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Arne and Berit play a game on a blackboard

Source: 2023 Abelkonkurransen Finale, Problem 2b

March 12, 2024
combinatorics

Problem Statement

Arne and Berit are playing a game. They have chosen positive integers mm and nn with n4n\geq 4 and m2n+1m \leq 2n + 1. Arne begins by choosing a number from the set {1,2,,n}\{1, 2, \dots , n \}, and writes it on a blackboard. Then Berit picks another number from the same set, and writes it on the board. They continue alternating turns, always choosing numbers that are not already on the blackboard. When the sum of all the numbers on the board exceeds or equals mm, the game is over, and whoever wrote the last number has won. For which combinations of mm and nn does Arne have a winning strategy?