MathDB
2012-2013 Winter OMO #27

Source:

January 16, 2013
Online Math Openanalytic geometrygeometryparallelogrammodular arithmetic

Problem Statement

Geodude wants to assign one of the integers 1,2,3,,111,2,3,\ldots,11 to each lattice point (x,y,z)(x,y,z) in a 3D Cartesian coordinate system. In how many ways can Geodude do this if for every lattice parallelogram ABCDABCD, the positive difference between the sum of the numbers assigned to AA and CC and the sum of the numbers assigned to BB and DD must be a multiple of 1111? (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 A,B,C,DA,B,C,D form a parallelogram ABCDABCD if and only if the midpoint of segment ACAC coincides with the midpoint of segment BDBD.) [hide="Clarifications"]
[*] The ``positive difference'' between two real numbers xx and yy is the quantity xy|x-y|. Note that this may be zero. [*] The last sentence was added to remove confusion about ``degenerate parallelograms.''
Victor Wang