Definition:Triangular Number/Definition 2

Definition
Triangular numbers are defined as:
 * $\ds T_n = \sum_{i \mathop = 1}^n i = 1 + 2 + \cdots + \paren {n - 1} + n$

for $n = 1, 2, 3, \ldots$

Also see

 * Equivalence of Definitions of Triangular Number