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A little game with symmetries

Source: European Mathematical Cup 2022, Senior Division, Problem 1

December 19, 2022
combinatoricsgamegame strategysymmetry

Problem Statement

Let n3n\geq 3 be a positive integer. Alice and Bob are playing a game in which they take turns colouring the vertices of a regular nn-gon. Alice plays the first move. Initially, no vertex is coloured. Both players start the game with 00 points.
In their turn, a player colours a vertex VV which has not been coloured and gains kk points where kk is the number of already coloured neighbouring vertices of VV. (Thus, kk is either 00, 11 or 22.)
The game ends when all vertices have been coloured and the player with more points wins; if they have the same number of points, no one wins. Determine all n3n\geq 3 for which Alice has a winning strategy and all n3n\geq 3 for which Bob has a winning strategy.