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