English

Zarankiewicz's problem for semilinear hypergraphs

Combinatorics 2021-07-27 v2 Logic

Abstract

A bipartite graph H=(V1,V2;E)H = \left(V_1, V_2; E \right) with V1+V2=n|V_1| + |V_2| = n is semilinear if ViRdiV_i \subseteq \mathbb{R}^{d_i} for some did_i and the edge relation EE consists of the pairs of points (x1,x2)V1×V2(x_1, x_2) \in V_1 \times V_2 satisfying a fixed Boolean combination of ss linear equalities and inequalities in d1+d2d_1 + d_2 variables for some ss. We show that for a fixed kk, the number of edges in a Kk,kK_{k,k}-free semilinear HH is almost linear in nn, namely E=Os,k,ε(n1+ε)|E| = O_{s,k,\varepsilon}(n^{1+\varepsilon}) for any ε>0\varepsilon > 0; and more generally, E=Os,k,r,ε(nr1+ε)|E| = O_{s,k,r,\varepsilon}(n^{r-1 + \varepsilon}) for a Kk,,kK_{k, \ldots,k}-free semilinear rr-partite rr-uniform hypergraph. As an application, we obtain the following incidence bound: given n1n_1 points and n2n_2 open boxes with axis parallel sides in Rd\mathbb{R}^d such that their incidence graph is Kk,kK_{k,k}-free, there can be at most Ok,ε(n1+ε)O_{k,\varepsilon}(n^{1+\varepsilon}) incidences. The same bound holds if instead of boxes one takes polytopes cut out by the translates of an arbitrary fixed finite set of halfspaces. We also obtain matching upper and (superlinear) lower bounds in the case of dyadic boxes on the plane, and point out some connections to the model-theoretic trichotomy in oo-minimal structures (showing that the failure of an almost linear bound for some definable graph allows one to recover the field operations from that graph in a definable manner).

Keywords

Cite

@article{arxiv.2009.02922,
  title  = {Zarankiewicz's problem for semilinear hypergraphs},
  author = {Abdul Basit and Artem Chernikov and Sergei Starchenko and Terence Tao and Chieu-Minh Tran},
  journal= {arXiv preprint arXiv:2009.02922},
  year   = {2021}
}

Comments

v.2: 32 pages; minor corrections throughout the article, updated references; accepted to the Forum of Mathematics, Sigma

R2 v1 2026-06-23T18:21:11.632Z