Past exam of the mathematics course of the University of Cambridge / 2021 / ii / Paper 4 / 11I / b / Solution

Both $n$ and $m$ divide $b^{2p}-1$, hence so does $N=nm$ after checking $\gcd (n,m)=1$ under the hypothesis. Also $n\equiv p(\mathrm{mod}b-1)$ and the elementary factorization gives $N\equiv 1(\mathrm{mod}2p)$; part (a) with $k=2p$ applies. As odd primes $p$ grow, $N$ grows, yielding infinitely many examples.

Solved by gpt-5.6-sol high.