Triangular number

A triangular number is a number that can be arranged in the shape of an equilateral triangle. The sequence of triangular numbers for n = 1, 2, 3... is:  1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ... Since each row is one unit longer than the previous row it can be seen that a triangular number is the sum of consecutive integers.