Pentatope number

Derivation of pentatope numbers from a left-justified Pascal's triangle.
  Natural numbers
  Triangular numbers
  Tetrahedral numbers
  Pentatope numbers
  5-simplex numbers
  6-simplex numbers
  7-simplex numbers

In number theory, a pentatope number is a number in the fifth cell of any row of Pascal's triangle starting with the 5-term row 1 4 6 4 1, either from left to right or from right to left. It is named because it represents the number of 3-dimensional unit spheres which can be packed into a pentatope (a 4-dimensional tetrahedron) of increasing side lengths.

The first few numbers of this kind are:

1, 5, 15, 35, 70, 126, 210, 330, 495, 715, 1001, 1365 (sequence A000332 in the OEIS)
A pentatope with side length 5 contains 70 3-spheres. Each layer represents one of the first five tetrahedral numbers. For example, the bottom (green) layer has 35 spheres in total.

Pentatope numbers belong to the class of figurate numbers, which can be represented as regular, discrete geometric patterns.[1]

  1. ^ Deza, Elena; Deza, M. (2012), "3.1 Pentatope numbers and their multidimensional analogues", Figurate Numbers, World Scientific, p. 162, ISBN 9789814355483