Prove that there are infinitely many primes such that each of them divides an integer of the form , but does not divide any integer of the form , where and are positive integers.
Solution
We show the stronger result that the set of prime such that for some (since , we can work with instead of , by restricting to even exponents), but for any , is infinite.
Note that if for some , then would imply that is a quadratic residue mod , which contradicts . (The latter can be easily checked using the law of quadratic reciprocity.)
Thus, it suffices to justify that the set of primes that divide for some is infinite. Suppose these primes are finite, say (there is at least one such prime, for instance, for ). Then, for , we have by Fermat's little theorem, which implies for each . However, must have a prime divisor (if all prime factors were , their product with multiplicities yields , a contradiction), distinct from each . The desired result follows.