Growth of regular partitions 3: strong regularity and the vertex partition

Abstract: This is part 3 in a series of papers about the growth of regular partitions in $3$-uniform hypergraphs. We consider here the strong regularity for $3$-uniform hypergraphs developed by Frankl, Gowers, Kohayakawa, Nagle, R\"{o}dl, Skokan, and Schacht. This type of regular decomposition comes with two components, a partition of vertices, and a partition of pairs of vertices. We define two corresponding growth functions associated to a hereditary property $\mathcal{H}$ of $3$-uniform hypergraphs: $T_{\mathcal{H}}$ which measures the size of the vertex component, and $L_{\mathcal{H}}$ which measures the size of the pairs component. In this paper, we give an almost complete description of the possible growth rates for $T_{\mathcal{H}}$: constant, polynomial, between single and double exponential, or wowzer. The only existing lower bound constructions for this type of hypergraph regularity were due to Moshkovitz and Shapira, who constructed examples requiring a wowzer-type lower bound on the vertex component in a weaker type of hypergraph regularity. The results of this paper rely crucially on the fact that a slightly simpler construction can be used to produce a wowzer type lower bound for $T_{\mathcal{H}}$. The key ingredient in this simpler example is a lower bound construction for strong graph regularity due to Conlon and Fox.