Problem

Source: 2017 Iran MO 3rd round, first combinatorics exam P3

Tags: Iran, combinatorics



Ali has $6$ types of $2\times2$ squares with cells colored in white or black, and has presented them to Mohammad as forbidden tiles. $a)$ Prove that Mohammad can color the cells of the infinite table (from each $4$ sides.) in black or white such that there's no forbidden tiles in the table. $b)$ Prove that Ali can present $7$ forbidden tiles such that Mohammad cannot achieve his goal.