Solved. Erdős and Gyárfás [EG97] proved $f(n)\ge\frac56(n-1)$ for all $n\ge4$ and $f(n)\le n$ for odd $n$, and reported $f(9)=8$ (claim page (Erdős and Gyárfás, 1997)); Erdős's own report [Er97b] (item 12) prints the weaker bounds $\frac23n<f(n)<n$ and says that he believed the upper bound closer to the truth while Gyárfás believed in the lower bound. Bennett, Cushman, Dudek and Prałat [BCDP22] proved $f(n)=\frac56n+o(n)$ (claim page (Bennett Cushman Dudek Pralat, 2022)), and Joos and Mubayi [JoMu22] gave a much shorter second proof of the same asymptotic (claim page (Joos and Mubayi, 2022)); both are refereed (J. Combin. Theory Ser. B 169 (2024) and Proc. Amer. Math. Soc. 152 (2024)). The site reads the instruction to determine the size of $f(n)$ as asking for the constant in $f(n)\sim cn$ and labels the problem solved; the exact value of $f(n)$ is not known. The site's thread holds one comment, a typographical remark of 23 July 2026.