Expander mixing lemma

The expander mixing lemma states that, for any two subsets S, T of a regular expander graph G, 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.


Let G = (V, E) be a d-regular graph with (un-)normalized second-largest eigenvalue lambda. Then for any two subsets S, T subseteq V, let E(S, T) denote the number of edges between S and T. We have

:|E(S, T) - frac{d |S| cdot |T{n}| leq lambda sqrt

For a proof, see link in references.


Recently, Bilu and Linial showed that the converse holds as well: if a graph satisfies the conclusion of the expander mixing lemma, that is, for any two subsets S, T subseteq V,

:|E(S, T) - frac{d |S| cdot |T{n}| leq alpha sqrt

then its second-largest eigenvalue is O(alpha(1+log(d/alpha))).


*Notes proving the expander mixing lemma. [http://algo.epfl.ch/handouts/en/algoM_lect24.pdf]
*Expander mixing lemma converse. [http://www.cs.huji.ac.il/~nati/PAPERS/raman_lift.pdf]

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