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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$$p>2$. I even got stuck in case $p=2$. Any idea?

Jie Fan
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