OPEN
This is open, and cannot be resolved with a finite computation.
Let $F(N)$ be the maximal size of $A\subseteq\{1,\ldots,N\}$ such that no $a\in A$ divides the sum of any distinct elements of $A\backslash\{a\}$. Estimate $F(N)$. In particular, is it true that\[F(N) > N^{1/2-o(1)}?\]
This was studied by Erdős, Lev, Rauzy, Sándor, and Sárközy
[ELRSS99], where they call such a property 'non-dividing', and prove the explicit bound\[F(N)<3N^{1/2}+1.\]In
[Er97b] Erdős credits Csaba with a construction that proves $F(N) \gg N^{1/5}$. Such a construction was also given in
[ELRSS99], where it is linked to the problem of non-averaging sets (see
[186]).
Indeed, every such set is non-averaging, and hence the result of Pham and Zakharov
[PhZa24] implies\[F(N) \leq N^{1/4+o(1)}.\]This shows the answer to the original question is no, but the general question of the correct growth of $F(N)$ remains open.
In
[Er75b] Erdős writes that he originally thought $F(N) <(\log N)^{O(1)}$, but that Straus proved that\[F(N) > \exp((\sqrt{\tfrac{2}{\log 2}}+o(1))\sqrt{\log N}).\]See also
[13].
This is discussed in problem C16 of Guy's collection
[Gu04].
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This page was last edited 30 September 2025.
Additional thanks to: Desmond Weisenberg
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