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partitions of first n naturals

Source: VI May Olympiad (Olimpiada de Mayo) 2000 L2 P1

September 19, 2022
combinatorics

Problem Statement

The set {1,2,3,4}\{1, 2, 3, 4\} can be partitioned into two subsets A={1,4}A = \{1, 4\} and B={3,2}B = \{3, 2\} with no common elements and such that the sum of the elements of AA is equal to the sum of the elements of B. Such a partition is impossible for the set {1,2,3,4,5}\{1, 2, 3, 4, 5\} and also for the set {1,2,3,4,5,6}\{1, 2, 3, 4, 5, 6\}. Determine all values of nn for which the set of the first nn natural numbers can be partitioned into two subsets with no common elements such that the sum of the elements of each subset is the same.