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sets equal in terms of integral parameter

Source: Mongolian MO 2002 Teachers P3

April 9, 2021
parameterization

Problem Statement

Find all positive integer nn for which there exist real number a1,a2,,ana_1,a_2,\ldots,a_n such that {ajai1i<jn}={1,2,,n(n1)2}.\{a_j-a_i|1\le i<j\le n\}=\left\{1,2,\ldots,\frac{n(n-1)}2\right\}.