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Locally common graphs

Authors Endre Csóka, Tamás Hubai, László Lovász
Publication date 2022-09-02

Goodman proved that the sum of the number of trianglesin a graph onnnodes and its complement is at least n3 / 24; in other words, this sum is minimized, asymptotically, by a random graph with edge density 1/2. Erdős conjectured that a similar inequality will hold for K4 in place of K3, but this was disproved by Thomason. But ananalogous statement does hold for some other graphs,which are called common graphs. Characterization ofcommon graphs seems, however, out of reach. Franek and Rödl proved that K4 is common in a weaker, local sense.Using the language of graph limits, we study two versions of locally common graphs. We sharpen a result of Jagger,Štovíček and Thomason by showing that no graph containing K4 can be locally common, but prove that all such graphs are weakly locally common. We also show that not all connected graphs are weakly locally common.

Keywords: graph homomorphisms, graph theory


E. Csóka, T. Hubai, and L. Lovász, Locally common graphs, J. Graph Theory. (2022), 1–12.

Endre Csóka (Alfréd Rényi Institute of Mathematics, Budapest, Hungary)
Tamás Hubai (Department of Computer Science,Institute of Mathematics, Eötvös LorándUniversity, Budapest, Hungary)
László Lovász (Alfréd Rényi Institute of Mathematics, Budapest, Hungary)