Prove that there are infinitely many positive integers such that the equation
has no solution over the integers.
Solution
We claim that if with a non-negative integer then the equation has no solution over the integers. Let's assume that there is one and get a contradiction. Indeed, by Fermat's little theorem,
and hence
because and are the only congruence classes modulo such that its squares are congruent to or modulo .
It follows that
which is a contradiction because neither of them is a quadratic residue as can be seen by direct inspection.
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