tetrahedral number
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Encyclopedic overview
A pyramid with side length 5 contains 35 spheres. Each layer represents one of the first five triangular numbers. A tetrahedral number, or triangular pyramidal number, is a figurate number that represents a pyramid with a triangular base and three sides, called a tetrahedron. The nth tetrahedral number, Ten, is the sum of the first n triangular numbers, that is,
T
Excerpted from Wikipedia’s “tetrahedral number” article, available under the CC BY-SA 4.0 licence.