Definition:Triangular Number

Triangular numbers are those denumerating a collection of objects which can be arranged in the form of an equilateral triangle.

They are otherwise called triangle numbers.

Or we can just say that a number is triangular.

They are often denoted $$T_1, T_2, T_3, \ldots$$, and they are formally defined as:
 * $$T_n = \sum_{i=1}^n i = 1 + 2 + \cdots + \left({n-1}\right) + n$$.

Thus $$T_0 = 0$$.



The first triangular number: $$T_1 = 1$$.



The second triangular number: $$T_2 = 1 + 2 = 3$$.



The third triangular number: $$T_3 = 1 + 2 + 3 = 6$$.



The fourth triangular number: $$T_4 = 1 + 2 + 3 + 4 = 10$$.



The fifth triangular number: $$T_5 = 1 + 2 + 3 + 4 + 5 = 15$$.

Recurrence Formula
It can be seen directly from the above that:
 * $$T_n = \begin{cases}

0 & : n = 0 \\ n + T_{n-1} & : n > 0 \end{cases}$$

Closed Form
From Closed Form for Triangular Numbers, we have:
 * $$T_n = \frac {n \left({n + 1}\right)} 2$$