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On the local geometry of graphs in terms of their spectra. (English) Zbl 1420.05042

Summary: In this paper we consider the relation between the spectrum and the number of short cycles in large graphs. Suppose \(G_1, G_2, G_3, \ldots\) is a sequence of finite and connected graphs that share a common universal cover \(T\) and such that the proportion of eigenvalues of \(G_n\) that lie within the support of the spectrum of \(T\) tends to 1 in the large \(n\) limit. This is a weak notion of being Ramanujan. We prove such a sequence of graphs is asymptotically locally tree-like. This is deduced by way of an analogous theorem proved for certain infinite sofic graphs and unimodular networks, which extends results for regular graphs and certain infinite Cayley graphs.

MSC:

05C10 Planar graphs; geometric and topological aspects of graph theory
05C50 Graphs and linear algebra (matrices, eigenvalues, etc.)
05C25 Graphs and abstract algebra (groups, rings, fields, etc.)
05C30 Enumeration in graph theory
05C38 Paths and cycles

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