Tetrahedral number

[Merna]

The {n}th [tetrahedral number] is the sum of the first n [Triangular] numbers. Triangular numbers are the sum of the first n integers (1, 3, 6, 10, 15, etc.), and taking the sum of the [triangular] numbers results in the [tetrahedral] numbers (1, 4, 10, 20, 35, etc.)