We determine the largest subset \(A \subseteq \{1,\dotsc,n\}\) such that for all \(a,b\in A\), the product \(ab\) is not squarefree. Specifically, the maximum size is achieved by the complement of the odd squarefree numbers.
This resolves a problem of Paul Erdős and András Sárközy from 1992.
Boris Alexeev, Dustin G. Mixon, and Will Sawin
The independence and clique cover numbers of the squarefree graph
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