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Number theory Difficulty 6.0 National olympiad Prove it

Example 7 Given any n2n \geqslant 2, prove: there exist nn distinct positive integers, such that the sum of any two of them divides the product of these nn numbers.

Solution

We arbitrarily take nn distinct positive integers a1,,ana_{1}, \cdots, a_{n}, and select a (positive integer) parameter KK, hoping that the product Kna1anK^{n} a_{1} \cdots a_{n} of Ka1,,KanK a_{1}, \cdots, K a_{n} is divisible by the sum of any two terms Kai+KajK a_{i}+K a_{j} (1i,jn,ij)(1 \leqslant i, j \leqslant n, i \neq j). Since n2n \geqslant 2, it is clear that taking
K=1i<jn(ai+aj)K=\prod_{1 \leq i<j \leqslant n}\left(a_{i}+a_{j}\right)

meets the requirement (note that Ka1,,KanK a_{1}, \cdots, K a_{n} are distinct).

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.