Mar 27: Nikolay Romanovskii, Novosibirsk State University

Hilbert's Nullstellensatz in algebraic geometry over rigid solvable groups
Abstract: The classical Hilbert's Nullstellensatz says: if $K$ is algebraically closed field and there is a system of polynomial equations over $K$, $\{f_i (x_1, \ldots, x_n) = 0 \ | \ i \in I \}$, then an equation $ f (x_1, \ldots, x_n) = 0 $ is a logical consequence of this system (satisfies all the solutions of the system in $K^n$) if and only if some nonzero power of $f$ belongs to the ideal $ (f_i \ | \ i \in I) $ of the ring $ K [x_1, \ldots, x_n]. $ One can say say that we give an algebraic method for constructing all logical  consequences of the given system of equations: $f$ is obtained from $f_i \ (i \in I)$ using the operations of addition, subtraction, multiplication by elements of $K [x_1, \ldots, x_n]$, and extraction of roots.
Our approach to Hilbert's theorem in algebraic geometry over groups is as follows.
1. We should consider some good class of equationally Noetherian groups, let it be a hypothetical class $\mathcal{K}$.
2. In this class, we need to define and allocate an algebraically closed objects and to prove that any group of $\mathcal {K}$ is embedded into some algebraically closed group. Hilbert's theorem should be formulated and proved for algebraically closed in $\mathcal{K}$ groups.
3. Further, let $G$ be an algebraically closed group in $\mathcal{K}$. We think about equations over $G$ as about expressions $v=1$, where $v$ is an element of the coordinate group of the affine space $G^n$.
4. Since an arbitrary closed subset of $G^n$ is defined in general not by a system of equations, but by a positive quantifier free formula (Boolean combination without negations of a finite set of equations) we should consider as basic blocks not equations, but positive formulas.
5. We should specify and fix some set of algebraic rules of deduction on the set of positive formulas over $G$.
6. If the above conditions Hilbert's theorem will consist in a statement that all logical consequences of given positive formula over $G$ are exactly the algebraic consequences.
We realized this approach in algebraic geometry over rigid solvable groups.

Friday, Oct 28, 16:00-17:00, Room 5417 Dima Savchuk (University of South Florida)

posted Oct 24, 2016, 6:34 PM by NY Group Theory

Title: Lamplighter groups and (bi)reversible automata from affine transformations of $\mathbb Z_p[[t]]$.

The ring $\mathbb Z_p[[t]]$ of formal power series over $\Z_p$ can be naturally identified with the boundary of $p$-ary rooted tree $T_p$. For each $a(t),b(t)\in\mathbb Z_p[[t]]$ with $b(t)$ being a unit, we consider the affine transformations of $\mathbb Z_p[[t]]$ defined by $f(t)\mapsto a(t)+f(t)\cdot b(t)$. This transformation defines automorphisms of $T_p$ that can be explicitly described by an automaton that is finite if and only if both $a(t)$ and $b(t)$ are rational power series. 

We prove that the multiplication by a power series corresponding to a rational function $p(t)/q(t)\in\mathbb Z_p(t)$ is defined by a finite automaton that is reversible if and only if $\deg p\leq \deg q$. In particular, if $\deg p=\deg q$ the corresponding automaton is bireversible. This covers several examples that were studied earlier. We also describe algebraic structure of corresponding self-similar groups generated by such automata and show that they are isomorphic to lamplighter groups of various ranks.

This is a joint result with Ievgen Bondarenko.

Friday, Oct 21, Pascal Weil, CNRS and Université de Bordeaux

posted Oct 18, 2016, 10:37 AM by NY Group Theory

The study of random algebraic objects sheds a different light on these objects, which complements the algebraic, but also the algorithmic points of view. I will discuss random finitely generated subgroups of free groups from several perspectives: when they are given by a random tuple of generators (of reduced words), and when they are given by a random Stallings graph. The Stallings graph of a subgroup H is a finite labeled graph uniquely associated with H, from which one can efficiently compute invariants of H. It is an interesting combinatorial object in and of itself, whose structure must be understood to enumerate and randomly generate Stallings graphs and subgroups. While both approaches to random subgroups, generators and Stallings graphs, are natural, they yield different distributions, and a different view of what ‘most subgroups’ look like.

Sep 30, Alexei Miasnikov (Stevens Institute), Room 5417, 16:00-17:00

posted Sep 23, 2016, 7:57 AM by NY Group Theory

“What the group rings know about the groups?”

How much information about a group G is contained in the group ring K(G) for an arbitrary field K? Can one recover the algebraic or geometric structure of G from the ring? Are the algorithmic properties of K(G) similar to that of G?  I will discuss all these questions in conjunction with the classical Kaplansky-type problems for some interesting classes of groups, in particular, for limit, hyperbolic, and solvable  groups.  At the end I will touch on the solution to the generalized 10th Hilbert problem in group rings and how equations in groups are related to equations in the group rings. The talk is based on joint results with O.Kharlampovich. 

Sep 23, Ben Steinberg, 16:00-17:00, Room 5417

posted Sep 19, 2016, 6:16 AM by NY Group Theory

Title: Homological and topological finiteness conditions for monoids


Homological and topological finiteness properties of groups has long been of interest in connection with topology.  Interest in homological finiteness conditions for monoids began with the Anick-Squier-Groves-Kobayashi theorem which says that a monoid with a finite complete rewriting system is of type $FP_{\infty}$.  Starting in the early nineties Pride, Otto, Kobayashi and Guba began to investigate homological finiteness properties of monoids in connection with complete rewriting systems (there is also some work of Ivanov and of Sapir). 

In group theory, one normally studies homological properties via topology by using Eilenberg-MacLane spaces.  For monoids, the work has been almost entirely algebraic in nature and for this reason progress on understanding finiteness conditions for such basic operations as free product with amalgamation has been slow.  

In this talk, we introduce the topological finiteness condition $F_n$ for monoids.  It extends the usual notion for groups and seems to be surprisingly robust.  We can then extend Ken Brown's topological proof of the Anick-Squier-Groves-Kobayashi theorem to monoids and we have made new progress on understanding finiteness properties of amalgamations, HNN extensions and HNN-like extensions (in the sense of Otto and Pride).  In the process we develop some very rudimentary Bass-Serre theory for monoids.

This is joint work with Bob Gray.

May 13. E. Zelmanov (U. San Diego), 16:00-17:00, Science Center, room 4102

posted May 5, 2016, 10:00 PM by NY Group Theory

Groups with Identities.

Abstract. We will discuss groups satisfying pro-p and prounipotent identities : examples, theory and possible applications. 

May 6 M. Bestvina (Utah), 16:00-17:00, Room 5417

posted May 2, 2016, 5:17 AM by NY Group Theory

Title: On the Farrell-Jones conjecture for mapping class groups
Abstract: I will try to describe what the Farrell-Jones conjecture is
about, and how one goes about proving it. Then I will try to outline a
proof of FJC for mapping class groups, which is work in progress,
joint with Arthur Bartels.

Apr.29 L. Babai (U. of Chicago), Science Center (4102), 16:00-18:00

posted Apr 20, 2016, 5:42 AM by NY Group Theory

Title:  A little group theory goes a long way: The group theory
   behind recent progress on the Graph Isomorphism problem
 One of the fundamental computational problems in the complexity
 class NP on Karp's 1973 list, the Graph Isomorphism problem (GI)
 asks to decide whether or not two given graphs are isomorphic.
 While program packages exist that solve this problem remarkably
 efficiently in practice (McKay, Piperno, and others), for
 complexity theorists the problem has been notorious for its
 unresolved asymptotic worst-case complexity: strong theoretical
 evidence suggests that the problem should not be NP-complete, yet
 the worst-case complexity has stood at $\exp(O(\sqrt{v\log v}))$
 (E. M. Luks, 1983) for decades, where $v$ is the number of
By addressing the bottleneck situation for Luks's algorithm, we
 recently reduced this ``moderately exponential'' upper bound to
 quasipolynomial, i.e., $\exp((\log v)^c)$.
The problem we actually solve in quasipolynomial time is more
 general: we solve the String Isomorphism problem (SI), introduced
 by Luks in his seminal 1980/82 paper in which he brough in-depth
 applications of group theory to bear on the GI and SI problems.
 The input to an instance of SI is a permutation group $G$ acting
 on a set $\Omega$ of $n$ elements, and a pair of strings, $x$ and
 $y$, over $\Omega$ (functions that map $\Omega$ to a finite
 alphabet).  The question is, does there exist a permutation in $G$
 that transforms $x$ into $y$ (``anagrams under group action'').
 ($G$ is given by a list of generators.)  As Luks pointed out, this
 problem is polynomial-time equivalent to the Coset Intersection
 problem: given two subcosets of the symmetric group $S_n$, decide
 whether or not their intersection is empty.
The following group theoretic lemma is at the heart of the new
Let $G$ be a permutation group of degree $n$ and $f$ an
 epimorphism of $G$ onto $A_k$, the alternating group of degree
 $k$.  We say that a point $p$ in the permutation domain on which
 $G$ acts is _affected_ by $f$ if the stabilizer $G_p$ is mapped by
 $f$ to a proper subgroup of $A_k$.  Let $U$ be the set of
 unaffected points and let $H$ be the pointwise stabilizer of $U$
 in $G.$
Unaffected Stabilizers Lemma.
    If $k > \max\{8, 2+log_2 n\}$ then $f$ maps $H$ onto $A_k.$
In the talk we outline the proof of this result and try to convey
 the basic idea, how, through the Lemma, the affected/unaffected
 dichotomy plays a central role in the design and analysis of
 the algorithm.
The proof of the lemma is elementary with reference to Schreier's
 Hypothesis that the outer automorphism group of every finite
 simple group is solvable.  Schreier's Hypothesis follows from the
 Classification of Finite Simple Groups (CFSG).  Under the slightly
 stronger assumption that $k > (log n)^c$ for some constant $c$,
 Laszlo Pyber recently announced a CFSG-free proof of the result.
The paper is available at  arXiv:1512.03547.

 Helpful reading:
E.M. Luks : Isomorphism of graphs of bounded valence can be
   tested in polynomial time.   J. Comp. Sys. Sci., 25:42--65, 1982.

M. Volkov (Ural Federal University and Hunter College CUNY), April 15, 16:00-17:00

posted Apr 8, 2016, 5:49 PM by NY Group Theory

Title: Algebraic properties of monoids of diagrams and 2-cobordisms.

Abstract: Partition of diageram monoids first appeared in 1937
in a paper by Brauer in which they serve as vector space bases
of certain associative algebras relevant in representation theory
of classical groups. Other species of diagram monoids were
invented by Temperley and Lieb in the context of statistical
mechanics in the 1970s and by Kauffman and Jones in the context
of knot theory in the 1980s. Since then diagram monoids have
revealed many other connections, e.g., with low-dimensional
topology, topological quantum field theory, quantum groups etc.
Recently, they have been intensively studied as purely algebraic
objects, and these studies have shown that diagram monoids are
quite interesting from this viewpoint as well.

In the talk, we first present geometric definitions for some
classes of infinite diagram monoids and then survey our results
on the finite basis problem for their identities. Whilst it is
not clear whether or not a study of the identities of infinite
diagram monoids may be of any use for any of their non-algebraic
applications, such a study has constituted an interesting challenge
from the algebraic viewpoint and required to develop new techniques.
We also report on a recent application of these new techniques to
the finite basis problem for the identities of monoids of 2-cobordisms.

M. Hagen (U. of Cambridge), Friday April 8, 16:00-17:00, Room 5417

posted Apr 2, 2016, 9:54 PM by NY Group Theory

Title: Curve complexes for cube complexes

Abstract: I'll discuss a hyperbolic space -- the "contact graph" --
associated to a CAT(0) cube complex.  Using the example of a
right-angled Artin group, I'll illustrate how the contact graph can help
one understand the large-scale geometry of a CAT(0) cube complex in very
much the same way that the curve graph of a surface can, by work of
Masu-Minsky, be used to understand the geometry of the mapping class
group.  This is joint work with Jason Behrstock and Alessandro Sisto.

Apr.1 A. Myropolska ( University of Paris-Sud) Room 5417, 16:00-17:00

posted Mar 27, 2016, 2:58 PM by NY Group Theory

Title: Nielsen and Andrews-Curtis equivalence in finitely generated groups
Abstract: Various aspects of geometric group theory lead to the study of the natural
action of Aut(F_n) on the set Epi(F_n, G) of generating n-tuples of a group
G generated by at least n elements. One of the main questions, raised in
the context, is the transitivity of this actions.
In the talk, we will give an introduction to the subject, then extensively
discuss its relation to the Andrews-Curtis conjecture and explain the
transitivity results for the class \emph{MN} of finitely generated groups
of which every maximal subgroup is normal (this includes nilpotent groups
and Grigorchuk-like groups).

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