1,606 research outputs found
Sets of Complex Unit Vectors with Two Angles and Distance-Regular Graphs
We study {0,\alpha}-sets, which are sets of unit vectors of in
which any two distinct vectors have angle 0 or \alpha. We investigate some
distance-regular graphs that provide new constructions of {0,\alpha}-sets using
a method by Godsil and Roy. We prove bounds for the sizes of {0,\alpha}-sets of
flat vectors, and characterize all the distance-regular graphs that yield
{0,\alpha}-sets meeting the bounds at equality.Comment: 15 page
Walks and the spectral radius of graphs
We give upper and lower bounds on the spectral radius of a graph in terms of
the number of walks. We generalize a number of known results.Comment: Corrections were made in Theorems 5 and 11 (the new numbers are
different), following a remark of professor Yaoping Ho
On the Expansion of Group-Based Lifts
A -lift of an -vertex base graph is a graph on
vertices, where each vertex of is replaced by vertices
and each edge in is replaced by a matching
representing a bijection so that the edges of are of the form
. Lifts have been studied as a means to efficiently
construct expanders. In this work, we study lifts obtained from groups and
group actions. We derive the spectrum of such lifts via the representation
theory principles of the underlying group. Our main results are:
(1) There is a constant such that for every , there
does not exist an abelian -lift of any -vertex -regular base graph
with being almost Ramanujan (nontrivial eigenvalues of the adjacency matrix
at most in magnitude). This can be viewed as an analogue of the
well-known no-expansion result for abelian Cayley graphs.
(2) A uniform random lift in a cyclic group of order of any -vertex
-regular base graph , with the nontrivial eigenvalues of the adjacency
matrix of bounded by in magnitude, has the new nontrivial
eigenvalues also bounded by in magnitude with probability
. In particular, there is a constant such that for
every , there exists a lift of every Ramanujan graph in
a cyclic group of order with being almost Ramanujan. We use this to
design a quasi-polynomial time algorithm to construct almost Ramanujan
expanders deterministically.
The existence of expanding lifts in cyclic groups of order
can be viewed as a lower bound on the order of the largest abelian group
that produces expanding lifts. Our results show that the lower bound matches
the upper bound for (upto in the exponent)
FPRAS for computing a lower bound for weighted matching polynomial of graphs
We give a fully polynomial randomized approximation scheme to compute a lower
bound for the matching polynomial of any weighted graph at a positive argument.
For the matching polynomial of complete bipartite graphs with bounded weights
these lower bounds are asymptotically optimal.Comment: 16 page
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