Maths Olympiad Prep

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, 2016

Number theory Difficulty 5.0 AIME Prove it Hong Kong

Find all triples of integers (m,p,q)(m, p, q) such that 2mp2+1=q72^m p^2 + 1 = q^7, where m>0m > 0, and pp and qq are prime numbers, or show that they do not exist.

Solution

There is no such triple.
First note that
2mp2=q71=(q1)(q6+q5+q4+q3+q2+q+1). 2^m p^2 = q^7 - 1 = (q-1)(q^6 + q^5 + q^4 + q^3 + q^2 + q + 1).
Since q6+q5+q4+q3+q2+q+1q^6 + q^5 + q^4 + q^3 + q^2 + q + 1 is odd and is larger than 11, we have q1=2mq-1 = 2^m or q1=2mpq-1 = 2^m p. The latter is impossible by Zsigmondy's theorem, since q71q^7 - 1 has a prime divisor not dividing q1q-1 (alternatively, one can obtain a contradiction by using some simple bounds). Thus, we have q=2m+1q = 2^m + 1.
If mm is odd, then q=2m+12+10(mod3)q = 2^m + 1 \equiv 2 + 1 \equiv 0 \pmod 3. As qq is a prime, we must have q=3q=3, and so m=1m=1. But then 2p2+1=21872p^2+1=2187 has no solution.
If mm is even, let m=2km = 2k. The equation becomes q7(2kp)2=1q^7 - (2^k p)^2 = 1. By Catalan's conjecture, there is no solution.

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