MathDB
I thought fairies were nice

Source: IMO Shortlist 2023 C5

July 17, 2024
IMO Shortlistcombinatorics

Problem Statement

Elisa has 20232023 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 CC with the following property: Elisa can ensure that the difference between the numbers of gems in any two chests never exceeds CC, regardless of how the fairy chooses the chests to unlock.