4140, 21 \, 147, \ldots$

where the first $(1)$ is the zeroth Bell number $B_0$.