2,090 research outputs found
Spectral radius of finite and infinite planar graphs and of graphs of bounded genus
It is well known that the spectral radius of a tree whose maximum degree is
cannot exceed . In this paper we derive similar bounds for
arbitrary planar graphs and for graphs of bounded genus. It is proved that a
the spectral radius of a planar graph of maximum vertex degree
satisfies . This result is
best possible up to the additive constant--we construct an (infinite) planar
graph of maximum degree , whose spectral radius is . This
generalizes and improves several previous results and solves an open problem
proposed by Tom Hayes. Similar bounds are derived for graphs of bounded genus.
For every , these bounds can be improved by excluding as a
subgraph. In particular, the upper bound is strengthened for 5-connected
graphs. All our results hold for finite as well as for infinite graphs.
At the end we enhance the graph decomposition method introduced in the first
part of the paper and apply it to tessellations of the hyperbolic plane. We
derive bounds on the spectral radius that are close to the true value, and even
in the simplest case of regular tessellations of type we derive an
essential improvement over known results, obtaining exact estimates in the
first order term and non-trivial estimates for the second order asymptotics
Median eigenvalues of bipartite subcubic graphs
It is proved that the median eigenvalues of every connected bipartite graph
of maximum degree at most three belong to the interval with a
single exception of the Heawood graph, whose median eigenvalues are
. Moreover, if is not isomorphic to the Heawood graph, then a
positive fraction of its median eigenvalues lie in the interval . This
surprising result has been motivated by the problem about HOMO-LUMO separation
that arises in mathematical chemistry.Comment: Accepted for publication in Combin. Probab. Compu
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