EDIT:
Found the proof here, so the problem is solved.
Hey, the following is meant to be an application of the Chinese Remainder Theorem.
A lattice point in the plane is a point , where m and n are integers.
A point is called if the straight line segment from to goes through some other integer lattice point.
Show that for every there exists a square in of side length M, so that all its lattice points are invisible.