MathDB
JBMO Shortlist 2022 C3

Source: JBMO Shortlist 2022

June 26, 2023
combinatoricsJuniorBalkanshortlist

Problem Statement

There are 200200 boxes on the table. In the beginning, each of the boxes contains a positive integer (the integers are not necessarily distinct). Every minute, Alice makes one move. A move consists of the following. First, she picks a box XX which contains a number cc such that c=a+bc = a + b for some numbers aa and bb which are contained in some other boxes. Then she picks a positive integer k>1k > 1. Finally, she removes cc from XX and replaces it with kckc. If she cannot make any mobes, she stops. Prove that no matter how Alice makes her moves, she won't be able to make infinitely many moves.