# Difference between revisions of "Geometric group theory seminar@SNU (Past)"

For current seminar, please visit http://snutop.cayley.kr

This is an archive page of SNU Geometry and Topology Seminar.

# 2017 - present

SNU Geometry, Topology and Dynamics Seminar http://snutop.cayley.kr

# 2016

## Keivan Mallahi-Karai (Jacobs University)

• December 12th (Monday), 4:00 pm - 5:00 pm, Bldg. 129-301
On the chromatic number of structured Cayley graphs
The famous Hadwider-Nielson problem asks for the smallest number $n$ such that the points of the plane can be partitioned into $n$ sets such that no two points in the same set are at distance $1$. This problem, among many similar problems, can be reformulated in terms of the chromatic number of certain Cayley graphs. In this talk, I will discuss two recent results in this direction: one pertains to the Borel chromatic number of the unit-distance graph associated to quadratic forms over local fields. The other result is a lower bound on the chromatic number of Cayley graphs of finite groups of Lie type.

## Lim Seonhee (Seoul National University)

• December 6th (Tuesday), 4:00 pm - 5:00 pm, Bldg. 129-406
Around quantitative Oppenheim conjecture
We will explain properties of geometry and dynamics on homogeneous space that are used to prove a quantitative version of Oppenheim conjecture, which was proved by Eskin-Margulis-Mozes for the real case. We will introduce analogous statement in S-arithmetic case, which we proved in a joint work with K. Mallahi-Karai and J. Han. We will report preliminary observations for the case of signature (2,2).

## Lei Zhao (Chern Institute of Mathematics)

• November 30 (Wednesday), 4:00 pm - 5:00 pm, Bldg. 129-406 (to be confirmed)
Relative equilibrium motions and real moment map geometry
Chenciner and Jimenez Perez showed that the range of the spectra of the angular momenta of all the rigid motions of a fixed central configuration in a general Euclidean space form a convex polytope. In this note we explain how this result follows from a general real convexity theorem of O Shea and Sjamaar in symplectic geometry. Finally, we provide a representation theoretic description of the pushforward of the normalized measure under the real moment map for Riemannian symmetric pairs.

## Hyowon Park (Seoul National University)

• November 22 (Tuesday), 4:00 pm - 5:00 pm, Bldg. 129-104
Finite index subgroups of right-angled Artin groups
Right-angled Artin groups are the graph product whose vertex groups are infinite cyclic groups, which are defined by finite simple graphs. A finite simple graph is called thin-chordal if it has no induced subgraphs that are isomorphic to either the cycle with 4 vertices or the path with 4 vertices. We will discuss group properties related to right-angled Artin groups from thin-chordal graphs. We show that a right-angled Artin group is defined by a thin-chordal graph if and only if every finite index subgroup of the group is a right-angled Artin group.

## Sang-hyun Kim (Seoul National University)

• November 16 (Wednesday), 4:30 pm - 5:30 pm, Bldg. 27-116
Flexibility of projective representations
For which countable group G, does the moduli space
X(G) = Aut(G) \ Hom(G,PSL(2,R)) / Inn(PSL(2,R))
contain (uncountably many distinct equivalence classes of) dense faithful representations? Groups with such properties are called flexible. We prove combination theorems for flexible groups, and show that most Fuchsian groups and all limit groups (possibly with torsion) are flexible. The diversity of quasi-morphisms on those groups will follow. Implications for word-hyperbolic groups and mapping class groups will also be discussed. Joint with Thomas Koberda and Mahan Mj.

## Paul Jung (KAIST) (Topology/dynamics joint seminar)

• November 8 (Tuesday), 4 pm - 5 pm, Bldg. 129-104
An alpha-stable limit theorem for Sinai billiards with cusps
We consider the dynamical system of Sinai billiards with a cusp where two walls of the billiard table meet at the vertex of a cusp and have zero one-sided curvature, thus forming a "flat point" at the vertex. For Holder continuous observables (random variables), we show that the properly normalized Birkhoff sums of stationary variables, with respect to the so-called ergodic billiard map, converge in distribution to a totally skewed alpha-stable law, for some alpha between 1 and 2.

## Yoshifumi Matsuda (Aoyama Gakuin University)

• October 24 (Monday), 4 pm - 5 pm, Bldg. 27-325
Bounded Euler number of actions of 2-orbifold groups on the circle
Burger, Iozzi and Wienhard defined bounded Euler number for actions of the fundamental group of connected oriented surfaces of finite type possibly with punctures on the circle by orientation preserving homeomorphisms. Bounded Euler number can be extended to actions of 2-orbifold groups. We discuss Milnor-wood type inequality and rigidity phenomenon involving bounded Euler number for actions of certain 2-orbifold groups, such as the modular group.

## JaeChoon Cha (Postech)

• October 6 (Thursday), 4 pm - 5 pm, Bldg. 129-101
4-manifold topology and disk embedding (Colloquium)
TBA

## BoGwang Jeon (Columbia University)

• August 8 (Monday), 11 am - 12 pm, Bldg. 129-301
The Unlikely Intersection Theory and the Cosmetic Surgery Conjecture
The main result of this talk is the following theorem:
Let $$M$$ be a 1-cusped hyperbolic 3-manifold whose cusp shape is not quadratic, and $$M(p/q)$$ be its $$p/q$$-Dehn filled manifold. If $$p/q$$ is not equal to $$p'/q'$$ for sufficiently large $$|p|+|q|$$ and $$|p'|+|q'|$$, there is no orientation preserving isometry between $$M(p/q)$$ and $$M(p'/q')$$.
This resolves the conjecture of C. Gordon, which is so called the Cosmetic Surgery Conjecture, for hyperbolic 3-manifolds belonging to the aforementioned class except for possibly finitely many exceptions for each manifold. We also consider its generalization to more cusped manifolds. The key ingredient of the proof is the unlikely intersection theory developed by E. Bombieri, D. Masser, and U. Zannier.

## Sejong Park (National University of Ireland Galway)

• August 2 (Tuesday), 4 - 5 pm, Bldg. 129-301
Toward a structure theorem for double Burnside algebras
Given a finite group $$G$$, the $$(G, G)$$- bisets form the double Burnside ring $$B(G, G)$$ with multiplication given by "tensor product over $$G$$". Unlike the Burnside ring $$B(G)$$, not much is known about the ring structure of $$B(G, G)$$. Boltje and Danz (2013) gave a "ghost map" for $$B(G, G)$$ over rationals when $$G$$ is cyclic. I will describe their result in a more conceptual form and show how this approach can be applied to some noncyclic cases. This is a joint work with Goetz Pfeiffer and Brendan Masterson.

## Insuk Seo (UC Berkeley)

• July 20 (Wednesday), 4 - 5 pm, Bldg. 129-301
Metastable behavior of the dynamics perturbed by a small random noise
We consider a class of stochastic processes, which can be regarded as the perturbation of deterministic dynamics in a potential field. These processes exhibit a phenomenon known as metastable behavior if the potential field has several local minima. Metastable behavior is the phenomenon in which a process starting from one of local minima arrives at the neighborhood of the global minimum after a sufficiently long time scale. The precise asymptotic analysis of this transition time has been known only for the reversible dynamics, based on the potential theory of reversible Markov processes. In this presentation, we review this metastability theory for reversible dynamics, and introduce our recent generalization of this theory for non-reversible processes. (joint work with C. Landim)

## Urs Frauenfelder (University of Augsburg)

• July 15 (Friday), 11am - 12 pm, Bldg. 129-301
Periodic orbits in the restricted three body problem and Arnold's J^+ invariant.
This is joint work with Kai Cieliebak. After regularization periodic orbits in the restricted three body problem become knots in RP^3. Their projection to position space is a curve in the plane. We show that Arnold's J^+ invariant is invariant under homotopies of periodic orbits and gives an additional invariant to their knot type in RP^3.

## Thomas Koberda (University of Virginia)

• July 14 (Thursday), 4 - 5 pm, Bldg. 129-301
Group actions on low dimensions I
• July 28 (Thursday), 4 - 5 pm, Bldg. 129-406
Group actions on low dimensions II

## Kasra Rafi (University of Toronto)

• July 12 (Tuesday), 4 - 5 pm, Bldg. 129-301
The Teichmüller diameter of the thick part of moduli space
We study the shape of the moduli space of a surface of finite type. In particular, we compute the asymptotic behavior of the Teichmüller diameter of the thick part of the moduli space. This turns out to be closely related to diameter of the space of trivalent graphs equipped with simultaneous whitehead move metric.

## Jaesuk Park

• May 20 (Friday), 2 - 4 pm, Bldg 25-103
Non-commutative quantum field theory and Chen’s iterated path integrals
Iterated path integrals, generalising the familiar line integrals, are functions in the path space of smooth manifold which notion has introduced and used to extend de Rham cohomology theory to a homotopy theory on the fundamental group level by by K.-T. Chen. The theory has found many interesting applications, beyond algebraic topology, in algebraic geometry and number theory. Theory of homotopy category of homotopy QFT algebras is this speaker’s attempts to understand quantum field theory mathematically. In this lecture I will show that iterated integral(s) is quantum expectation of a (0+0)-dimensional non-commutative QFT obtained by certain quantisation of the algebra of differential forms on a manifold such that Chen’s homotopy functionals are equivalentto quantum correlation functions. Both Chen’s theory and NC QFT theory will be introduced beforehand.

## Mounir Nisse

• May 19 (Thursday), 4 - 6 pm, Bldg 25-103
Amoebas and Coamoebas
Amoebas (resp. coamoebas) are the image under the logarithmic (resp. argument) map of algebraic (or analytic) varieties of the complex algebraic torus.They inherit some algebraic, geometric, and topological properties of the variety itself. In this introductory talk, I will define these two objects and tell you some of their nice properties and focalize myself on coamoebas. I will give several examples in the case of plane algebraic curves.
• May 20 (Friday), 4 - 6 pm, Bldg 25-103
On Coamoebas of Algebraic Hypersurfaces
I will describe coamoebas of algebraic complex hypersurfaces using their decomposition as pairs-of-pants shown by Mikhalkin. This talk will be focus on plane algebraic curves.

## Koji Fujiwara (Kyoto University)

• May 11 (Wednesday), 5 - 6 pm, Bldg 129-104
Handlebody subgroups in mapping class groups
• May 12 (Thursday), 4 - 5 pm (colloquium), Bldg 129-101
Quasi-homomorphisms into non-commutative groups
A function from a group $$G$$ to integers $$\mathbb{Z}$$ is called a quasi-morphism if there is a constant $$C$$ such that for all $$g$$ and $$h$$ in $$G$$, $$|f(gh)-f(g)-f(h)| < C$$. Surprisingly, this idea has been useful. I will overview the theory of quasi-morphisms including applications.

Then we discuss a recent work with M. Kapovich when we replace the target group from $$\mathbb{Z}$$ to a non-commutative group, for example, a free group.

## Min Lee (University of Bristol)

• May 4 (Wednesday) 2-3pm, 129-406
Effective equidistribution of primitive rational points on expanding horospheres
The limit distribution of primitive rational points on expanding horospheres on SL(n,Z)\SL(n, R) has been derived in the recent work by M. Einsiedler, S. Mozes, N. Shah and U. Shapira. For n=3, in our joint project with Jens Marklof, we prove the effective equidistribution of q-primitive points on expanding horospheres as q→∞. Our proof relies on Fourier analysis and Weil's bound on Kloosterman sums. We will also discuss applications in number theory.

## Jae Choon Cha (Postech)

• Apr 27 (Wednesday), 5 - 6 pm, Bldg. 129-104
Quantitative topology and Cheeger-Gromov universal bounds
I will begin with a quick introduction to the Cheeger-Gromov rho invariants from a topological viewpoint, and then present recent quantitative results on how they are related to triangulations and Heegaard splittings of 3-manifolds. I will also discuss quantitative bordism theory and an algebraic notion of controlled chain homotopy, which are the key ingredients of the proofs. Applications to topology of dimension 3 and 4 will be discussed if time permits.

## Richard M. Weiss (Tufts University)

• Apr 5 (Tuesday), 5 - 6 pm, , Bldg. 129-104.
Simple groups and buildings

We will describe the role of buildings in the study of simple groups, both finite and algebraic. We will focus on buildings of rank two, which can be easily defined in graph theoretical terms, and on groups of small rank. In particular, we will describe how the Suzuki groups, the Ree groups and a third family in this sequence arise in this context.

## Masato Mimura (Tohoku University)

• Mar 22 (Tuesday), 3:30 - 4:45 pm, Bldg. 24-207
Expanders and Margulis' construction
Expanders (or, a family of expander graphs) are "sparse but high-connected" finite graphs. They have been showed to serve as key objects in many fields of pure/applied mathematics: for instance, geometric group theory, coarse geometry, low-dimensional topology, rigidity, random walks, and computer sciences. In the first talk, we introduce this concept. The first concrete example of expanders was provided by G. A. Margulis in 1973, and his construction is based on Cayley graphs of (finite) groups and Kazhdan's property (T) for finitely generated groups, which is equivalent to the fixed point property (FHilb) relative to Hilbert spaces. We explain his argument.
• Mar 23 (Wednesday), 2 - 3 pm, , Bldg. 129-406
Strong algebraization of fixed point properties
Since Shalom's breakthrough of Bounded generations and Kazhdan's property (T) in 1999, it had been a big problem to remove any form of "bounded generation" condition from algebraization processes in proving fixed point property. In this talk, we provide the affirmative resolution of this problem (arXiv:1505.06728).
Applications of our main theorem ("strong algebraization") include non-commutative universal lattices, and (Kac-Moody-)Steinberg groups over finitely generated unital ring.
• Mar 24 (Thursday), 3:30 - 4:45 pm, Bldg. 24-207
Non-commutative universal lattices and unbounded rank expanders
From Margulis' construction of examples above, the following natural problem arises: "Can special linear groups over finite fields of unbounded rank form expanders?" This was a long-standing problem, and M. Kassabov resolved this problem in the affirmative in 2005. Later, M. Ershov and A. Jaikin in 2010, furthermore, obtained the ideal answer to the problem above is, in fact, in the scope of Margulis' argument. They did it by establishing property (T) for the groups so-called non-commutative universal lattices (NCUL).
In the second talk, we focus on the NCUL, and its relation on the problem above. We discuss some difficulty in establishing property (T) for that group.

## Ilya Gekhtman (Yale Univ. and Univ. of Bonn)

• March 18 (Friday) 2-3pm, 129-301. Dynamics of convex cocompact subgroups of mapping class groups

## Giulio Tiozzo (Yale Univ.)

• March 16 (Wednesday) 2-3pm, 129-406. Random walks on weakly hyperbolic groups
• Mar 17 (Thursday) 4-5pm, 129-101 auditorium (Colloquium talk)

Random walks on weakly hyperbolic groups

Given a group of isometries of a metric space, one can draw a random sequence of group elements, and look at its action on the space. What are the asymptotic properties of such a random walk?

The answer depends on the geometry of the space. Starting from Furstenberg, people considered random walks of this type, and in particular they focused on the case of spaces of negative curvature.

On the other hand, several groups of interest in geometry and topology act on spaces which are not quite negatively curved (e.g., Teichmuller space) or on spaces which are hyperbolic, but not proper (such as the complex of curves).

We shall explore some results on the geometric properties of such random walks. For instance, we shall see a multiplicative ergodic theorem for mapping classes (which proves a conjecture of Kaimanovich), as well as convergence and positive drift for random walks on general Gromov hyperbolic spaces. This also yields the identification of the measure-theoretic boundary with the topological boundary.

## Junehyuk Jung (KAIST)

• Mar 16 (Wednesday), 5 - 6 pm, Bldg. 129-104. Nodal domains of eigenfunctions on chaotic billiards
Abstract

In this talk I’ll first go over some problems and related results in quantum chaos. Then I’ll explain how one can combine Quantum Ergodicity and Bochner’s theorem to prove that the number of nodal domains of quantum ergodic sequence of even eigenfunctions tends to infinity as the eigenvalue λ → +∞. In particular, this implies that the number of nodal domains of Maass-Hecke eigenforms grows with the eigenparameter. This talk is based on the joint works with S. Zelditch and with S. Jang.

## Hyungryul Baik (MPI Bonn)

• Mar 3, 3:30 - 4:45, Room 24-207, Orderability
• Mar 8, 3:30 - 4:45, Room 24-207, Laminarity
• Mar 8, 5:00 - 6:00, Room 129-301, Convergence group

Abstract

Thursday, March 3, 3:30 - 4:45, Room 24-207 : Orderability

- Dynamical criterion
- application of Kurosh's theorem to free product
- Burns-Hale theorem
- application to 3-manifold groups via Scott's core theorem


Tuesday, March 3, 3:30 - 4:45, Room 24-207 : Laminarity

- Geodesic laminations on the surface
- characterization of geodesics laminations as saturated subsets of PTS
- characterization of geodesic laminations as subset of Mobius band
- constructing many very full laminations on surfaces
- constructing virtual actions of fibered 3-manifold groups


Tuesday, March 3, 5:00 - 6:00, Room 129-301 : Convergence group

- Convergence group theorem
- Convergence property from laminarity
- Moore's theorem and Abstract Cannon-Thurston map
- Convergence property on S^2 and the connection to Cannon's conjecture


## 강정수

• February 16 - 18, 4:30 - 6pm, Room 129-301.
• Symplectic capacities, Old and New I, II, III

Abstract

We give introduction to symplectic capacities and its recent developments.

## Thomas Koberda (University of Virginia)

• January 7, 4 pm - 6 pm, 129-301, Exotic quotients of surface groups

Abstract

I will explain how to use TQFT representations of mapping class groups to produce linear representations of surface groups in which every simple closed curve has finite order, but which have infinite image. As a corollary, I will show how to produce covers of surfaces where the integral homology is not generated by pullbacks of simple closed curves on the base. This talk represents joint work with R. Santharoubane.

## HyunKyu Kim (KIAS)

• Topological approach to seed-trivial mutation sequences in cluster algebras
• January 4, 2 pm, 129-301.

# 2015

At SNU.

## November 25, Hyun-shik Shin (KAIST)

• Pseudo-Anosov mapping classes not arising from Penner's construction
In this talk, we will discuss one property that is shared by all pseudo-Anosov mapping classes from Penner's construction. That is, all Galois conjugates of stretch factors of pseudo-Anosov mapping classes arising from Penner’s construction lie off the unit circle. As a consequence, we show that for all but a few exceptional surfaces, there are examples of pseudo-Anosov mapping classes so that no power of them arises from Penner’s construction. This resolves a conjecture of Penner. This is a joint work with Balazs Strenner.
• 4 pm, 129-104.

## August 9–13, Geometric Topology Fair in Korea

### Minicourses

• Fujiwara, Koji (Kyoto University), Contracting geodesics and acylindrical actions
• Wilton, Henry (University of Cambridge), Profinite completions of 3-manifold groups

### Research Talks

• Kim, Sungwoon (KIAS), Simplicial volume, Barycenter method and Bounded cohomology
• Kuessner, Thilo (KIAS), Chern-Simons invariants of 3-manifold groups in SL(4,R)
• Kwon, Sanghoon (Seoul National University), Effective Mixing and Counting in Trees
• Ohshika, Ken'ichi (Osaka University), Accumulation of closed 3-manifolds within character varieties

## January 16–21, Thomas Koberda (Yale)

• Lecture I. The relationship between right-angled Artin groups and mapping class groups
In this general talk, we will give an overview of work joint with S. Kim concerning the relationship between right-angled Artin groups and mapping class groups. We will consider the broad question of, what right-angled Artin subgroups does a mapping class group or a right-angled Artin group admit?
• Lecture II. The curve complex for a right-angled Artin group
In this talk we will give a more in depth discussion of the analogy between right-angled Artin groups and mapping class groups, through the comparison of the geometry of the extension graph and the curve complex.
• Lecture III. Convex cocompactness for subgroups of right-angled Artin groups
In this talk we will discuss a new result joint with J. Mangahas and S. Taylor which characterizes finitely generated subgroups of right-angled Artin groups which have quasi-isometric orbit maps on the extension graph. These are analogous to convex cocompact subgroups of mapping class groups as defined by B. Farb and L. Mosher, and which are still rather poorly understood despite having attracted so much attention in recent years.
• Jan 16 (2pm), 19 (4pm), 21 (4pm), Bldg 129-301

## January 13, BoGwang Jeon (Columbia)

• Hyperbolic three manifolds of bounded volume and trace field degree
In this talk, I present my recent proof of the conjecture that there are only a finite number of hyperbolic three manifolds of bounded volume and trace field degree.
• 4 pm, 129-301

# 2014

At SNU.

## December 9, Piotr Przytycki (McGill University)

• Balanced wall for random groups
Gromov showed that one way to obtain a word-hyperbolic group is to choose a presentation "at random". I will survey random group properties in Gromov's model at various values of the density parameter. We will then focus on Ollivier-Wise cubulation of random groups for density parameter <1/5. I will indicate how to construct new walls that work at higher densities. This is joint work with John Mackay.
• 5 pm.

### Related classes at SNU

• 12/09, 12/10
• Systolic group I, II

### Related talk at KAIST

• 2014-12-11 (15:00 - 16:00)
• Arcs intersecting at most once
We prove that on a punctured oriented surface with Eulercharacteristic chi < 0, the maximal cardinality of a set of essential simple arcs that are pairwise non-homotopic and intersecting at most once is 2|chi|(|chi|+1). This gives a cubic estimate in |chi| for a set of curves pairwise intersecting at most once on a closed surface.

## November 13, Alden Walker (University of Chicago)

• Random groups contain surface subgroups
Gromov asked whether every one-ended hyperbolic group contains a surface subgroup. I'll explain this question and sketch the proof that a random group (an example of a one-ended hyperbolic group) contains a surface subgroup. I'll give all necessary background and motivation on random groups. This is joint work with Danny Calegari.
• 2PM, 129-104.

## November 4, Jason Behrstock (CUNY)

• Higher dimensional filling and divergence for mapping class groups
We will discuss filling and divergence functions. We will describe their behaviors for mapping class groups of surfaces and show that these functions exhibit phase transitions at the rank, in analogy to the corresponding result for symmetric spaces. This work is joint with Cornelia Drutu.

## October 14, Michael Brandenbursky, (CRM, Univ. Montreal)

• Concordance group and stable commutator length in braid groups
In this talk I will define quasi-homomorphisms from braid groups to the concordance group of knots and examine its properties and consequences of its existence. In particular, I will provide a relation between the stable four ball genus in the concordance group and the stable commutator length in braid groups, and produce examples of infinite families of concordance classes of knots with uniformly bounded four ball genus. I will also provide applications to the geometry of the infinite braid group. In particular, I will show that its commutator subgroup admits a stably unbounded conjugation invariant norm. This answers an open problem posed by Burago, Ivanov and Polterovich. If time permits I will describe an interesting connection between the concordance group of knots and number theory. This work is partially joint with Jarek Kedra.
• 129-104

## October 7, Moon Duchin (Tufts)

• Geodesics in nilpotent groups
Perhaps the simplest non-abelian infinite group to understand is the Heisenberg group H(Z). Given a generating set as our "alphabet," a geodesic in the group is an efficient "spelling" of a group element. It is quite challenging to understand these precisely for an arbitrary choice of generators, but the large-scale geometric structure of the group makes it possible.
• 129-104

## October 2, Sang-jin Lee (Konkuk Univ)

• Embedding of RAAG into braid groups
• 27-220

## September 30, Alessandro Sisto (ETH)

• Acylindrically hyperbolic groups
Acylindrically hyperbolic groups form an extensive class of groups that contain, for example, non-elementary (relatively) hyperbolic groups, mapping class groups of hyperbolic surfaces and non-Abelian RAAGs. Their defining feature is that they admit a "non-trivial enough" action on some hyperbolic space. Despite the generality of the notion, many results can be proven about them, for example that they are SQ-universal, meaning that if G is acylindrically hyperbolic then any countable group embeds in some quotient of G (in particular, G has uncountably many pairwise non-isomorphic quotients). We will discuss geometric properties of acylindrically hyperbolic groups, focusing on ingredients that one can use to prove SQ-universality.
• 129-301

## August 7–12, Geometry on Groups and Spaces (ICM satellite)

### Plenary Talks

Click the titles for lecture videos.

• Michelle Bucher-Karlsson (Université de Genève), Volume and characteristic numbers of representations of hyperbolic manifolds (video missing)

### Long Sessions

• Thierry Barbot (Avignon), Construction of flat spacetimes in expansion with particles
• Ara Basmajian (City Univ. of New York), Hyperbolic surface identities
• Suhyoung Choi (KAIST), The convex real projective orbifolds with radial or totally geodesic ends: The closedness and openness of deformations
• Francois Dahmani (Institut Fourier, Grenoble), Conjugacy problem for some automorphisms of free groups
• Pallavi Dani (Louisiana State), Quasi-isometry and commensurability for right-angled Coxeter groups
• Sergio Fenley (Florida State Univ), Closed orbits of pseudo-Anosov flows: cardinality and length growth
• Daniel Groves (Univ. of Illinois at Chicago), The Malnormal Special Quotient Theorem
• Martin Kassabov (Cornell), Hopf algebras and invariants of the Johnson cockerel
• Fanny Kassel (CNRS & Universite Lille 1), Anosov representations and proper actions
• Thilo Kuessner (KIAS), Proportionality principle for noncompact manifolds
• Eiko Kin (Osaka Univ.), Dynamics of the monodromies of the fibrations on the magic 3-manifold
• Seonhee Lim (Seoul National Univ.), Martin boundary and Brownian motion on hyperbolic manifolds
• Sara Maloni (Brown), Polyhedra inscribed in quadrics, anti-de Sitter and half-pipe geometry
• Jason Manning (Cornell), Dehn filling and ends of groups
• Eduardo Martinez-Pedroza (Memorial), On Subgroups of Non-positively Curved Groups
• Hiroshige Shiga (Tokyo Institute of Technology), Teichmüller curves and holomorphic maps on Riemann surfaces
• Stephan Tillmann (The Univ. of Sydney), Thurston norm via Fox calculus
• Andrey Vesnin (Sobolev Institute of Mathematics), On Jorgensen numbers of hyperbolic 3-orbifold groups
• Henry Wilton (Cambridge), 3-manifolds in random groups
• Genkai Zhang (Chalmers University of Technology and Gothenburg University), Kähler metric over Hitchin component

### Short Sessions

• Ilesanmi Adeboye (Wesleyan), The Area of Projective Surfaces
• Juan Alonso (Universidad de la Republica - Uruguay), Measure free factors of free groups
• Yago Antolin (Vanderbilt), Finite generating sets of relatively hyperbolic groups
• Lucien Clavier (Cornell), Geometric limits of cyclic subgroups of PSL2(R)
• Matthew Durham (Univ. Illinois at Chicago), Elliptic Actions on Teichmüller Space
• Steven Frankel (Yale), Quasigeodesic flows and dynamics at infinity
• Simion Filip (Univ. of Chicago), Hodge theory and rigidity in Teichmuller dynamics
• David Hume (Universite Catholique de Louvain), Group approximation in Cayley topology and coarse geometry
• Woojin Jeon (KIAS), Some applications of Cannon-Thurston map
• Jingyin Huang (New York Univ.), Quasi-isometry classification of right-angled Artin group with finite outer automorphism group
• Sungwoon Kim (KIAS), Simplicial volume and its applications
• Gye-Seon Lee (Univ. of Heidelberg), Andreev’s theorem on projective Coxeter polyhedra
• HyoWon Park (UNiv. of Utah), A metric on an outer space for 2-dimensional right-angled Artin groups
• Catherine Pfaff (Universite d'Aix-Marseille), A Dense Geodesic Ray in Certain Subcomplexes of Quotiented Outer Space
• Jaipong Pradthana (Chiang Mai University), Totally geodesic surfaces in closed hyperbolic 3-manifolds
• Alessandro Sisto (ETH), Bounded cohomology of acylindrically hyperbolic groups
• Alden Walker (Univ. Chicago), Homologically essential surface subgroups of random groups
• Maxime Wolff (Paris 6), The modular action on PSL(2,R)-characters in genus two
• Tengren Zhang (Univ. of Michigan), Degeneration of convex real projective structures on surfaces

# 2013

Geometric group seminar at KAIST.

## August 12- 16, KAIST Geometric Topology Fair

• The 11th KAIST Geometric Topology Fair
• venue: Jeonju Hanok Village
• webpage
• booklet

### Lecture series

• Mladen Bestvina (University of Utah, USA), The geometry of mapping class groups and Out(Fn)
• Dave Witte Morris (University of Lethbridge, Canada), Arithmetic subgroups of SL(n,R)
• Joan Porti (Universitat Autònoma de Barcelona, Spain), Dynamics at infinity of symmetric spaces

### Research talks

• Delaram Kahrobaei (Graduate Center, CUNY), Polycyclic groups: A Secure Platform for Ko-Lee Protocol
• Youngju Kim (KIAS), Quasiconformal deformations of Schottky groups in complex hyperbolic space
• Thilo Kuessner (KIAS), Proportionality principle for simplicial volume
• Sangyop Lee (Chung-Ang University), Twisted torus knots
• Jung Hoon Lee (Chonbuk National University), Topologically minimal surfaces
• Seonhee Lim (Seoul National University), Subword complexity and Sturmian colorings of trees
• Ken'ichi Ohshika (Osaka U, Japan), Geometric limits and deformation spaces of Kleinian groups
• Catherine Pfaff (Universite Aix-Marseille, France), Stratifying the set of fully Irreducible elements of Out(Fr)

## July 9 - 11, Thomas Koberda (Yale)

• Two lectures on curve graphs for right-angled Artin groups
• Lecture I. Curve graphs for right-angled Artin groups I: right-angled Artin group actions on the extension graph
I will discuss basics of extension graphs for right-angled Artin groups and the actions of right-angled Artin groups on their extension graphs. The main result in this lecture will be a version of the Nielsen--Thurston classification for right-angled Artin groups. Joint with S. Kim.
• Lecture II. Curve graphs for right-angled Artin groups II: the large and small scale geometry of the extension graph
I will discuss various aspects of the geometry of the extension graph. I will discuss vertex link projections, the bounded geodesic image theorem, and a distance formula for right-angled Artin groups. Joint with S. Kim.
• July 9 and 11, 4-5 pm. Building E6-1, Room 4415.

## May 20 - June 10, Igor Mineyev (UIUC)

• Ten lectures on Groups, Cell Complexes and l2-Homology
One of the longest outstanding conjectures in group theory was Hanna Neumann Conjecture (1957):
if H and K are subgroups of a free group, then rank(H∩K)-1 ≤ (rank(H)-1) (rank(K)-1).
A long list of group theorists attempted to prove this conjecture, and also have contributed to related results. Mineyev first resolved this question completely in a paper at Annals of Mathematics (2012). In this series of ten lectures, he will give the main construction of the proof and its applications.
• Topics
Cell Complexes and Group Actions.
The L2 Homology and L2 Betti Numbers.
The Hanna Neumann Conjecture.
Orderability of Groups.
Submultiplicativity and the Deep-Fall Property.
The Atiyah Problem.
• May 20 - June 10. MWF 11 - 12:30 pm. Building E6-1, Room 2411.
• One-credit intensive course (MAS 583).

## May 30, Irene Peng (POSTECH)

• 4:30 - 5:30 pm. Room 1501.
• Amenability and all that
Amenability is one of those properties of group that has many different characterizations. I will discuss what it means in terms of invariant means, random walks and C* algebras. If time permits, I will also describe some related notions such as property rapid decay in the C* algebra setting.

## June 4, Hyungryul Baik (Cornell)

• 4:30 pm - 5:30 pm. Room 4415.
• Circular-Orderability of Three-Manifold Groups and Laminations of the Circle
We will discuss the connection between the circular-orderability of the fundamental group of a 3-manifold M and the existence of certain codimension-1 foliations on M via Thurston's universal circle theory. This theory provides a motivation to study group actions on the circle with dense invariant laminations. As an one lower dimensional example, we will give a complete characterization of Fuchsian groups in terms of its (topological) invariant laminations.

## April 10 - 15, Jason Fox Manning (U of Buffalo)

• Four Lectures on Hyperbolic Dehn Fillings of Groups and Spaces
• Lecture 1: The Gromov-Thurston 2\pi Theorem.
In the first lecture, I'll describe an explicit construction of negatively curved metrics on closed 3-manifolds obtained by Dehn filling of cusped hyperbolic manifolds. I also plan to sketch an application by Cooper and Long to finding surface subgroups of 3-manifolds. I'll talk about how to extend the 2\pi Theorem to cusped hyperbolic manifolds of dimension larger than 3.
• Lecture 2: Relatively hyperbolic groups.
I'll define and give examples of relatively hyperbolic groups, and talk about what it means to do Dehn filling on a group pair.
• Lecture 3: The relatively hyperbolic Dehn filling theorem.
I'll state the main theorem and sketch a proof.
• Lecture 4: Quasiconvex subgroups and Dehn filling.
I'll define relatively quasiconvex subgroups, and talk about how to do Dehn filling while preserving quasiconvexity.
• April 10 (W), 11 (Th), 12 (Fr), 16 (T). 4 pm - 5:15 pm. Room 4415.

## May 3 - 9, Cameron McA. Gordon (U Texas at Austin)

• Four Lectures on Dehn Surgery and Three-Manifold Groups
• Seminar: Dehn Surgery
The lectures will be an introduction to Dehn surgery. This is a construction, going back to Dehn in 1910, for producing closed 3-manifolds from knots. A natural generalization is Dehn filling, in which some torus boundary component $T$ of a 3-manifold $M$ is capped off with a solid torus $V$. If $\alpha$ is the isotopy class of the loop on $T$ that bounds a disk in $V$, the resulting filled manifold is denoted by $M(\alpha)$. Generically, the topological and geometric properties of $M$ persist in $M(\alpha)$; in particular if $M$ is hyperbolic then $M(\alpha)$ is usually also hyperbolic. If this fails then the filling is said to be {\it exceptional}. We will outline a program to classify the triples $(M;\alpha,\beta)$ with $M(\alpha)$ and $M(\beta)$ exceptional, describing what is known in this direction and what remains to be done.
• Colloquium: Left-Orderability of Three-Manifold Groups
We will discuss connections between three notions in 3-dimensional topology that are, roughly speaking, algebraic, topological, and analytic. These are: the left-orderability of the fundamental group of a 3-manifold M, the existence of certain codimension 1 foliations on M, and the Heegaard Floer homology of M.
• Reference
Park City Lectures Dehn Surgery and 3-Manifolds, in Low Dimensional Topology, ed. T.S. Mrowka and P.S. Ozsvath, IAS/Park City Mathematics Series, Vol. 15, AMS 2009.
• Seminar May 3 (F), 7 (T), 8 (W). 4:30 - 5:30 pm. Room 4415
• Colloquium May 9 (Th). 4:30 - 5:30 pm. Building E6, 1501.

# 2012

At KAIST.

## August 13 - 17, the 10th KAIST Geometric Topology Fair

### Lecture Series

• Michael Davis (Ohio State Univ), Graph products, RACGs and RAAGs
• Koji Fujiwara (Tohoku Univ), Group actions on quasi-trees
• Alan Reid (Univ of Texas at Austin), 3-manifold groups, covering spaces and LERF

### Research Talks

• Jinseok Cho (KIAS), Introduction to Kashaev volume conjecture
• Kanghyun Choi (KAIST), The definability criterion for cocompact convex projective polyhedral reflection groups
• Stefan Friedl (Univ. of Cologne), Minimal genus surfaces in 4-manifolds with a free circle action
• Thilo Kuessner (KIAS), Invariants preserved by mutation
• Sang-hyun Kim (KAIST), Hyperbolic aspects of right-angled Artin groups
• Taehee Kim (Konkuk Univ.), Knot concordance and invariants from derived covers
• Sang-Jin Lee (Konkuk Univ.), Braid group of type (de,e,r)
• Gye-Seon Lee (Seoul National Univ.), Real projective deformations of hyperbolic reflection orbifolds
• Seonhee Lim (Seoul National Univ.), Ford circles and Farey maps for function field
• Ken’ichi Ohshika (Osaka University), Actions of isomorphism groups of Heegaard splittings on projective lamination spaces

## August 21 - 30, Genevieve S. Walsh (Tufts)

• Four Lectures on Introduction to Hyperbolic Orbifolds and Knot Commensurability.
• Aug 21, 23, 28, 30. TTh 4 - 5 pm. Room 3433.
• Lecture 1: 2-dimensional orbifolds
In this lecture we will define and describe orbifolds and set notation. In particular, we will discuss orbifold Euler characteristic, orbifold covers, good orbifolds, bad orbifolds, and the orbifold fundamental group. Explicit examples of spherical, Euclidean and hyperbolic 2-orbifolds will be given. We will also prove that there is a smallest closed hyperbolic 2-orbifold.
• Lecture 2: 3-dimensional orbifolds
Here we will explore 3-dimensional orbifolds, restricting mainly to good orbifolds. Although we will give explicit examples of many different types of 3-dimensional orbifolds, the focus will be on hyperbolic 3-orbifolds. To this end, we will discuss hyperbolic isometries and the geometry of hyperbolic orbifolds and hyperbolic orbifolds. We will discuss how useful orbifolds are to the study of 3-manifolds, and give a statement of geometrization.
• Lecture 3: Commensurability
Commensurability is an equivalence relation on manifolds and orbifolds which is a refinement of geometrization. Here we will describe the current study of commensurability of hyperbolic manifolds, focusing on commensurability of knot complements. We will describe hyperbolic knot complements and their symmetry groups, and discuss the commensurator group and the orbifold commensurator quotient of a hyperbolic non-arithmetic knot complement.
• Lecture 4: Some results on commensurability of knot complements
A conjecture of Reid and Walsh asserts that there are at most 3 hyperbolic knot complements in any commensurability class. Here we discuss this conjecture, and give results under certain circumstances. The problem naturally divides itself into two cases, the case of hidden symmetries and the case of no hidden symmetries, and we discuss both. The new results presented here are joint with M. Boileau, S. Boyer, and R. Cebanu.
• References
1. Not Knot Part 1 (video)
2. Not Knot Part 2 (video)
3. Thurston's course notes, Chapter 13
4. Michael Kapovich, Hyperbolic manifolds and discrete groups, Chapter 6
5. PDF "Orbifolds and commensurability"

## September 5–7, Ian Agol (UC Berkeley)

• Three Lectures on Virtual Haken Conjecture.
• September 5, 6, 7 (WThF), 4 - 5 pm. Room 3433.
• Prequel Lecture September 5, Wednesday, 4 – 5 pm @ Room 3433
An Invitation to Cube Complexes by Sang-hyun Kim (Slides)
We survey basic facts on cube complexes and discuss how those facts are related to the study of subgroups of right-angled Artin groups.
• Lecture I
We will discuss the proof of the virtual Haken conjecture and related questions. The first lecture will be an overview and an explanation of how to reduce the problem to a conjecture of Wise in geometric group theory.
• Lecture II
The second lecture will be on the RFRS condition and virtual fibering for hyperbolic 3-manifolds.
• Lecture III
The third lecture will be on the proof of Wise's conjecture, that cubulated hyperbolic groups are virtually special.
• The following video lectures @ KAIST by Alan Reid are not prerequisites, but provide valuable information on the background and a big picture surrounding this problem:
3-manifold groups, covering spaces and LERF
• Selected References
1. F. Haglund and D. Wise, Special cube complexes, Geom. Funct. Anal. (2007) 1–69.
2. I. Agol, Criteria for virtual fibering, J. Topol. 1 (2008), no. 2, 269–284.
3. D. Wise, Research announcement: the structure of groups with a quasiconvex hierarchy., Electron. Res. Announc. Math. Sci. 16 (2009), 44–55.
4. CBMS lecture videos, notes and related preprints: http://comet.lehman.cuny.edu/behrstock/cbms/program.html

## September 25–October 4, Thomas Koberda (Yale)

• Four Lectures on Mapping Class Groups and Right-Angled Artin Groups
1. An Introduction to Right-Angled Artin Groups and Mapping Class Group In this lecture, we will begin with some basic facts about right-angled Artin groups and mapping class groups. The goal is to provide a foundation for various new results concerning the structure and geometry of right-angled Artin groups, mapping class groups, and their subgroups.
2. An Introduction to Right-Angled Artin Groups and Mapping Class Groups In this lecture, we will discuss the primary result of [3], which roughly says that if we take any collection of mapping classes, say {f1,...,fk} and replace them by sufficiently high powers {f1^N,...,fk^N}, they generate a right-angled Artin subgroup of the mapping class group of the expected type. Unless otherwise noted, all examples and statements can be found with proof (or appropriate reference) in [3].
3. Right-Angled Artin Subgroups of Right-Angled Artin Groups In this lecture, we will discuss the primary results of [2]. In that article, the authors develop a general theory for determining when there exists an embedding A(X) -> A(Y) for two graphs X and Y.
4. A Dictionary Between Mapping Class Groups and Right-Angled Artin Groups Via Curve Complexes In this lecture, we will primarily be discussing the results of [1], together with appropriate background. The general principle we would like to explore is that right-angled Artin groups behave a lot like mapping class groups from the point of view of their actions on their extension graphs and curve complexes respectively.
• References
1. Sang-hyun Kim and Thomas Koberda. Actions of right-angled Artin groups on quasi–trees. In preparation.
2. Sang-hyun Kim and Thomas Koberda. Embedability of right-angled Artin groups. Preprint.
3. Thomas Koberda. Right-angled Artin groups and a generalized isomorphism problem for finitely generated subgroups of mapping class groups. To appear in Geom. Funct. Anal.
• September 25, 27, October 2, 4. T 4 - 5 pm, Th 2:30 - 3:30. Room 3433

# 2011

At KAIST.

## August 8–12, The 9th KAIST Geometric Topology Fair

### Lecture Series

• Danny Calegari (Caltech), Random rigidity in free groups
• Jason F. Manning (SUNY Buffalo), Recognizing low-dimensional manifold groups
• Mark V. Sapir (Vanderbilt), Embedding finitely generated groups into finitely presented groups
• Efim Zelmanov (UC San Diego), Asymptotic group theory - pro-p group; property T and expanders

### Research Talks

• Sungwoon Kim (KIAS), Simplicial volume and bounded group cohomology
• Sang-hyun Sam Kim (KAIST), Embeddability between right-angled Artin groups
• Kihyoung Ko (KAIST), Graph braid groups: its 10 year history
• Donghi Lee (Pusan National University), Combinatorial group theory applied to 2-bridge link groups
• Seonhee Lim (Seoul National University), Commensurizer group and its growth

## September 1 - 16, Jon McCammond (UC Santa Barbara)

• Ten Lectures on Coxeter Groups and Reflection Symmetry
• September 1 - 16, 2011 (except for Saturday, Sunday and 09/12, 09/13) MWF 4 - 5:30 pm, TTh 1 - 2:30 pm
• One-credit intensive course.
• Symmetry and Abstraction - Why do mathematicians see only 17 types of wallpaper?, a public Lecture
Coxeter groups are a central object of study in many parts of mathematics. They include the groups of symmetries of the regular polytopes, the finite reflection groups and the Weyl groups at the core of the study of Lie groups and Lie algebras. They have many remarkable properties including the fact that they have faithful linear representations and a proper cocompact action by isometries on piecewise Euclidean space of nonpositive curvature. In this course I will focus on laying the foundations for the geometry, topology and combinatorics of Coxeter groups.
• Prerequisites
The prerequisites are only Linear Algebra and Abstract Algebra (i.e. groups). Some familiarity with groups given by generators and relations, and fundamental groups and covering spaces would be nice but probably not absolutely necessary.
• Lecture Plan
1 & 2 Regular polytopes and spherical Coxeter groups
3 & 4 Lie groups and Euclidean Coxeter groups
5 & 6 Coxeter groups in general
7 & 8 Linear representations and basic facts
9 & 10 Non-positively curved spaces and geometric actionsGrading
• Letter grades are given based on (1) attendance / participation (2) a short paper due 09/01/2011.