A Refutation of the Pach-Tardos Conjecture for 0-1 Matrices

Abstract: The theory of forbidden 0-1 matrices generalizes Turan-style (bipartite) subgraph avoidance, Davenport-Schinzel theory, and Zarankiewicz-type problems, and has been influential in many areas, such as discrete and computational geometry, the analysis of self-adjusting data structures, and the development of the graph parameter twin width. The foremost open problems in this area is to resolve the Pach-Tardos conjecture from 2005, which states that if a forbidden pattern $P\in{0,1}^{k\times l}$ is the bipartite incidence matrix of an acyclic graph (forest), then $\mathrm{Ex}(P,n) = O(n\log^{C_P} n)$, where $C_P$ is a constant depending only on $P$. This conjecture has been confirmed on many small patterns, specifically all $P$ with weight at most 5, and all but two with weight 6. The main result of this paper is a clean refutation of the Pach-Tardos conjecture. Specifically, we prove that $$\mathrm{Ex}(S_0,n),\mathrm{Ex}(S_1,n) \geq n2^{{\Omega(\sqrt{\log} n})}$$, where $S_0,S_1$ are the outstanding weight-6 patterns. We also prove sharp bounds on the entire class of alternating patterns $(P_t)$, specifically that for every $t\geq 2$, $$\mathrm{Ex}(P_t,n)=\Theta(n(\log n/\log\log n)<sup>t)$$. This is the first proof of an asymptotically sharp bound that is $\omega(n\log n)$.