Is the maximum size of a set $A ⊆ \{1, \dots, N\}$ such that $ab + 1$ is never squarefree
(for all $a, b ∈ A$) achieved by taking those $n ≡ 7 \pmod{25}$?
This asks whether `Erdos848 N` holds for all $N$ (formulated using `A ⊆ Finset.range N`).
This was solved for all sufficiently large $N$ by Sawhney in this note. In fact, Sawhney proves
something slightly stronger, that there exists some constant $c>0$ such that if
$\lvert A\rvert \geq (\frac{1}{25}-c)N$ and $N$ is large then $A$ is contained in either
$\{ n\equiv 7\pmod{25}\}$ or $\{n\equiv 18\pmod{25}\}$.
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