A pyramid with side length 5 contains 35 spheres. Each layer represents one of the first five triangular numbers.
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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,
The tetrahedral numbers are:
- 1, 4, 10, 20, 35, 56, 84, 120, 165, 220, ... (sequence A000292 in the OEIS)
The formula for the nth tetrahedral number is represented by the 3rd rising factorial of n divided by the factorial of 3:
The tetrahedral numbers can also be represented as binomial coefficients:
Tetrahedral numbers can therefore be found in the fourth position either from left or right in Pascal's triangle.
Proofs of formula
This proof uses the fact that the nth triangular number is given by
It proceeds by induction.
- Base case
- Inductive step
The formula can also be proved by Gosper's algorithm.
Tetrahedral numbers can be modelled by stacking spheres. For example, the fifth tetrahedral number (Te5 = 35) can be modelled with 35 billiard balls and the standard triangular billiards ball frame that holds 15 balls in place. Then 10 more balls are stacked on top of those, then another 6, then another three and one ball at the top completes the tetrahedron.
When order-n tetrahedra built from Ten spheres are used as a unit, it can be shown that a space tiling with such units can achieve a densest sphere packing as long as n ≤ 4.
Tetrahedral roots and tests for tetrahedral numbers
By analogy with the cube root of x, one can define the (real) tetrahedral root of x as the number n such that Ten = x:
which follows from Cardano's formula. Equivalently, if the real tetrahedral root n of x is an integer, then x is the nth tetrahedral number.
Number of gifts of each type and number received each day and their relationship to figurate numbers
Te12 = 364 is the total number of gifts "my true love sent to me" during the course of all 12 verses of the carol, "The Twelve Days of Christmas". The cumulative total number of gifts after each verse is also Ten for verse n.
The number of possible KeyForge three-house combinations is also a tetrahedral number, Ten−2 where n is the number of houses.