2006 Romanian NMO Problems/Grade 8/Problem 2

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Problem

Let $n$ be a positive integer. Prove that there exists an integer $k$ , $k\geq 2$ , and numbers $a_i \in \{ -1, 1 \}$ , such that $n = \sum_{1\leq i < j \leq k } a_ia_j$ .

Solution

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2006 Romanian NMO Problems/Grade 8/Problem 2

From AoPSWiki

Problem

Solution

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