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Number theory Difficulty 4.8 AIME Prove it Hong Kong

Find infinitely many positive integers mm such that for each such mm, the number 2m118191m\frac{2^{m-1}-1}{8191m} is an integer.

Solution

By Dirichlet's theorem, there are infinitely many primes pp of the form 13k+113k + 1. We claim that we can set m=pm = p whenever p>8191p > 8191.

Firstly, pp divides 2p112^{p-1} - 1 by the Fermat little theorem. Secondly, we have
2p11=213k1=(2131)(213(k1)+213(k2)++1), 2^{p-1} - 1 = 2^{13k} - 1 = (2^{13} - 1)(2^{13(k-1)} + 2^{13(k-2)} + \dots + 1),
which is divisible by 2131=81912^{13} - 1 = 8191. As pp and 81918191 are relatively prime, the number 2p118191p\frac{2^{p-1}-1}{8191p} is an integer.

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