ISL 2015 N4

Suppose that $a_{0}, a_{1}, \ldots$ and $b_{0}, b_{1}, \ldots$ are two sequences of positive integers satisfying $a_{0}, b_{0} \geqslant 2$ and $$a_{n+1}=\operatorname{gcd}\left(a_{n}, b_{n}\right)+1, b_{n+1}=\operatorname{lcm}\left(a_{n}, b_{n}\right)-1$$ for all $n \geqslant 0$. Prove that the sequence $\left(a_{n}\right)$ is eventually periodic; in other words, there exist integers $N \geqslant 0$ and $t>0$ such that $a_{n+t}=a_{n}$ for all $n \geqslant N$. (France)