MathDB
Game strategy with flipping coins

Source: 2023 China Southeast Grade 11 P8

July 31, 2023
combinatorics

Problem Statement

Let n{n} be a fixed positive integer. A{A} and B{B} play the following game: 20232023 coins marked 1,2,,20231, 2, \dots, 2023 lie on a circle (the marks are considered in module 20232023) and each coin has two sides. Initially, all coins are head up and A{A}'s goal is to make as many coins with tail up. In each operation, A{A} choose two coins marked k{k} and k+3k+3 with head up (if A{A} can't choose, the game ends) and B{B} choose a coin marked k+1k+1 or k+2k+2 and flip it. If at some moment there are n{n} coins with tail up, A{A} wins. Find the largest n{n} such that A{A} has a winning strategy.