Pyramidal number

A pyramidal number is the number of points in a pyramid with a polygonal base and triangular sides.

[1] The term often refers to square pyramidal numbers, which have a square base with four sides, but it can also refer to a pyramid with any number of sides.

It is possible to extend the pyramidal numbers to higher dimensions.

The formula for the nth r-gonal pyramidal number is where r ∈

[1] This formula can be factored: where Tn is the nth triangular number.