- Expander graph
In

combinatorics , an**expander graph**is asparse graph which has high connectivity properties, quantified using vertex or edge expansion as described below. Expander constructions have spawned research in pure and applied mathematics, with several applications totheoretical computer science , design of robustcomputer network s, and the theory oferror-correcting code s.**Definitions**There are several different ways to measure the expansion properties of a finite, undirected

multigraph $G$.**Edge expansion**The "edge expansion" (also "isoperimetric number") $h(G)$ of a graph $G$ is defined as: $h(G)\; =\; min\_\{1\; le\; |S|le\; frac\{n\}\{2\}\; \}\; frac$where $Gamma(S)$ stands for the set of vertices with at least one neighbor in $S$. In a variant of this definition (called "unique neighbor expansion")$Gamma(S)$ stands for the set of vertices in $V$ with "exactly" one neighbor in $S$.

**pectral expansion**When $G$ is regular, a

linear algebra ic definition of expansion is possible based on the eigenvalues of theadjacency matrix $A=A(G)$ of $G$ (where $A\_\{ii\}$ is the number of loops at the $i$th vertex). Because $A$ is symmetric, theSpectral theorem implies that $A$ has $n$ real-valued eigenvalues $lambda\_0\; ge\; lambda\_1\; ge\; ldots\; ge\; lambda\_\{n-1\}$. Because $G$ is regular, $lambda\_0=d$ where $d$ is the degree of regularity of $G$. In some contexts, the "spectral gap " of $G$ is defined to be $d-lambda\_1$. In other contexts, the spectral gap refers to $d-lambda$, where $lambda=max\{|lambda\_1|,ldots,\; |lambda\_\{n-1\}|\}$.The normalized version of this definition, where we consider the matrix $A/d$ and get eigenvalues between -1 and 1, is also widely used and more convenient in the statement of some results.

Frequently one will refer to the "second-largest eigenvalue" of a graph G, which refers to the max of $left|lambda\_1\; ight|$ and $left|lambda\_\{n-1\}\; ight|$. Depending on the context it may be either the normalized version (taking value between 0 and 1) or the un-normalized version (taking value between 0 and d).

**Expander families**A family $mathcal\{G\}\; =\; \{G\_1,\; G\_2,\; ldots\; \}$ of $d$-regular graphs is an "edge expander family" if there is a constant $c\; >\; 0$ such that $h(G)\; ge\; c$ for each $G\; in\; mathcal\{G\}$. Typically we want $d$ to be constant, though it is sometimes also interesting to consider $d\; =\; log\; |G|$ or even $d\; =\; |G|^\{O(1)\}$. Similarly, $mathcal\{G\}$ is a "vertex expander family" if there is a constant $c\; >\; 1$ such that $g\_\{1/2\}(G)\; ge\; c$ for each $G\; in\; mathcal\{G\}$, and $mathcal\{G\}$ is a "spectral expander family" if some positive constant is a lower bound for the spectral gap of each $G\; in\; mathcal\{G\}$.

These definitions can be extended to the case of

directed graph s. A directed graph can also be interpreted as a "balanced"bipartite graph (with all edges going from one copy of $V$ to another copy). The definition of bipartite expanders can further be generalized to the case of "unbalanced" bipartite graphs.**Relationship between the different definitions**The expansion parameters defined above are related to each other. In particular, for any graph $G$, $h(G)\; ge\; g\_\{1/2\}(G)\; -\; 1$. Consequently, every vertex expander family is also an edge expander family.

Similarly, when $G$ is $d$-regular, there is a relationship between $h(G)$ and the spectral gap $d\; -\; lambda\_1$ of $G$. An inequality due to "Cheeger and Buser in the continuous case and Tanner, Alon, and Milman in the discrete case" [

*http://www.math.ias.edu/~boaz/ExpanderCourse/lecture02.ps*] states that: $frac\{1\}\{2\}(d\; -\; lambda\_1)\; le\; h(G)\; le\; sqrt\{2d(d\; -\; lambda\_1)\}$

As a result, a family $mathcal\{G\}$ of graphs is an edge expander family if and only if $mathcal\{G\}$ is a spectral expander family.

**Examples of expanders**A random $d$-regular graph has good expansion, with high probability.

Ramanujan graph s are a family of $d$-regular expander graphs with $d$ being constant and with explicit constructions, that have essentially the largest possible spectral gap. Algebraic constructions based onCayley graph s are known for various variants of expander graphs. Most recently, combinatorial constructions of expanders have also been discovered.**Applications and useful properties**The original motivation for expanders is to build economical robust networks (phone or computer): an expander with bounded valence is precisely an asymptotic robust graph with number of edges growing linearly with size (number of vertices), for all subsets.

Expander graphs have found extensive applications in

computer science , in designingalgorithm s,error correcting code s,extractor s,pseudorandom generator s,sorting network s and robustcomputer network s. They have also been used in proving many important results incomputational complexity theory , such as SL=L and thePCP theorem . Incryptography too, expander graphs are used to constructhash function s.The following are some properties of expander graphs that have proven useful in many areas.

**Expander mixing lemma**The expander mixing lemma states that, for any two subsets of the vertices $S,\; T\; subseteq\; V$, the number of edges between $S$ and $T$ is approximately what you would expect in a random $d$-regular graph, i.e. $d\; |S|\; cdot\; |T|\; /\; n$.

**Expander walk sampling**The Chernoff bound states that, when sampling many independent samples from a random variables in the range $[-1,\; 1]$, with high probability the average of our samples is close to the expectation of the random variable. The expander walk sampling lemma, due to Gillman and Ajtai-Komlós-Szemerédi, states that this also holds true when sampling from a walk on an expander graph. This is particularly useful in the theory of

derandomization , since sampling according to an expander walk uses much fewer random bits than sampling independently.**References***

**External links*** [

*http://www.ams.org/notices/200407/what-is.pdf Brief introduction in Notices of the American Mathematical Society*]

* [*http://www.qinfo.org/people/nielsen/blog/archive/notes/expander_graphs.pdf Introductory paper by Michael Nielsen*]

* [*http://www.cs.huji.ac.il/~nati/PAPERS/expander_survey.pdf A thorough survey of the field by Hoory, Linial & Wigderson*]

* [*http://www.math.ias.edu/~boaz/ExpanderCourse/ Lecture notes from a course on expanders*]

* [*http://ttic.uchicago.edu/~prahladh/teaching/spring05/index.html Lecture notes from yet another course on expanders*]

* [*http://www.yann-ollivier.org/specgraph/specgraph.html Definition and application of spectral gap*]

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