Find all positive integers $a$ and $b$ such that $ ab+1 \mid a^2-1$
Problem
Source: Regional Olympiad - Federation of Bosnia and Herzegovina 2015
Tags: number theory, Divisibility
Source: Regional Olympiad - Federation of Bosnia and Herzegovina 2015
Tags: number theory, Divisibility
Find all positive integers $a$ and $b$ such that $ ab+1 \mid a^2-1$