Let be integers with and a_ m+n =a_ m +a_ n +m n for all positive integers m

MATHEMATICS · Subjective · AoPS · Mathematics · Functions · Functions

Problem

Let

\left\{a_{n}\right\}

be integers with

a_{1}=1

and

a_{m+n}=a_{m}+a_{n}+m n \quad \text { for all positive integers } m, n .

Compute

a_{12}