Determine all triples of integers which satisfy the equation
Solution
Answer: .
Substituting , we obtain the equation
which suffices to be solved in integers such that and have equal parity.
We shall consider the equivalent equation
If both and are even then is even, whence the l.h.s. is divisible by 4. Thus , but this is impossible since is not a quadratic residue modulo 4.
Let now both and be odd; then is a positive odd number. Consider
two cases:
* If then and . Since whereas the product equals the positive number , we must have . The possibilities and give and , respectively. In both cases .
* If then . Squares of odd numbers give remainder 1 upon division by 4, whence . Consequently there exists a prime divisor of such that . But then , which is impossible since is not a quadratic residue modulo .