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Balkan Mathematical Olympiad Shortlist

counting and probability

Problem

Let and let be a circle. For every nonempty subset of the set , denote by the sum of elements of the set , and define ( is the empty set). Is it possible to join every subset of with some point on the circle so that following conditions are fulfilled: 1. Different subsets are joined with different points; 2. All joined points are vertices of a regular polygon; 3. If are some of the joined points, , such that is a regular -gon, then 2014 divides ?
Solution
We will prove that this is possible. Total number of subsets of the set is . On circle we arbitrarily choose points which are vertices of a regular -gon. We join subsets of the set and chosen points in the following manner: if we join subset with some point on , we join subset with a point symmetric to the point joined with with respect to the center of (number is even, so this is possible). If are some of the points joined with subsets, which are vertices of a regular -gon, then it follows that , so is a number divisible by , say . That is why all points can be divided in pairs of symmetric points with respect to center of . Using that we get , so all conditions are fulfilled.

Techniques

Recursion, bijectionRotationFactorization techniques