A pyramidal number is the number of points in a pyramid with a polygonal base and triangular sides. 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.[1] The numbers of points in the base and in layers parallel to the base are given by polygonal numbers of the given number of sides, while the numbers of points in each triangular side is given by a triangular number. It is possible to extend the pyramidal numbers to higher dimensions.
The formula for the th -gonal pyramidal number is
| r= | |
| P | |
| n |
| 3n2+n3(r-2)-n(r-5) | |
| 6 |
,
This formula can be factored:
| |||||
| P | =\left( | ||||
| n |
| n(n+1) | \right)\left( | |
| 2 |
| n(r-2)-(r-5) | |
| 3 |
\right)=Tn ⋅
| n(r-2)-(r-5) | |
| 3 |
,
where is the th triangular number.
The first few triangular pyramidal numbers (equivalently, tetrahedral numbers) are:
1, 4, 10, 20, 35, 56, 84, 120, 165, 220, ...
The first few square pyramidal numbers are:
1, 5, 14, 30, 55, 91, 140, 204, 285, 385, 506, 650, 819, ... .
The first few pentagonal pyramidal numbers are:
1, 6, 18, 40, 75, 126, 196, 288, 405, 550, 726, 936, 1183, 1470, 1800, 2176, 2601, 3078, 3610, 4200, 4851, 5566, 6348, 7200, 8125, 9126 .
The first few hexagonal pyramidal numbers are:
,,,,,,, 372, 525, 715, 946, 1222, 1547, 1925 .
The first few heptagonal pyramidal numbers are:[2]
1, 8, 26, 60, 115, 196, 308, 456, 645, 880, 1166, 1508, 1911, ...