Question 39
An $n\times n$ grid square is initially colored black and white like a chessboard, with at least one black corner cell. In one move choose a $2\times2$ square and change each of its four cells simultaneously: white to black, black to green, green to white. Find all $n$ for which finitely many moves can produce the opposite black-and-white chessboard coloring. (B. Trushin)