Expansion properties of random Cayley graphs and vertex transitive graphs via matrix martingales

  • Authors:
  • Demetres Christofides;Klas Markström

  • Affiliations:
  • Department of Pure Mathematics and Mathematical Statistics, Centre for Mathematical Sciences, Cambridge CB3 0WB, UK;Department of Mathematics and Mathematical Statistics, Umeå Universitet SE-901 87, Umeå, Sweden

  • Venue:
  • Random Structures & Algorithms
  • Year:
  • 2008

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Abstract

The AlonRoichman theorem states that for every ε 0there is a constant c(ε), such that the Cayley graphof a finite group G with respect to c(ε)log|G| elements of G, chosen independently and uniformlyat random, has expected second largest eigenvalue less thanε. In particular, such a graph is an expander with highprobability.Landau and Russell, and independently Loh and Schulman, improvedthe bounds of the theorem. Following Landau and Russell we give anew proof of the result, improving the bounds even further. Whenconsidered for a general group G, our bounds are in a sensebest possible. We also give a generalization of the AlonRoichmantheorem to random coset graphs.Our proof uses a Hoeffding-type result for operator valuedrandom variables, which we believe can be of independent interest.© 2007 Wiley Periodicals, Inc. Random Struct. Alg., 2008