\begin{definition}[Definition:Hexagonal Pyramidal Number]
'''Hexagonal pyramidal numbers''' are those denumerating a collection of objects which can be arranged in the form of a hexagonal pyramid.
The $n$th '''hexagonal pyramidal number''' $P_n$ is defined as:
:$\ds P_n = \sum_{k \mathop = 1}^n H_k$
where $H_k$ is the $k$th hexagonal number.
\end{definition}