Definition:Heptagonal Number/Definition 2

Definition
Heptagonal numbers are defined as:
 * $\ds H_n = \sum_{i \mathop = 1}^n \paren {5 i - 4} = 1 + 6 + \cdots + \paren {5 \paren {n - 1} - 4} + \paren {5 n - 4}$

for $n = 1, 2, 3, \ldots$

Also see

 * Equivalence of Definitions of Heptagonal Number