DEPARTMENT COLLOQUIUM
November 6, 2002

  Avi Wigderson
IAS and the Hebrew University

Expansion of graphs can be given equivalent definitions in combinatorial and algebraic terms. This is the most basic connection between combinatorics and algebra illuminated by expanders and the quest to construct them. The talk will survey how fertile this connection has been to both fields, focusing on recent results. In particular, we'll explain the zig-zag product of graphs, how it leads to an elementary combinatorial construction of expanders, and its relation to semi-direct product in groups.