2012-2013 Winter OMO #27
Source:
January 16, 2013
Online Math Openanalytic geometrygeometryparallelogrammodular arithmetic
Problem Statement
Geodude wants to assign one of the integers to each lattice point in a 3D Cartesian coordinate system. In how many ways can Geodude do this if for every lattice parallelogram , the positive difference between the sum of the numbers assigned to and and the sum of the numbers assigned to and must be a multiple of ? (A lattice point is a point with all integer coordinates. A lattice parallelogram is a parallelogram with all four vertices lying on lattice points. Here, we say four not necessarily distinct points form a parallelogram if and only if the midpoint of segment coincides with the midpoint of segment .)
[hide="Clarifications"][*] The ``positive difference'' between two real numbers and is the quantity . Note that this may be zero.
[*] The last sentence was added to remove confusion about ``degenerate parallelograms.''Victor Wang