Triangular number

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The triangular numbers are the numbers $\displaystyle T_n$ which are the sum of the first $\displaystyle n$ natural numbers from $\displaystyle 1$ to $\displaystyle n$.

Using the sum of an arithmetic series formula, a formula can be calculated for $\displaystyle T_n$:

$T_n = \displaystyle\sum_{i=1}n = 1 + 2 \cdots n = \frac{n(n+1)}2$

The rather simple recursive definition can be easily found by noting that $\displaystyle T_{n} = 1 + 2 \cdots (n-1) + n = (1 + 2 \cdots n-1) + n = T_{n-1} + n$.

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