The non-principal pair of posets conjecture for the incidence graph of K4K_4

Let G4=(A,B,E)G_4=(A,B,E) be the bipartite incidence graph of the complete graph K4K_4, with A=E(K4)A=E(K_4), B=V(K4)B=V(K_4), and (e,v)E(e,v)\in E exactly when vev\in e. Let PG4,AP_{G_4,A} and PG4,BP_{G_4,B} be the associated posets, and let  ⁣La(n,{PG4,A,PG4,B})\mathop{}\!\mathrm{La}(n,\{P_{G_4,A},P_{G_4,B}\}) denote the maximum size of a family in 2[n]2^{[n]} containing neither poset. The non-principal pair of posets conjecture.

 ⁣La(n,PG4,A) ⁣La(n,{PG4,A,PG4,B})=Ω((nn/2)).\mathop{}\!\mathrm{La}(n,P_{G_4,A})-\mathop{}\!\mathrm{La}(n,\{P_{G_4,A},P_{G_4,B}\})=\Omega\left(\binom{n}{n/2}\right).

This predicts that the family of posets {PG4,A,PG4,B}\{P_{G_4,A},P_{G_4,B}\} is not asymptotically principal: forbidding both posets costs a positive-order middle-layer term beyond forbidding PG4,AP_{G_4,A} alone. The source gives no resolution, so the conjecture remains open.

Sources & referencesView supporting material

Primary source

Dániel Gerbner and Balázs Patkós, “A note on vertex Turán problems in the Kneser cube”, arXiv:2402.02525 (2024).

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