Definition:Pentagonal Number

Definition
Pentagonal numbers are those denumerating a collection of objects which can be arranged in the form of a regular pentagon.

They can be denoted:
 * $P_1, P_2, P_3, \ldots$

Also known as
Pentagonal numbers are also known as pentagon numbers.

Or we can just say that a number is pentagonal.

Also see

 * Closed Form for Pentagonal Number‎s: $P_n = \dfrac {n \left({3 n - 1}\right)} 2$