Question 11
(GBR) IMO2. Let $a_1,a_2,a_3,\ldots$ be any infinite increasing sequence of positive integers. (For every integer $i\gt 0$, $a_{i+1}\gt a_i$.) Prove that there are infinitely many $m$ for which positive integers $x,y,h,k$ can be found such that $0\lt h\lt k\lt m$ and $a_m=xa_h+ya_k$.