Let be an odd prime. Does there exist a permutation of satisfying
for all pairwise distinct ?
Nineteenth IMAR Mathematical Competition
Solution
The answer is in the affirmative. To define the desired permutation, let be a quadratic non-residue modulo , let , , and let . Clearly, the form a permutation of ; moreover, , , and, since is a quadratic non-residue modulo , , .
To prove , let first be all (strictly) less than . For convenience, write for congruence modulo . Then
Let now one of be equal to . Since the left-hand member of is antisymmetric in , we may and will assume that , so . Then
The latter is non-zero modulo , and follows, unless , in which case , and
since . This completes the argument and concludes the proof.
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