Questions tagged [expander-graphs]
An expander graph is a graph in which small sets of vertices have large 'boundary'. Ramanujan graphs are examples of expanders.
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Could unramified Galois groups satisfy a version of property tau?
This is an experiment: there is a question I want to mention in an article I'm writing, and I am not sure it's a sensible question, so I will ask it here first, in the hopes that if it's insensible ...
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Embedding expanders in CAT(0) spaces
It is well-known that expanders are hard to embed into Hilbert (or $\ell^p$) spaces - any embedding of an expander with $n$ vertices has distortion $\Omega(\log n)$.
Can anyone provide a reference (...
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An expander (?) graph
For a prime $p$, consider the graph on the vertex set ${\mathbb F}_p$, in
which every vertex $z$ is adjacent to $z\pm 1$ and also to $z^{-1}$ (unless
$z=0$). I was told that this graph is known to be ...
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Groups without property (T) but all finite quotients are expanders
What is an example of a group $G$ which
1- is finitely generated by $S$,
2- does not have property (T),
3- admits infinitely many finite quotients which do not factor through an homomorphism $G \...
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Variant of an Expander graph: Probability that S random points cast a shadow/projection of size at most S/2 on each face of a cube.
Consider an integer cube of size $\sqrt{k} \times \sqrt{k} \times \sqrt{k}$, where $k$ is an asymptotically large perfect square number. Place k points in this cube at uniformly random locations, i.e.,...
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An introductory text on expanders
I am looking for a book that covers expander graphs rigorously. Preferably a book aimed at beginners.
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Are there two-sided $\varepsilon$-expanders with independent sets of size $(1-\varepsilon)n$?
Terry Tao's notes on expander graphs has the following exercise:
Exercise 13 Let $G$ be a $k$-regular graph on $n$ vertices that is a two-sided $\epsilon$-expander for some $n > k \geq 1$ and $\...
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Connections between Riemann hypothesis for curves over finite fields and Ramanujan property for graphs
this question relates to the beautiful construction of expander graphs using Cayley graphs of $PGL_2(\mathbb{F}_q)$, as exposited by Davidoff-Sarnak-Valette in their book, Elementary Number Theory, ...
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Does it make sense to talk about expansion in irregular graphs?
Usually, one defines an expander graph to be a regular graph satisfying one of the following properties:
Either the edge-expansion is large, or
the spectral gap is large, or
the mixing time is at most ...
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Where does $2\sqrt{d-1}$ come from in Ramanujan graphs?
Ramanujan graphs are the best spectral expanders: $\lambda_2 \le 2\sqrt{d-1}$. I'm looking for some intuition for this value $2\sqrt{d-1}$.
Friedman showed that every random $d$-regular graph ...
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How to prove a random d-regular graph is an expander with prob >= 0.5?
How to prove a random $d$-regular graph is an expander with prob $\ge 0.5$?
Context: Many resources, like
http://math.mit.edu/~fox/MAT307-lecture22.pdf
state the theorem in the general case, but then ...
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Cheeger Numbers for 3-regular Graphs
A student wanted a challenging Graph Theory programming project and I had
him try to determine the maximum value of the Cheeger number (isoperimetric number) among all 3-regular graphs of order $n$, ...
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Do there exist "expanding" $1$-skeletons of simple $4$-polytopes?
Let $\{ G_n \}_{n \ge 1}$ be a sequence of graphs such that the number of vertices of $G_n$ tends to $\infty$ as $n \to \infty$. We say that $\{ G_n \}_{n \ge 1}$ is an expander family if
$\lambda_2( ...
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Explicit constructions of Ramanujan graphs
I am trying to find a list of all the explicit constructions of Ramanujan graphs. By a Ramanujan graph I mean a $k$-regular multi-graph $G$ such that all the non-trivial eigenvalues $\lambda$ of the ...
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When are (Abelian) Cayley graphs also expanders?
I want to ask the question in two parts,
(1)
Is there some fundamental distinguishing property between Abelian and non-Abelian Cayley graphs? (say some specific proof technique which distinguishes ...
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Constructing Ramanujan graphs from elliptic curves
Is there an exposition which explains how to do this step-by-step? (I see stray references and allusions to such a thing being possible but can't locate anything concretely)
Something to do with ``...
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Constructing expanders in Z/pZ
Fix a positive integer $k>0$. For $p>k$ a prime, let $A_p$ be a subset of the finite field $\mathbb{Z}/p\mathbb{Z}$ of size $k$ which contains a primitive element.
Define $G_p$ to be the (di)...
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Understanding Gillman's proof of the Chernoff bound for expander graphs
My question is about the proof of Claim 1 in this paper: Gillman (1993).
At the end of the proof, the author says:
The matrix product $U^\top\sqrt{D^{-1}}(P+(\mathrm{e}^x-1)B(0)-\mu I)\sqrt{D}U$, ...
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Existence of connected set with large edge boundary
Let $\Gamma=(V,E)$ be a finite connected graph.
Pretty standard notation. Given a set $S\subset V$, write $\Gamma|_S$ for the restriction of $\Gamma$ to $S$, i.e., the subgraph $(S,\{\{v,w\}\in E: v,w\...
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Arbitrary-dimensional expanders?
Rephrasing expansion (slightly). Consider the following slightly tweaked version of the usual definition of a (spectral) expander graph.
(We write a weighted graph as $(V,\beta)$, where the weight $\...
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Matching polynomials and Ramanujan graphs
Is it purely coincidental that the same number $2\sqrt{d-1}$ appears in these two following apparently disparate concepts?
A $d-$regular graph is said to be called Ramanujan if its adjacency ...
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More expanders?
Having received several exhausting answers to my recent question about
the expansion properties of a certain graph, I now wonder whether anything is
known on the following graphs of a similar nature:
...
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Non-Cayley expander graphs
When I search about expander graphs in google I see a lot of articles about expander Cayley graphs. Now my questions are as follows:
Are all expander regular graphs are Cayley, or there is a special ...
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Construction of graphs of high girth and chromatic number
Are there any concrete constructions of graphs of both high girth and chromatic number?
Of course there is the seminal paper of Erdős which proves the existence of such graphs via the probabilistic ...
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Group of Lie type as expanders: explicit estimates
In a paper Finite simple groups as expanders by M. Kassabov, A. Lubotzky and N. Nikolov there is a theorem, stating that there exists $\varepsilon>0$ and $k\in\mathbb{N}$, such that for every non-...
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The Bilu-Linial conjecture and Ramanujan graphs
The Bilu-Linial conjecture claims that every $d-$regular graph has a $2-$lift such that for the signing matrix has its eigenvalues between $[-2\sqrt{d-1},2\sqrt{d-1}]$ (the ``signing matrix" is the ...
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What are the best bounds to date on the maximum girth of a cubic graph?
The girth of a graph is the length of its smallest cycle. Studying the maximum possible girth for a $k$-regular graph on $n$ vertices is a very well-studied problem.
In the 1988 paper "Ramanujan ...
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Minimum size of regular graph with no short cycles
For $d \geq 3$ (degree) and $r \geq 3$ (radius), say that a $d$-regular (finite, simple, non-oriented) graph $G$ is $r$-almost-tree if it contains no cycle of length $\leq 2 r$: in other words, we ...
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Spectral radius of Markov averaging operator on graphs
The definition of Markov operator which I am familiar with:
For a graph $G=(V,E)$, Markov's operator upon a function
$\varphi:V\rightarrow\mathbb{C}$ , $\varphi\in L^2(G,\nu)$ ($\nu(v):=\deg(v)$) ...
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Sums over products over short paths in an expander graph
Let $\Gamma=(V,E)$ be an undirected graph of degree $d$. (Say $d$ is a large constant and the number of vertices $n=|V|$ is much larger.) Let $W_0$ be the space of functions $f:V\to \mathbb{C}$ with ...
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Reference request: maximal Cheeger constant for 3-regular graphs
Given a connected graph $G$, the Cheeger constant $h(G)$ (a.k.a. Cheeger number or isoperimetric number) roughly measures the "bottleneckedness" of $G$. See Wikipedia for the precise definition.
I ...
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Expanding graphs from iterated zig-zag product
Let $\Gamma \, zz\, G$ denote the zig-zag product of graphs $\Gamma$ and $G$.
Reingold, Vadhan, and Wigderson proved that the second largest eigenvalue of the normalized adjacency matrix of the zig-...
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Property $(T)$ for $\mathrm{SL}_2(\mathbb{Z}) \ltimes \mathbb{Z}^2$
(This is in part a request for references and in part a somewhat pedagogical question.)
I gave a course on expanders seven years ago, and I am giving a course on expanders again now. We will soon do ...
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Spectral norm and "operator norm" for hypergraphs
Consider a $d$-regular, $k$-uniform hypergraph: the elements $S$ of its set $E$ of edges are subsets of $V$ of size $k$, and each vertex $v\in V$ is in $d$ edges. We can then define its adjacency ...
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Analogues of relative property $(\tau)$ for Schreier graphs
Suppose I have an expanding family of Schreier graphs $Z_n=\text{Sch}(G_n,S_n,X_n)$ of groups $G_n=\underbrace{G\wr\ldots\wr G}_{\text{$n$ times}}$ acting on sets $S_n=S^n$ by generating sets $X_n$, ...
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Random walk and isoperimetric constant
I assume that a result of the following kind is known, and I would really appreciate a reference for it... Or at least, some hints as to where to start looking.
Theorem(?): Let $\varepsilon>0$ ...
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Ramanujan graphs from varieties over finite fields
Let $G$ be a $d$-regular graph. Let $d= \lambda_1 \ge \lambda_2 \ge \dots \ge \lambda_n \ge -d $ be the eigenvalues of the adjacency matrix of $G$, and set $\lambda = \max (|\lambda_2| , |\lambda_n|) $...
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How many edges guarantee an expander?
Complete graphs are good expanders. Deleting few edges from a complete graph leaves a good expander. What's the proportion of edges that one needs to remove to make the graph a bad expander? Or, is ...
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Do balls in expander graphs have small expansion?
Consider a $d$-regular infinite transitive expander graph $G$, and let $B_r$ be a ball of radius $r$ in $G$. Can one place any upper bounds on the expansion of $B_r$?
My intuition is that $B_r$ will ...
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Expansion in hypergraphs
Is there a useful concept of expansion in hypergraphs, generalizing the concept for graphs (see: expander graphs)?
Of course, expander graphs can be characterized in several qualitatively equivalent ...
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How does one calculate/estimate/guarantee the girth of a non-Abelian Cayley graph?
This question is in reference to this other question,
Can someone point out references (or explain!) which give techniques of being able to prove for any Cayley graph this property of having a girth ...
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Can connectivity be less than min cut/degree?
Suppose we have a graph with min-cut $\lambda$ and minimum degree $> \lambda$.
Is it possible for there to be a vertex that is at most $\lambda$-connected to every other vertex in the graph?
...
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Expansion of random subgraphs of a bi-regular bipartite graph
Let $G = (L, R, E)$ be a bi-regular bipartite graph, with $|L|=n$ and $|R| = C \cdot n$, where $C$ is a large constant. Let $d$ be its (constant) right-degree.
We know $G$ is a good spectral expander. ...
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Examples of 3-transitive expander family of Schreier graphs
What are examples of expander family of 3-transitive Schreier graphs?
Meaning for an action that is 3-transitive.
It is better to have an option for randomization. We know that choosing 2 elements ...
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Expansion of product of simple Lie group
(a quite technical question if you want to skip).
I am looking at the paper Breuillard, Green, Guralnick, and Tao - Expansion in finite simple groups of Lie type; Specifically, proposition 8.4.
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Expander graphs with many 4-cycles
The question is not strictly well-defined. But it goes like this:
Could you find an infinite family of graphs $G_i$, that are all $\epsilon$-expanders, but have many 4-cycles?
$\epsilon$ should ...
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Asymptotic results in unbalanced left $d$-regular expander graphs
Let $U = [n]$ and $V = [m]$ be sets of nodes with $n > m$ and $E = U\times V$ be a set of edges. Let $\mathcal{N}(S)$ be the set of neighbors of a subset $S$ from $U$ or $V$.
Call a graph $G = (U, ...
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How should one define expansion for irregular graphs?
This question is related to a previous question:
Does it make sense to talk about expansion in irregular graphs?
A family of expanders can be defined as a sequence of graphs whose spectral gap is ...
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A particular argument in the review on expanders by Hoory-Linial-Wigderson
I am thinking about the third bullet point on page 455 here, http://www.ams.org/journals/bull/2006-43-04/S0273-0979-06-01126-8/
Can someone explain what is the argument there which seems to conclude ...
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Exploring the Intersection of Expander Graphs, Number Theory, Representation Theory and Recent Computer Science Developments [closed]
I have a solid understanding of the basics of expander graphs and their properties and the recent development of High-Dimensional Expanders and their application to Random Walks, along with other ...
