Game strategy with flipping coins
Source: 2023 China Southeast Grade 11 P8
July 31, 2023
combinatorics
Problem Statement
Let be a fixed positive integer. and play the following game: coins marked lie on a circle (the marks are considered in module ) and each coin has two sides. Initially, all coins are head up and 's goal is to make as many coins with tail up. In each operation, choose two coins marked and with head up (if can't choose, the game ends) and choose a coin marked or and flip it. If at some moment there are coins with tail up, wins. Find the largest such that has a winning strategy.