A little game with symmetries
Source: European Mathematical Cup 2022, Senior Division, Problem 1
December 19, 2022
combinatoricsgamegame strategysymmetry
Problem Statement
Let be a positive integer. Alice and Bob are playing a game in which they take turns colouring the vertices of a regular -gon. Alice plays the first move. Initially, no vertex is coloured. Both players start the game with points.In their turn, a player colours a vertex which has not been coloured and gains points where is the number of already coloured neighbouring vertices of . (Thus, is either , or .)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 for which Alice has a winning strategy and all for which Bob has a winning strategy.