A pyramidal number is a figurate number that represents a pyramid with a polygonal base and a given number of triangular sides. The term usually refers to square pyramidal numbers, which have a square base with four sides, but it can also refer to:
as well as to pyramids with higher numbers of sides
The formula for an r-gonal pyramidal number is:
with r ∈ ℕ, r ≥ 3.
This formula can be factorized as follows:
References
Pyramidal number Wikipedia(Text) CC BY-SA