Tel Aviv University Colloquium 
 
Hello everyone,
 
The next colloquium talk will be held on Monday, 28/11/2011, 12:15,
Schreiber 006, Tel Aviv University. The speaker is
 
     Ariel Yadin (Ben-Gurion University)
 
and the title of his talk is
 
     Harmonic growth on groups and stationary random graphs
 
His abstract is given below. Tea and coffee at 12:00, same room.
 
Hope to see you there. For information about future colloqiua, see
 <http://www.math.tau.ac.il/~klartagb/colloq_2011.html>
 
You are all welcome to suggest colloquium speakers, especially for the end
of January and for the second semester.
 
Yours,
Bo'az
 
--------------
 
Abstract: We study the structure of harmonic functions on certain
homogeneous graphs: Cayley graphs, and stationary random graphs which are
homogeneous "on average". Harmonic functions have been used to understand
the geometry of these objects. Two notable examples are: Kleiner's proof of
Gromov's theorem regarding polynomial volume growth groups, and the
invariance principle (CLT) for super-critical percolation clusters on Z^d.
 
We consider the following question: What is the minimal growth of a
non-constant harmonic function on a graph G (as above)? This question is of
course closely related to the Liouville property and Poisson-Furstenberg
boundary; a non-Liouville graph just means that there exist _bounded_
non-constant harmonic functions. A classical result of Kaimanovich & Vershik
relates the Liouville property on groups to sublinear entropy of the random
walk.
 
We show a simple but very useful inequality regarding harmonic functions and
entropy on a group. This inequality allows us to deduce many results rather
simply, among them:
1. A quantified version of one direction of Kaimanovich & Vershik
2. Groups (and stationary graphs) of polynomial growth have no sub-linear
non-constant harmonic functions.
3. Uniqueness of the "corrector" for super-critical percolation on Z^d.
 
Another question we address is how small can harmonic growth of groups be?
We are able to construct a group with a logarithmic harmonic growth. We also
have an argument why certain naive approaches cannot construct smaller than
logarithmic harmonic growth.
 
No prior knowledge is assumed, and we will define everything in the talk.
Joint work with Itai Benjamini, Hugo Duminil-Copin and Gady Kozma.
 
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Technion Math Net-2 (TECHMATH2)
Editor: Michael Cwikel   <techm@math.technion.ac.il> 
Announcement from:  <klartagb@post.tau.ac.il>