111 research outputs found
Ramanujan Complexes and bounded degree topological expanders
Expander graphs have been a focus of attention in computer science in the
last four decades. In recent years a high dimensional theory of expanders is
emerging. There are several possible generalizations of the theory of expansion
to simplicial complexes, among them stand out coboundary expansion and
topological expanders. It is known that for every d there are unbounded degree
simplicial complexes of dimension d with these properties. However, a major
open problem, formulated by Gromov, is whether bounded degree high dimensional
expanders, according to these definitions, exist for d >= 2. We present an
explicit construction of bounded degree complexes of dimension d = 2 which are
high dimensional expanders. More precisely, our main result says that the
2-skeletons of the 3-dimensional Ramanujan complexes are topological expanders.
Assuming a conjecture of Serre on the congruence subgroup property, infinitely
many of them are also coboundary expanders.Comment: To appear in FOCS 201
Hypergraph expanders from Cayley graphs
We present a simple mechanism, which can be randomised, for constructing
sparse -uniform hypergraphs with strong expansion properties. These
hypergraphs are constructed using Cayley graphs over and have
vertex degree which is polylogarithmic in the number of vertices. Their
expansion properties, which are derived from the underlying Cayley graphs,
include analogues of vertex and edge expansion in graphs, rapid mixing of the
random walk on the edges of the skeleton graph, uniform distribution of edges
on large vertex subsets and the geometric overlap property.Comment: 13 page
Coboundary expanders
We describe a natural topological generalization of edge expansion for graphs
to regular CW complexes and prove that this property holds with high
probability for certain random complexes.Comment: Version 2: significant rewrite. 18 pages, title changed, and main
theorem extended to more general random complexe
High Dimensional Random Walks and Colorful Expansion
Random walks on bounded degree expander graphs have numerous applications,
both in theoretical and practical computational problems. A key property of
these walks is that they converge rapidly to their stationary distribution.
In this work we {\em define high order random walks}: These are
generalizations of random walks on graphs to high dimensional simplicial
complexes, which are the high dimensional analogues of graphs. A simplicial
complex of dimension has vertices, edges, triangles, pyramids, up to
-dimensional cells. For any , a high order random walk on
dimension moves between neighboring -faces (e.g., edges) of the complex,
where two -faces are considered neighbors if they share a common
-face (e.g., a triangle). The case of recovers the well studied
random walk on graphs.
We provide a {\em local-to-global criterion} on a complex which implies {\em
rapid convergence of all high order random walks} on it. Specifically, we prove
that if the -dimensional skeletons of all the links of a complex are
spectral expanders, then for {\em all} the high order random walk
on dimension converges rapidly to its stationary distribution.
We derive our result through a new notion of high dimensional combinatorial
expansion of complexes which we term {\em colorful expansion}. This notion is a
natural generalization of combinatorial expansion of graphs and is strongly
related to the convergence rate of the high order random walks.
We further show an explicit family of {\em bounded degree} complexes which
satisfy this criterion. Specifically, we show that Ramanujan complexes meet
this criterion, and thus form an explicit family of bounded degree high
dimensional simplicial complexes in which all of the high order random walks
converge rapidly to their stationary distribution.Comment: 27 page
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