Hypergraphs with many extremal configurations

Abstract

For every positive integer $t$ we construct a finite family of triple systems ${\mathcal{M}}_t$, determine its Turán number, and show that there are $t$ extremal ${\mathcal{M}}_t$-free configurations that are far from each other in edit-distance. We also prove a strong stability theorem: every ${\mathcal{M}}_t$-free triple system whose size is close to the maximum size is a subgraph of one of these $t$ extremal configurations after removing a small proportion of vertices. This is the first stability theorem for a hypergraph problem with an arbitrary (finite) number of extremal configurations. Moreover, the extremal hypergraphs have very different shadow sizes (unlike the case of the famous Turán tetrahedron conjecture). Hence a corollary of our main result is that the boundary of the feasible region of ${\mathcal{M}}_t$ has exactly $t$ global maxima.

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