OPEN
What is the size of the largest $A\subseteq \{1,\ldots,n\}$ such that if $a\leq b\leq c\leq d\in A$ are such that $abcd$ is a square then $ad=bc$?
A question of Erdős, Sárközy, and Sós. Erdős claims that Sárközy proved that $\lvert A\rvert =o(n)$.
See also [121].
