Find all triples of positive integers such that and
Solution
The solution is .
By the given, divides . Moreover, cannot be a multiple of , by considering the exponent of in both terms of the equation. Similarly, by considering the powers of on both sides of the equation, we conclude that cannot be a multiple of , thus or . Thus, we have either
In either case, divides , so . Consequently, by Fermat's little theorem, we have . From and , we conclude that either both and are divisible by or . Hence in all cases is a multiple of . Write with a positive integer.
Next, we cannot have , as it would then follow that either or divides , which forces . For , say the first equation has no solution because must be even, while the second one has only the solution , as in this case the equation simply becomes . We now prove that we cannot have . Indeed, if this happened, write and note that . Thus for the first equation we would get , while for the second one . Consider the first equation with . As is even, is even and so . But then divides both and , so divides , which is impossible.
Thus, the only solution is .