Conjectured by Erdős, Sós, and Sárközy [ESS95], who proved\[F_2(N)=\left(\frac{6}{\pi^2}+o(1)\right)N,\]\[F_3(N) = (1-o(1))N,\]and also established asymptotics for $F_k(N)$ for all even $k\geq 4$ (in particular $F_k(N)\asymp N/\log N$ for all even $k\geq 4$). Erdős [Er38] earlier proved that $F_4(N)=o(N)$ - indeed, if $\lvert A\rvert \gg N$ and $A\subseteq \{1,\ldots,N\}$ then there is a non-trivial solution to $ab=cd$ with $a,b,c,d\in A$.

Erdős (and independently Hall [Ha96] and Montgomery) also asked about $F(N)$, the size of the largest $A\subseteq\{1,\ldots,N\}$ such that the product of no odd number of $a\in A$ is a square. Ruzsa [Ru77] observed that $1/2<\lim F(N)/N <1$. Granville and Soundararajan [GrSo01] proved an asymptotic\[F(N)=(1-c+o(1))N\]where $c=0.1715\ldots$ is an explicit constant.

This problem was answered in the negative by Tao [Ta24], who proved that for any $k\geq 4$ there is some constant $c_k>0$ such that $F_k(N) \leq (1-c_k+o(1))N$.

See also [888].

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This page was last edited 17 October 2025. View history