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Playing a game of musical chairs

Source: 2023 Dutch BxMO TST, Problem 3

March 12, 2024
combinatorics

Problem Statement

We play a game of musical chairs with nn chairs numbered 11 to nn. You attach nn leaves, numbered 11 to nn, to the chairs in such a way that the number on a leaf does not match the number on the chair it is attached to. One player sits on each chair. Every time you clap, each player looks at the number on the leaf attached to his current seat and moves to sit on the seat with that number. Prove that, for any mm that is not a prime power with1<mn 1 < m \leq n, it is possible to attach the leaves to the seats in such a way that after mm claps everyone has returned to the chair they started on for the first time.