\begin{definition}[Definition:Hexagonal Number/Sequence]
The sequence of hexagonal numbers, for $n \in \Z_{\ge 0}$, begins:
:$0, 1, 6, 15, 28, 45, 66, 91, 120, 153, 190, 231, \ldots$
{{OEIS|A000384}}
\end{definition}