Find all triples of integers (m,p,q) such that 2mp2+1=q7, where m>0, and p and q are prime numbers, or show that they do not exist.
Solution
There is no such triple. First note that 2mp2=q7−1=(q−1)(q6+q5+q4+q3+q2+q+1). Since q6+q5+q4+q3+q2+q+1 is odd and is larger than 1, we have q−1=2m or q−1=2mp. The latter is impossible by Zsigmondy's theorem, since q7−1 has a prime divisor not dividing q−1 (alternatively, one can obtain a contradiction by using some simple bounds). Thus, we have q=2m+1. If m is odd, then q=2m+1≡2+1≡0(mod3). As q is a prime, we must have q=3, and so m=1. But then 2p2+1=2187 has no solution. If m is even, let m=2k. The equation becomes q7−(2kp)2=1. By Catalan's conjecture, there is no solution.
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