Appalachian set theory
Appalachian set theory
Saturday, November 22, 2008
9:30 a.m. - 6 p.m. (first coffee at 8:45 a.m.)
406 Malott Hall ("the math building")
Vladimir Pestov : "An introduction to hyperlinear and sofic groups"
List of participants in this workshop
Lecture notes by Vladimir Pestov and Aleksandra Kwiatkowska (PDF; Final version 11/09)
This workshop will be dedicated to two new classes of (discrete, countable) groups, introduced fairly recently, the hyperlinear and sofic groups. Their interest for set theorists, model theorists and logicians stems from the fact that groups from both classes are most naturally defined as subgroups of ultraproducts of certain classical groups (even if alternative descriptions are also possible).
The ultraproducts in question are metric ultraproducts, formed very much like ultraproducts of Banach spaces. Hyperlinear groups can now be defined as those groups isomorphic to subgroups of ultraproducts of unitary groups of finite rank U(n) equipped with the Euclidean distance between matrices (normalized so as to make U(n) sit on a unit sphere), with regard to a non-principal ultrafilter on natural numbers. Similarly, sofic groups are those groups isomorphic to subgroups of metric ultraproducts of symmetric groups Sn of finite rank n, equipped with the normalized Hamming distance between permutations (counting the number of coordinates where two permutations differ).
Hyperlinear groups are motivated by Connes' Embedding Conjecture, which is one of the central unresolved problems in theory of operator algebras, while sofic groups were introduced by Gromov in connection with Gootschalk's Surjunctivity Conjecture (stating that no shift dynamical system AG, where G is a countable group and A is a finite discrete space, contains a proper isomorphic copy of itself). It is still unknown, for example, if every group is hyperlinear and/or sofic, and generally open questions outnumber results.
The workshop lectures will roughly follow the recent survey article by the speaker:
However, there will be some additional material not found in the survey, and some topics may be presented differently.
Hyperlinear and sofic groups: a brief guide. - arXiv:0804.3968v7 [math.GR], 28 pages, June 2008, accepted, to appear in The Bulletin of Symbolic Logic.
For those interested in background reading:
- Basics of ultrafilters and ultraproducts as found in many sources on model theory, e.g., Bell and Slomson's Models and Ultraproducts which book is
available on Amazon.com for an incredible $14.78 (in paperback).
Maybe the soon-to-be published Model Theory for Metric Structures by Ben Yaacov, Berenstein, Henson, and Usvyatsov is even more relevant, especially since one
can still download it for free. I suggest an ε-neighbourhood of pages 22-25, for any value of
- The very beginnings of topological and symbolic dynamics, which can be found in practically any book treating abstract topological dynamics. For instance,
everything we'll ever need can be found in Glasner's Ergodic Theory via Joinings or Auslander's Minimal Flows and Their Extensions, but both books are
an overkill. I need to think a bit of something more accessible here. Here is a list of notions whose knowledge will help: action of a group on a compact space,
morphism of G-spaces, a shift (symbolic dynamical system), topological entropy of a subshift.
- Some infinite group theory, including basic constructions of group theory, examples (e.g. finite groups of permutations), and such concepts as an amenable
(discrete) group and residually finite group. Many other classes of groups will be mentioned in passing in connection with open problems (hyperbolic groups,
one-relator groups, topologically amenable groups...)
For instance, Pierre de la Harpe's Topics in Geometric Group Theory is an excellent book both to read and to leaf through, and it is
not expensive either. There are many other sources of course.
Basic topological group theory notions: a topological group, left-invariant and bi-invariant metrics, the unitary group U(n) equipped with the uniform and
Abstract Harmonic Analysis, Vol. I by Hewitt and Ross is an excellent source. The unitary group, as well as a number of other topics on the list, is treated in
Essential Results in Functional Analysis
by Robert Zimmer.
- Rudiments of C*-algebras and von Neumann algebras, for instance, the first few chapters of Vaughan Jones' 2003 lectures available on the web for download. However, the main concepts, results and open questions pertaining
to hyperlinear groups can be stated and understood without referring to operator algebras: they are important for understanding the origin and motivation.
Travel and lodging
We had blocks of fifteen rooms set aside at each of the following two hotels
until October 21.
In both cases, the group name is "Appalachian set theory",
the arrival date is Friday, November 21,
and the departure date is Sunday, November 23.
$99 per night plus 13% tax
1020 Ellis Hollow Rd.
Ithaca, NY 14850
Hilton Garden Inn
$119 per night plus 13% tax
130 E. Seneca St.
Ithaca, NY 14850
- Other useful websites include:
- Getting to Malott Hall:
Malott is at the corner of Tower Road and Garden Ave.
You should enter Malott via the doors which are
closest to the Tower Rd/Garden Ave intersection. Room 251 is one of the
first lecture halls you will encounter. This
entrance should be unlocked. The other entrance (near 406) likely will be
The Best Western offers a shuttle service to Cornell (you should let
them know in advance).
The walk to the department is a little over 1.5 miles.
You can also find a map of campus here:
Generally you do not need a parking permit to park on Campus
on the weekend. There are, however, a large number of exceptions.
Often they are marked with "reserved" or with a "moon" symbol and
additional, self explanatory text. There is a parking lot adjacent to
Malott (accessible via Garden Ave), but parking is limited there and it
fills up quickly on the weekend. Additional parking can be found either
East of Malott on Tower road or in the parking garage off of Hoy road
(soon after you enter Campus if you enter where Dryden turns South and
- Coffee shop: There will be coffee, juice, bagels, etc. at the
department starting at 8:45am. There is also a coffee shop (the "Queen
of Tarts") on Maple (on the walking route) near Veterans Pl. There is
also an excellent bagel shop at College and Oak (it also has a branch in
the shopping plaza where the Best Western is located).
Participant travel support
Funds provided by the National Science Foundation will be used
to reimburse some participant transportation and lodging expenses.
Priority will be given to students
and faculty who do not hold federal research grants.
Please request such funds as far in advance of the meeting as possible
by sending the following information to
James Cummings and
Ernest Schimmerling by email.
- Your name, university affiliation,
mailing address, phone number and email address
- Your professional status and
- undergraduate students: please describe your background in set theory
- graduate students: please tell us your year and the name of
your thesis advisor if you have one
- faculty: please tell us whether you hold a federal research grant
- A brief statement about your interest in the workshop