Problem

Source: Rioplatense Olympiad 2014 level 3 P4

Tags: number theory, Divisors, sets of integers



A pair (a,b) of positive integers is Rioplatense if it is true that $b + k$ is a multiple of $a + k$ for all $k \in\{ 0 , 1 , 2 , 3 , 4 \}$. Prove that there is an infinite set $A$ of positive integers such that for any two elements $a$ and $b$ of $A$, with $a < b$, the pair $(a,b)$ is Rioplatense.