回複：紅色單位正方形問題解答

Please clarify. I assume you meant (a), but can be (b) or (c). thanks.


(a) Is it possible to color red some of the unit squares of a 2009x2009 grid
so that every non-red unit square shares a side with exactly one red square,
without two red unit squares share any sides?


(b) Is it possible to color red some of the unit squares of a 2009x2009 grid
so that every unit square, even the red ones, shares a side with exactly one red square,


(c) Is it possible to color red some of the unit squares of a 2009x2009 grid
so that every non-red unit square shares a side with exactly one red square,
but two red unit squares can share any sides?