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Baltic Way 2019 number theory
Problem
Chessboard has a form of torus, where . Prove that queens in the cells (, multiplication modulo ) do not attack each other.

Solution
It is clear that each vertical line contains 1 queen. Since is invertible modulo , each horizontal line contains 1 queen. Each diagonal line also contains 1 queen because is invertible modulo .
Techniques
Inverses mod n