MathDB
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 100×100100 \times 100 board with all unit squares initially white. Alice sets up the game by coloring exactly kk 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 1010 red squares and color all of the remaining squares in it red. What is the smallest kk 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