58,276 research outputs found
Strongly walk-regular graphs
We study a generalization of strongly regular graphs. We call a graph
strongly walk-regular if there is an such that the number of walks of
length from a vertex to another vertex depends only on whether the two
vertices are the same, adjacent, or not adjacent. We will show that a strongly
walk-regular graph must be an empty graph, a complete graph, a strongly regular
graph, a disjoint union of complete bipartite graphs of the same size and
isolated vertices, or a regular graph with four eigenvalues. Graphs from the
first three families in this list are indeed strongly -walk-regular for
all , whereas the graphs from the fourth family are -walk-regular
for every odd . The case of regular graphs with four eigenvalues is the
most interesting (and complicated) one. Such graphs cannot be strongly
-walk-regular for even . We will characterize the case that regular
four-eigenvalue graphs are strongly -walk-regular for every odd ,
in terms of the eigenvalues. There are several examples of infinite families of
such graphs. We will show that every other regular four-eigenvalue graph can be
strongly -walk-regular for at most one . There are several examples
of infinite families of such graphs that are strongly 3-walk-regular. It
however remains open whether there are any graphs that are strongly
-walk-regular for only one particular different from 3
Walk entropies on graphs
Entropies based on walks on graphs and on their line-graphs are defined. They are based on the summation over diagonal and off-diagonal elements of the thermal Green’s function of a graph also known as the communicability. The walk entropies are strongly related to the walk regularity of graphs and line-graphs. They are not biased by the graph size and have significantly better correlation with the inverse participation ratio of the eigenmodes of the adjacency matrix than other graph entropies. The temperature dependence of the walk entropies is also discussed. In particular, the walk entropy of graphs is shown to be non-monotonic for regular but non-walk-regular graphs in contrast to non-regular graphs
A complete characterization of plateaued Boolean functions in terms of their Cayley graphs
In this paper we find a complete characterization of plateaued Boolean
functions in terms of the associated Cayley graphs. Precisely, we show that a
Boolean function is -plateaued (of weight ) if and only
if the associated Cayley graph is a complete bipartite graph between the
support of and its complement (hence the graph is strongly regular of
parameters ). Moreover, a Boolean function is
-plateaued (of weight ) if and only if the associated
Cayley graph is strongly -walk-regular (and also strongly
-walk-regular, for all odd ) with some explicitly given
parameters.Comment: 7 pages, 1 figure, Proceedings of Africacrypt 201
New bounds for the max--cut and chromatic number of a graph
We consider several semidefinite programming relaxations for the max--cut
problem, with increasing complexity. The optimal solution of the weakest
presented semidefinite programming relaxation has a closed form expression that
includes the largest Laplacian eigenvalue of the graph under consideration.
This is the first known eigenvalue bound for the max--cut when that is
applicable to any graph. This bound is exploited to derive a new eigenvalue
bound on the chromatic number of a graph. For regular graphs, the new bound on
the chromatic number is the same as the well-known Hoffman bound; however, the
two bounds are incomparable in general. We prove that the eigenvalue bound for
the max--cut is tight for several classes of graphs. We investigate the
presented bounds for specific classes of graphs, such as walk-regular graphs,
strongly regular graphs, and graphs from the Hamming association scheme
Three-weight codes over rings and strongly walk regular graphs
We construct strongly walk-regular graphs as coset graphs of the duals of
codes with three non-zero homogeneous weights over for
a prime, and more generally over chain rings of depth , and with a residue
field of size , a prime power. Infinite families of examples are built from
Kerdock and generalized Teichm\"uller codes. As a byproduct, we give an
alternative proof that the Kerdock code is nonlinear.Comment: 28 pages, 6 table
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