Let $p$ be a prime number, $a,b\in\mathbb Z$ and $ab\ne0$. I guess there exists only finitely many $k\in\mathbb N$ for which $(1+ap)^k(1+bp)\equiv 1\pmod{p^k}$. I think we may do as $(1+bp)((1+ap)^k-1)+bp\equiv 0\pmod{p^k}$. We need to consider the case $p=2$ và $p>2$. I even got stuck in case $p=2$. Any idea?
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Perhaps $p=2,a=b=-1$ is an exception – Tong Lingling May 22 '25 at 15:46
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1These two posts may help. https://math.stackexchange.com/q/5035006/966209 and https://math.stackexchange.com/q/3631619/966209 – Tong Lingling May 22 '25 at 18:42