I thought fairies were nice
Source: IMO Shortlist 2023 C5
July 17, 2024
IMO Shortlistcombinatorics
Problem Statement
Elisa has treasure chests, all of which are unlocked and empty at first. Each day, Elisa adds a new gem to one of the unlocked chests of her choice, and afterwards, a fairy acts according to the following rules:[*]if more than one chests are unlocked, it locks one of them, or
[*]if there is only one unlocked chest, it unlocks all the chests.Given that this process goes on forever, prove that there is a constant with the following property: Elisa can ensure that the difference between the numbers of gems in any two chests never exceeds , regardless of how the fairy chooses the chests to unlock.