Triangular number

A triangular number is a number that can be arranged in the shape of a triangle (by convention, the first triangular number is 1):

1:

l

3:

l
ll

6:

l
ll
lll

10:

l
ll
lll
llll

15:

l
ll
lll
llll
lllll

The formula for the nth triangular number is therefore (1+2+3+...+n-2+n-1+n). One of the most famous of all triangular numbers is 666 (the 36th triangular number).

See also square_number, polygonal_number.