Problem A. 587. (April 2013)
A. 587. Find all positive integers n for which there exists a sequence a_{1}<a_{2}<...<a_{n} of positive integers with the property that the sums a_{i}+a_{j} with 1i<jn are distinct and among them each modulo 4 residue occurs the same number of times.
(Proposed by Ilya Bogdanov, Moscow)
(5 pont)
Deadline expired on 10 May 2013.
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