\begin{definition}[Definition:Heptagonal Number/Definition 2]
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 $n = 1, 2, 3, \ldots$
\end{definition}