MathDB
Labeling of the vertices of a regular 2012-gon

Source: Tuymaada 2012, Problem 5, Day 2, Juniors

July 21, 2012
modular arithmeticratioarithmetic sequencecombinatorics proposedcombinatorics

Problem Statement

The vertices of a regular 20122012-gon are labeled A1,A2,,A2012A_1,A_2,\ldots, A_{2012} in some order. It is known that if k+k+\ell and m+nm+n leave the same remainder when divided by 20122012, then the chords AkAA_kA_{\ell} and AmAnA_mA_n have no common points. Vasya walks around the polygon and sees that the first two vertices are labeled A1A_1 and A4A_4. How is the tenth vertex labeled?
Proposed by A. Golovanov