In a chess board we call a group of queens independant if no two are threatening each other. In an $n$ by $n$ grid, we put exaxctly one queen in each cell ofa greed. Let us denote by $M_n$ the minimum number of independant groups that hteir union contains all the queens. Let $k$ be a positive integer, prove that $M_{3k+1} \le 3k+2$ Proposed by Alireza Haghi