Definition:Heptagonal Number/Definition 2

Definition
Heptagonal numbers are defined as:
 * $\displaystyle H_n = \sum_{i \mathop = 1}^n \left({5 \left({i - 1}\right) + 1}\right) = 1 + 6 + \cdots + \left({5 \left({n - 2}\right) + 1}\right) + \left({5 \left({n - 1}\right) + 1}\right)$

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

Also see

 * Equivalence of Definitions of Heptagonal Number