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.

Feb 24, 16:00-17:00, Room 5417 I. Kapovich (UIUC)

posted Feb 19, 2017, 10:51 AM by NY Group Theory

 Dynamics on free-by-cyclic groups

We develop a counterpart of the Thurston-Fried-McMullen ``fibered face'' theory in the setting of free-by-cyclic groups, that is, mapping tori groups of automorphisms of finite rank free groups.  We obtain information about the BNS invariant of such groups, and construct a version of McMullen's ``Teichmuller polynomial'' in the free-by-cyclic context. The talk is based on joint work with Chris Leininger and Spencer Dowdall.

Feb 10, 16:00-17:00, Room 5417 J. Behrstock (CUNY)

posted Feb 6, 2017, 11:23 AM by NY Group Theory

 TitleAsymptotic dimension of mapping class groups

Abstract: The goal of this talk will be to describe our recent result proving that the 
asymptotic dimension of the mapping class group of a closed surface is at most quadratic in the genus (building on and strengthening a prior result of  Bestvina-Bromberg giving an exponential estimate). We obtain this result as a special case of a result about the asymptotic dimension of a general class of spaces, which we call hierarchically hyperbolic; this class includes  hyperbolic spaces, mapping class groups, Teichmueller spaces endowed with either the Teichmuller or the Weil-Petersson metric, fundamental groups of non-geometric 3-manifolds, RAAGs, etc. We will discuss the general framework and a sketch of how this machinery provides new tools for studying special subclasses, such as mapping class groups. The results discussed are joint work with Mark Hagen and Alessandro Sisto.

Feb 3, A. Miasnikov (Stevens), 4:15-5:15, Room 5417

posted Jan 28, 2017, 7:59 PM by NY Group Theory

“Homogeneous and isotypic groups and algebras”



The type of a tuple of elements of a group (or a ring) G  is the set of all formulas of the group (ring) language that hold on the tuple.  For example, elements in an algebraically closed field are either algebraic (in which case the type is determined by the minimal polynomial) or transcendental (the type is the correspondent set of all polynomial inequalities). Group elements may have much more interesting and complex types.   We say that G is defined by its types if any group (ring) isotypic to G, i.e., having the same types of elements as in G, is isomorphic to G.   I will discuss finitely generated groups defined by their types – there are a lot of such groups!  

A group (ring) G is homogeneous if any two isotypic tuples of elements of G are conjugated by an automorphism of G.  Homogeneous structures are well studied in model theory, usually they arise as completions or limits in some classes (fields of complex or real numbers, or Fraisse limits, or saturated structures).  My focus will be on homogeneous finitely generated groups and their algebras.  Finitely generated homogeneous groups (rings) are defined by their types, but the converse does not hold even for some hyperbolic groups.

The original motivation of this work goes back to model theory and universal algebraic geometry, but recent developments show that the topic is interesting in its own right.   The talk is based on joint results with Olga Kharlampovich and Nikolay Romanovskii.

Dec 9, MAD, Room C-201, 2:00-5:10

posted Dec 6, 2016, 9:58 PM by NY Group Theory

Dec 2. M. Gromov (NYU and IHES)

posted Nov 21, 2016, 9:33 PM by NY Group Theory

Isoperimetric inequalities in group algebras

Nov 18. Rachel Skipper (Binghamton University)

posted Nov 14, 2016, 4:02 AM by NY Group Theory

Title: The congruence subgroup problem for a family of branch groups
Abstract: A group, G, acting on a regular rooted tree has the congruence
subgroup property (CSP) if every subgroup of finite index contains the
stabilizer of a level of the tree. When the subgroup structure of G
resembles that of the full automorphism group of the tree, additional
tools are available for determining if G has the CSP.
In this talk, we look at the Hanoi towers group which has fails to have
the CSP in a particular way. Then we will generalize this construction to
a new family of groups and discuss the CSP for them.

Friday Nov 11 Shamgar Gurevich (Madison and Yale)

posted Nov 7, 2016, 6:16 PM by NY Group Theory

Title: Small Representations of finite classical groups.
Abstract: Suppose you have a finite group G and you want to study certain related structures (random walks, expander graphs, word maps, etc.). In many cases, this might be done using sums over the characters of G. A serious obstacle in applying these formulas seemed to be lack of knowledge over the low dimensional representations of G. In fact, the “small" representations tend to contribute the largest terms to these sums, so a systematic knowledge of them might lead to proofs of some important conjectures. The “standard" method to construct representations of finite classical group is due to Deligne and Lusztig (1976). However, it seems that their approach has relatively little to say about the small representations.  
This talk will discuss a joint project with Roger Howe (Yale), where we introduce a language to define, and a new method for systematically construct, the small representations of finite classical groups. 
I will demonstrate our theory with concrete motivations and numerical data obtained with John Cannon (MAGMA, Sydney) and Steve Goldstein (Scientific computing, Madison).

Nov 4, Denis Ovchinnikov (Stevens Institute)

posted Oct 28, 2016, 12:06 PM by NY Group Theory

Title: Random nilpotent groups. 


Notion of a random (finitely presented) group gives a well-established  approach to think about the question "what most groups look like". In standard models (few relator and density models), with probability 1 (or, more precisely,  asymptotically almost surely), these groups turn out to be either hyperbolic or trivial. This way, if one desires to study the question "what most groups in class N look like", for some class of non-hyperbolic groups N, classical model cannot be applied directly.

I will discuss the question above for the class of (finitely generated) nilpotent groups, provide an outline of known approaches to define a random nilpotent group, and present our results about typical groups in some of these models. 

The talk is based on joint work with Albert Garreta-Fontelles and Alexei Miasnikov.

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.

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