Definition:Heptagonal Number/Definition 1

Definition
Heptagonal numbers are defined as:
 * $H_n = \begin{cases}

0 & : n = 0 \\ H_{n-1} + 5 n - 4 & : n > 0 \end{cases}$ for $n = 1, 2, 3, \ldots$

Also see

 * Equivalence of Definitions of Heptagonal Number