Definition:Pentagonal Number/Definition 3

Definition
Pentagonal numbers are defined as the sequence:
 * $\forall n \in \N: P_n = P \left({5, n}\right) = \begin{cases}

0 & : n = 0 \\ P \left({5, n - 1}\right) + 3 \left({n - 1}\right) + 1 & : n > 0 \end{cases}$ where $P \left({k, n}\right)$ denotes the $k$-gonal numbers.

Also see

 * Equivalence of Definitions of Pentagonal Number