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IMO Shortlist 2011, Combinatorics 4

Source: IMO Shortlist 2011, Combinatorics 4

July 12, 2012
arithmetic sequencecombinatoricsIMO ShortlistAdditive combinatorics

Problem Statement

Determine the greatest positive integer kk that satisfies the following property: The set of positive integers can be partitioned into kk subsets A1,A2,,AkA_1, A_2, \ldots, A_k such that for all integers n15n \geq 15 and all i{1,2,,k}i \in \{1, 2, \ldots, k\} there exist two distinct elements of AiA_i whose sum is n.n.
Proposed by Igor Voronovich, Belarus