virtuell

A challenging mathematical problem is to introduce meaningful curvature notions on discrete spaces like graphs (which do not have any smooth structure). An analytic Ricci type curvature, based on Bochner's identity involving a Laplacian (or more generally the generator of a Markov semigroup) and going back to Bakry and Emery in 1985, was initially studied in the context of graphs by Elworthy, Schmuckenschlaeger, Lin/Yau and others. The first aim of this talk is to motivate and to introduce this curvature notion. We will then present a graph theoretical version of Buser's inequality. Buser's inequality links non-negative Ricci curvature, a fundamental isoperimetric constant named after Jeff Cheeger, and the first positive eigenvalue of the Laplace operator. Buser provided back in 1982 two proofs of his inequality (both of them geometric in flavour - one was using Geometric Measure Theory and the Heintze-Karcher inequality while the other was more elementary). An alternative analytical proof in the graph setting and based on the heat semigroup was later given by Klartag/Kosma/Ralli/Tetali in 2016. They utilized a heat semigroup reformulation of Bakry-Emery curvature and their proof is analogous to a corresponding analytical proof by Ledoux in 2004 in the manifold case. This is one of the many instances when the seemingly innocent heat equation is of central importance in completely different and new contexts. If everything goes according to plan, I will finish the talk with an optimal ratio estimate between higher eigenvalues and the first Laplace eigenvalue by combining an improved Cheeger inequality for graphs due to Kwok/Lau/Lee/Oveis Gharan/Trevisan (2013) with the above mentioned Buser inequality. This talk is a review of joint work with Shiping Liu (USTC, Hefei, China).

Norbert Peyerimhoff

Durham University