JBMO Shortlist 2021 C4
Source: JBMO Shortlist 2021
July 2, 2022
JuniorBalkanshortlist2021combinatoricsColoringboard
Problem Statement
Alice and Bob play a game together as a team on a board with all unit squares initially white. Alice sets up the game by coloring exactly of the unit squares red at the beginning. After that, a legal move for Bob is to choose a row or column with at least red squares and color all of the remaining squares in it red. What is the
smallest such that Alice can set up a game in such a way that Bob can color the entire board red after finitely many moves?Proposed by Nikola Velov, Macedonia