Saturdays, 1pm; on Zoom
Topic: Geometric Group Theory
Source: Office Hours with a Geometric Group Theorist edited by Matt Clay and Dan Margalit
Contact UMS (Email Anda Tenie)
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Date | Speaker | Title | Abstract |
June 6 |
Anda Tenie
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Introduction |
We will first introduce Summer UMS and then decide on the logistics for the next weeks. We will go over some potential textbooks we could cover and then pick one by vote. Every member will then have the opportunity to sign up to give a talk.
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June 13 |
Anda Tenie
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Groups viewed as Symmetries |
We will begin the talk with a general discussion of groups focusing on viewing them as the group of symmetries of some object. We will then talk about infinite groups, group homomorphisms as well as group presentations. Throughout this lecture, I will also mention some interesting examples such as the Coxeter groups, Braid groups, and Lamplighter groups.
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June 20 |
Zhenfeng Tu
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Cayley Graphs and Groups as Metric Spaces |
In this talk, we will begin with a brief review of group actions and basic concepts of graphs. We will then introduce graph automorphisms, Cayley graphs, and, in particular, the Cayley graph of F_2. I will also mention how to introduce matrices on graphs and some relationships between geometric properties and algebraic properties of groups.
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June 27 |
Anton Wu
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Subgroup of a free group is free: a topological proof |
We'll define the fundamental group and see that the fundamental group of any connected finite graph is a free group. Then we'll show how, given any finitely-generated subgroup of a free group, we can construct a graph whose fundamental group is isomorphic to this subgroup; this construction will not only prove the famous result that every such subgroup is a free group but also answer specific algebraic questions about the subgroup using the properties of the graph.
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July 4 |
Iris Rosenblum-Sellers
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Understanding the automorphism group of a free group geometrically |
We start by defining the automorphism group of a group, a way to describe the symmetries of a group itself; we address inner and outer automorphisms as well as a few finite cases. We then move to the specific case of the automorphism group of a free group, and in particular, the geometric description of the free group Fn as the rose on n petals. We use this picture to describe the *dynamics* of Aut(Fn); what the repeated application of a single automorphism does to a loop in the rose, by studying their lengths using tools from linear algebra.
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July 11 |
Destine Lee
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The Ping-Pong Lemma |
What can be learned about a group by studying its action on a space? We encountered one response to this question in Office Hour 2: A group is torsion free if it has a free action by isometries on Euclidean space. The ping-pong lemma is another result in the same spirit but instead provides sufficient conditions for a group to be free. After I present the statement and proof of the ping-pong lemma, we take a short detour into the world of Möbius transformations and Schottky groups. Finally, I apply the ping-pong lemma to show that Schottky groups are free, a baby version of Maskit's 1967 "A characterization of Schottky groups."
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July 18 |
Cassandra Marcussen
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Asymptotic Dimension |
In this talk, we introduce asymptotic dimension, interpreting the topological notion of dimension for finitely generated groups. We start by considering dimension using the geometry of certain spaces. We then move to thinking about dimension in regards to how open sets overlap in spaces. Then, we discuss large-scale geometry, considering different Cayley graphs for the integers (Z) and their impact on our understanding of the dimension of Z. After introducing some topological notions of dimension, we then finally define asymptotic dimension of metric spaces and of a group, using the intuition and questions raised previously to formulate a large-scale analog of dimension.
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July 25 |
Anda Tenie
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Mapping Class Groups |
The goal of this talk is to introduce the Mapping class group, that is, the group of symmetries of a surface. We will start with a brief introduction to surfaces and maps between them and we will then define the mapping class group as a quotient of some subgroup of Homeo(S). We will be particularly interested in the generating set for this group. We will state the general result of finite generation by Dehn twist for surfaces and then see that this holds in some particular cases.
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