Online calculator which helps you calculate the tetrahedral number.
A tetrahedral number is a figure number that represents a pyramid, the base of which lies the triangle.
An example of the first few tetrahedral numbers:
1, 4, 10, 20, 35, 56, 84, 120, 165, 220, 286, 364, 455, 560, 680, 816, 969, ... (A000292 sequence in OEIS).