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Coloring a 30*30 table

Source: Iran MO 3rd round 2016 finals -Combinatorics P3

September 6, 2016
modular arithmeticcombinatoricstableColoring

Problem Statement

A 30×3030\times30 table is given. We want to color some of it's unit squares such that any colored square has at most kk neighbors. ( Two squares (i,j)(i,j) and (x,y)(x,y) are called neighbors if ix,jy0,1,1(mod30)i-x,j-y\equiv0,-1,1 \pmod {30} and (i,j)(x,y)(i,j)\neq(x,y). Therefore, each square has exactly 88 neighbors) What is the maximum possible number of colored squares if::
a)k=6a) k=6
b)k=1b)k=1