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painted orthogonal coordinate system

Source: Vietnam TST 2001 for the 42th IMO, problem 5

June 26, 2005
analytic geometryfloor functionceiling functionIMO Shortlistcombinatorics unsolvedcombinatorics

Problem Statement

Let an integer n>1n > 1 be given. In the space with orthogonal coordinate system OxyzOxyz we denote by TT the set of all points (x,y,z)(x, y, z) with x,y,zx, y, z are integers, satisfying the condition: 1x,y,zn1 \leq x, y, z \leq n. We paint all the points of TT in such a way that: if the point A(x0,y0,z0)A(x_0, y_0, z_0) is painted then points B(x1,y1,z1)B(x_1, y_1, z_1) for which x1x0,y1y0x_1 \leq x_0, y_1 \leq y_0 and z1z0z_1 \leq z_0 could not be painted. Find the maximal number of points that we can paint in such a way the above mentioned condition is satisfied.