= Triangular Numbers =

Triangular numbers are the number of objects arranged in an equilateral
triangle.

The first few are

{{{
0, 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171,
190, 210, 231, 253, 276, 300, 325, 351, 378, 406, 435, 465, 496, 528, 561, 595,
630, 666...
  }}}