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MathNet 2012 Question 51

Math Olympiad Vault · MathNet 2012

Question 51

Let $k\ge2$ be an integer.

1) Let $n\gt k$ be an integer. Do there exist $n$ positive integers such that any $k$ of them are never collectively coprime, but any $k+1$ of them are always collectively coprime?

2) Does there exist an infinite sequence of positive integers satisfying the two conditions above?