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                * Princeton Discrete Math Seminar *
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Date: Thursday 25th February, 3:00 in Fine Hall 224.
 
Speaker: Doron Puder (IAS)

Title: Ramanujan Coverings of Graphs

Ramanujan graphs are optimal expander graphs, and their existence and
construction have been the focus of much research during the last three 
decades. We prove that every bipartite Ramanujan graph has a d-covering 
(a.k.a. d-lift) which is also Ramanujan. This generalizes the d=2 case, 
a recent major breakthrough in the subject due to Marcus, Spielman and 
Srivastava. The main tools we use are the Peter-Weyl theory in group 
representations, as well as the theory of interlacing polynomials.
All notions will be explained. Joint work with Chris Hall and Will Sawin.

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Next week: Laci Babai on Tuesday

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