Definition:Square Pyramidal Number

Definition
Square pyramidal numbers are those denumerating a collection of objects which can be arranged in the form of a square pyramid.

The $n$th square pyramidal number $P_n$ is defined as:
 * $\displaystyle P_n = \sum_{k \mathop = 1}^n S_k$

where $S_k$ is the $k$th square number.