Pyramidal number
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Wikipedia : http://en.wikipedia.org/wiki/Pyramidal_numberA 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:
- Triangular pyramidal number (three sides)
- Square pyramidal number (four sides)
- Pentagonal pyramidal number (five sides)
- Hexagonal pyramidal number (six sides)
- Heptagonal pyramidal number (seven sides)
as well as to pyramids with higher numbers of sides ^{[1]}
The formula for an r-gonal pyramidal number is:
with r ∈ ℕ, r ≥ 3.
This formula can be factorized as follows:
References
- ^ Sloane, N.J.A. (ed.). "Sequence A002414". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
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