A note on the $t$-partite link problem of Füredi

Abstract

Motivated by the Erdős--Sós bipartite link conjecture, Füredi (Oberwolfach, 2004) asked for the asymptotic maximum edge density $\ddot{I}{€}_{\mathrm{link}}(t)$ of $3$-graphs in which the link graph of every vertex is $t$-partite. Goldwasser's recursive blow-up construction based on projective planes gives the lower bound $\ddot{I}{€}_{\mathrm{link}}(t)\ge 1-t^{-1}-(2+o_t(1))t^{-2}$ whenever $t-1$ is a prime power. In this note, we prove the upper bound $\ddot{I}{€}_{\mathrm{link}}(t)\le 1-t^{-1}-t^{-2}/12$ for every $t\ge 2$. Together with Goldwasser's construction, this determines, up to a constant factor, the correct order of the gap between $\ddot{I}{€}_{\mathrm{link}}(t)$ and the trivial averaging upper bound $1-t^{-1}$ for all prime-power values of $t-1$. In fact, our argument applies in the more general setting of $3$-graphs with no generalized daisies, equivalently, $3$-graphs in which the link graph of every vertex is $K_{t+1}$-free. We also establish an analogous upper bound for the positive $(r-1)$-codegree Turán density of generalized daisies.

14 pages. Comments are welcome