Department colloquium: Alex Lubotzky (Hebrew University)
Title: High dimensional expanders: from Ramanujan graphs to Ramanujan complexes
Speaker: Alex Lubotzky (Maurice and Clara Weil chair in mathematics), Einstein Institute of Mathematics, Hebrew University
Abstract: Expander graphs in general, and Ramanujan graphs, in particular, have played a major role in combinatorics and computer science in the last 4 decades and more recently also in pure math.
Approximately 10 years ago a theory of Ramanujan complexes was developed by Li, Lubotzky-Samuels-Vishne and others.
In recent years a high dimensional theory of expanders is emerging. The notions of geometric and topological expanders were defined by Gromov in 2010 who proved that the complete d-dimensional simplical complexes are such. He raised the basic question of existence of such bounded degree complexes of dimension d>1.
Ramanujan complexes were shown to be geometric expanders by Fox-Gromov-Lafforgue-Naor-Pach in 2013, but it was left open if they are also topological expanders.
By developing new isoperemetric methods for "locally minimal small" F_2-co-chains, it was shown recently by Kaufman-Kazdhan-Lubotzky for small dimensions and Evra-Kaufman for all dimensions that the d-skeletons of (d+1)-dimensional Ramanujan complexes provide bounded degree topological expanders. This answers Gromov's original problem, but still leaves open whether the Ramanujan complexes themselves are topological expanders.
We will describe these developments and the general area of high dimensional expanders and some of it open problems.