## Tuesday Seminar on Topology

Seminar information archive ～09/26｜Next seminar｜Future seminars 09/27～

Date, time & place | Tuesday 17:00 - 18:30 056Room #056 (Graduate School of Math. Sci. Bldg.) |
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Organizer(s) | KAWAZUMI Nariya, KITAYAMA Takahiro, SAKASAI Takuya |

**Seminar information archive**

### 2009/10/20

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Torus fibrations and localization of index

**吉田 尚彦**(明治大学大学院理工学研究科)Torus fibrations and localization of index

[ Abstract ]

I will describe a localization of index of a Dirac type operator.

We make use of a structure of torus fibration, but the mechanism

of the localization does not rely on any group action. In the case of

Lagrangian fibration, we show that the index is described as a sum of

the contributions from Bohr-Sommerfeld fibers and singular fibers.

To show the localization we introduce a deformation of a Dirac type

operator for a manifold equipped with a fiber bundle structure which

satisfies a kind of acyclic condition. The deformation allows an

interpretation as an adiabatic limit or an infinite dimensional analogue

of Witten deformation.

Joint work with Hajime Fujita and Mikio Furuta.

I will describe a localization of index of a Dirac type operator.

We make use of a structure of torus fibration, but the mechanism

of the localization does not rely on any group action. In the case of

Lagrangian fibration, we show that the index is described as a sum of

the contributions from Bohr-Sommerfeld fibers and singular fibers.

To show the localization we introduce a deformation of a Dirac type

operator for a manifold equipped with a fiber bundle structure which

satisfies a kind of acyclic condition. The deformation allows an

interpretation as an adiabatic limit or an infinite dimensional analogue

of Witten deformation.

Joint work with Hajime Fujita and Mikio Furuta.

### 2009/10/13

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Instanton Floer homology for lens spaces

**笹平 裕史**(東京大学大学院数理科学研究科)Instanton Floer homology for lens spaces

[ Abstract ]

Let Y be an oriented closed 3-manifold and P be an SU(2)-bundle on Y. Under a certain condition, instanton Floer homology for Y can be defined as the Morse homology of the Chern-Simons functional. The condition is that all flat connections on P are irreducible. When there is a reducible flat connection on P, instanton Floer homology is not defined in general.

Since the fundamental group of a lens sapce is commutative, all flat connections on the lens space are reducible. In this talk I will introduce instanton Floer homology for lens spaces. I also show calculations for some lens spaces.

Let Y be an oriented closed 3-manifold and P be an SU(2)-bundle on Y. Under a certain condition, instanton Floer homology for Y can be defined as the Morse homology of the Chern-Simons functional. The condition is that all flat connections on P are irreducible. When there is a reducible flat connection on P, instanton Floer homology is not defined in general.

Since the fundamental group of a lens sapce is commutative, all flat connections on the lens space are reducible. In this talk I will introduce instanton Floer homology for lens spaces. I also show calculations for some lens spaces.

### 2009/09/29

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Symbol of the Conway polynomial and Drinfeld associator

**Sergei Duzhin**(Steklov Mathematical Institute, Petersburg Division)Symbol of the Conway polynomial and Drinfeld associator

[ Abstract ]

The Magnus expansion is a universal finite type invariant of pure braids

with values in the space of horizontal chord diagrams. The Conway polynomial

composed with the short circuit map from braids to knots gives rise to a

series of finite type invariants of pure braids and thus factors through

the Magnus map. We describe explicitly the resulting mapping from horizontal

chord diagrams on 3 strands to univariante polynomials and evaluate it on

the Drinfeld associator obtaining a beautiful generating function whose

coefficients are integer combinations of multple zeta values.

The Magnus expansion is a universal finite type invariant of pure braids

with values in the space of horizontal chord diagrams. The Conway polynomial

composed with the short circuit map from braids to knots gives rise to a

series of finite type invariants of pure braids and thus factors through

the Magnus map. We describe explicitly the resulting mapping from horizontal

chord diagrams on 3 strands to univariante polynomials and evaluate it on

the Drinfeld associator obtaining a beautiful generating function whose

coefficients are integer combinations of multple zeta values.

### 2009/07/14

17:00-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

The Cannon-Thurston maps and the canonical decompositions

of punctured-torus bundles over the circle.

**作間 誠**(広島大学)The Cannon-Thurston maps and the canonical decompositions

of punctured-torus bundles over the circle.

[ Abstract ]

To each once-punctured-torus bundle over the circle

with pseudo-Anosov monodromy, there are associated two tessellations of the complex plane:

one is the triangulation of a horosphere induced by the canonical decomposition into ideal

tetrahedra, and the other is a fractal tessellation

given by the Cannon-Thurston map of the fiber group.

In this talk, I will explain the relation between these two tessellations

(joint work with Warren Dicks).

I will also explain the relation of the fractal tessellation and

the "circle chains" of double cusp groups converging to the fiber group

(joint work with Caroline Series).

If time permits, I would like to discuss possible generalization of these results

to higher-genus punctured surface bundles.

To each once-punctured-torus bundle over the circle

with pseudo-Anosov monodromy, there are associated two tessellations of the complex plane:

one is the triangulation of a horosphere induced by the canonical decomposition into ideal

tetrahedra, and the other is a fractal tessellation

given by the Cannon-Thurston map of the fiber group.

In this talk, I will explain the relation between these two tessellations

(joint work with Warren Dicks).

I will also explain the relation of the fractal tessellation and

the "circle chains" of double cusp groups converging to the fiber group

(joint work with Caroline Series).

If time permits, I would like to discuss possible generalization of these results

to higher-genus punctured surface bundles.

### 2009/06/30

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Torsion volume forms and twisted Alexander functions on

character varieties of knots

**北山 貴裕**(東京大学大学院数理科学研究科)Torsion volume forms and twisted Alexander functions on

character varieties of knots

[ Abstract ]

Using non-acyclic Reidemeister torsion, we can canonically

construct a complex volume form on each component of the

lowest dimension of the $SL_2(\\mathbb{C})$-character

variety of a link group.

This volume form enjoys a certain compatibility with the

following natural transformations on the variety.

Two of them are involutions which come from the algebraic

structure of $SL_2(\\mathbb{C})$ and the other is the

action by the outer automorphism group of the link group.

Moreover, in the case of knots these results deduce a kind

of symmetry of the $SU_2$-twisted Alexander functions

which are globally described via the volume form.

Using non-acyclic Reidemeister torsion, we can canonically

construct a complex volume form on each component of the

lowest dimension of the $SL_2(\\mathbb{C})$-character

variety of a link group.

This volume form enjoys a certain compatibility with the

following natural transformations on the variety.

Two of them are involutions which come from the algebraic

structure of $SL_2(\\mathbb{C})$ and the other is the

action by the outer automorphism group of the link group.

Moreover, in the case of knots these results deduce a kind

of symmetry of the $SU_2$-twisted Alexander functions

which are globally described via the volume form.

### 2009/06/23

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

The Meyer functions for projective varieties and their applications

**久野 雄介**(東京大学大学院数理科学研究科)The Meyer functions for projective varieties and their applications

[ Abstract ]

Meyer function is a kind of secondary invariant related to the signature

of surface bundles over surfaces. In this talk I will show there exist uniquely the Meyer function

for each smooth projective variety.

Our function is a class function on the fundamental group of some open algebraic variety.

I will also talk about its application to local signature for fibered 4-manifolds

Meyer function is a kind of secondary invariant related to the signature

of surface bundles over surfaces. In this talk I will show there exist uniquely the Meyer function

for each smooth projective variety.

Our function is a class function on the fundamental group of some open algebraic variety.

I will also talk about its application to local signature for fibered 4-manifolds

### 2009/06/16

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

The abelianization of the level 2 mapping class group

**佐藤 正寿**(東京大学大学院数理科学研究科)The abelianization of the level 2 mapping class group

[ Abstract ]

The level d mapping class group is a finite index subgroup of the mapping class group of an orientable closed surface. For d greater than or equal to 3, the abelianization of this group is equal to the first homology group of the moduli space of nonsingular curves with level d structure.

In this talk, we determine the abelianization of the level d mapping class group for d=2 and odd d. For even d greater than 2, we also determine it up to a cyclic group of order 2.

The level d mapping class group is a finite index subgroup of the mapping class group of an orientable closed surface. For d greater than or equal to 3, the abelianization of this group is equal to the first homology group of the moduli space of nonsingular curves with level d structure.

In this talk, we determine the abelianization of the level d mapping class group for d=2 and odd d. For even d greater than 2, we also determine it up to a cyclic group of order 2.

### 2009/06/09

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

A finite-dimensional construction of the Chern character for

twisted K-theory

**五味 清紀**(京都大学大学院理学研究科)A finite-dimensional construction of the Chern character for

twisted K-theory

[ Abstract ]

Twisted K-theory is a variant of topological K-theory, and

is attracting much interest due to applications to physics recently.

Usually, twisted K-theory is formulated infinite-dimensionally, and

hence known constructions of its Chern character are more or less

abstract. The aim of my talk is to explain a purely finite-dimensional

construction of the Chern character for twisted K-theory, which allows

us to compute examples concretely. The construction is based on

twisted version of Furuta's generalized vector bundle, and Quillen's

superconnection.

This is a joint work with Yuji Terashima.

Twisted K-theory is a variant of topological K-theory, and

is attracting much interest due to applications to physics recently.

Usually, twisted K-theory is formulated infinite-dimensionally, and

hence known constructions of its Chern character are more or less

abstract. The aim of my talk is to explain a purely finite-dimensional

construction of the Chern character for twisted K-theory, which allows

us to compute examples concretely. The construction is based on

twisted version of Furuta's generalized vector bundle, and Quillen's

superconnection.

This is a joint work with Yuji Terashima.

### 2009/06/02

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Graph homology: Koszul duality = Verdier duality

**Alexander Voronov**(University of Minnesota)Graph homology: Koszul duality = Verdier duality

[ Abstract ]

Graph cohomology appears in computation of the cohomology of the moduli space of Riemann surfaces and the outer automorphism group of a free group. In the former case, it is graph cohomology of the commutative and Lie types, in the latter it is ribbon graph cohomology, that is to say, graph cohomology of the associative type. The presence of these three basic types of algebraic structures hints at a relation between Koszul duality for operads and Poincare-Lefschetz duality for manifolds. I will show how the more general Verdier duality for certain sheaves on the moduli spaces of graphs associated to Koszul operads corresponds to Koszul duality of operads. This is a joint work with Andrey Lazarev.

Graph cohomology appears in computation of the cohomology of the moduli space of Riemann surfaces and the outer automorphism group of a free group. In the former case, it is graph cohomology of the commutative and Lie types, in the latter it is ribbon graph cohomology, that is to say, graph cohomology of the associative type. The presence of these three basic types of algebraic structures hints at a relation between Koszul duality for operads and Poincare-Lefschetz duality for manifolds. I will show how the more general Verdier duality for certain sheaves on the moduli spaces of graphs associated to Koszul operads corresponds to Koszul duality of operads. This is a joint work with Andrey Lazarev.

### 2009/05/26

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Configuration space integrals and the cohomology of the space of long embeddings

**境 圭一**(東京大学大学院数理科学研究科)Configuration space integrals and the cohomology of the space of long embeddings

[ Abstract ]

It is known that some non-trivial cohomology classes, such as finite type invariants for (long) 1-knots (Bott-Taubes, Kohno, ...) and invariants for codimension two, odd dimensional long embeddings (Bott, Cattaneo-Rossi, Watanabe) are given as configuration space integrals associated with trivalent graphs.

In this talk, I will describe more cohomology classes by means of configuration space integral, in particular those arising from non-trivalent graphs and a new formulation of the Haefliger invariant for long 3-embeddings in 6-space, in relation to Budney's little balls operad action and Roseman-Takase's deform-spinning.

This is in part a joint work with Tadayuki Watanabe.

It is known that some non-trivial cohomology classes, such as finite type invariants for (long) 1-knots (Bott-Taubes, Kohno, ...) and invariants for codimension two, odd dimensional long embeddings (Bott, Cattaneo-Rossi, Watanabe) are given as configuration space integrals associated with trivalent graphs.

In this talk, I will describe more cohomology classes by means of configuration space integral, in particular those arising from non-trivalent graphs and a new formulation of the Haefliger invariant for long 3-embeddings in 6-space, in relation to Budney's little balls operad action and Roseman-Takase's deform-spinning.

This is in part a joint work with Tadayuki Watanabe.

### 2009/05/19

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Geometric quantization of integrable systems

**Mark Hamilton**(東京大学大学院数理科学研究科, JSPS)Geometric quantization of integrable systems

[ Abstract ]

The theory of geometric quantization is one way of producing a "quantum system" from a "classical system," and has been studied a great deal over the past several decades. It also has surprising ties to representation theory. However, despite this, there still does not exist a satisfactory theory of quantization for systems with singularities.

Geometric quantization requires the choice of a polarization; when using a real polarization to quantize a regular enough manifold, a result of Sniatycki says that the quantization can be found by counting certain objects, called Bohr-Sommerfeld fibres. However, there are many types of systems to which this result does not apply. One such type is the class of completely integrable systems, which are examples coming from mechanics that have many nice properties, but which are nontheless too singular for Sniatycki's theorem to apply.

In this talk we will explore one approach to the quantization of integrable systems, and show a Sniatycki-type relationship to Bohr-Sommerfeld fibres. However, some surprising features appear, including infinite-dimensional contributions and strong dependence on the polarization.

This is joint work with Eva Miranda.

The theory of geometric quantization is one way of producing a "quantum system" from a "classical system," and has been studied a great deal over the past several decades. It also has surprising ties to representation theory. However, despite this, there still does not exist a satisfactory theory of quantization for systems with singularities.

Geometric quantization requires the choice of a polarization; when using a real polarization to quantize a regular enough manifold, a result of Sniatycki says that the quantization can be found by counting certain objects, called Bohr-Sommerfeld fibres. However, there are many types of systems to which this result does not apply. One such type is the class of completely integrable systems, which are examples coming from mechanics that have many nice properties, but which are nontheless too singular for Sniatycki's theorem to apply.

In this talk we will explore one approach to the quantization of integrable systems, and show a Sniatycki-type relationship to Bohr-Sommerfeld fibres. However, some surprising features appear, including infinite-dimensional contributions and strong dependence on the polarization.

This is joint work with Eva Miranda.

### 2009/05/12

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Discrete subgroups of the group of circle diffeomorphisms

**松田 能文**(東京大学大学院数理科学研究科)Discrete subgroups of the group of circle diffeomorphisms

[ Abstract ]

Typical examples of discrete subgroups of the group of circle diffeomorphisms

are Fuchsian groups.

In this talk, we construct discrete subgroups of the group of

orientation-preserving

real analytic cirlcle diffeomorphisms

which are not topologically conjugate to finite coverings of Fuchsian groups.

Typical examples of discrete subgroups of the group of circle diffeomorphisms

are Fuchsian groups.

In this talk, we construct discrete subgroups of the group of

orientation-preserving

real analytic cirlcle diffeomorphisms

which are not topologically conjugate to finite coverings of Fuchsian groups.

### 2009/04/28

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

The ambient metric in conformal geometry

**平地 健吾**(東京大学大学院数理科学研究科)The ambient metric in conformal geometry

[ Abstract ]

In 1985, Charles Fefferman and Robin Graham gave a method for realizing a conformal manifold of dimension n as a submanifold of a Ricci-flat Lorentz metric on a manifold of dimension n+2, which is now called the ambient space. Using this correspondence, one can construct many examples of conformal invariants and conformally invariant operators. However, if n is even, their construction of the ambient space is obstructed at the jet of order n/2 and thereby the application of the ambient space was limited. In this talk, I'll recall basic ideas of the ambient space and then explain how to avoid the difficulty and go beyond the obstruction. This is a joint work with Robin Graham.

In 1985, Charles Fefferman and Robin Graham gave a method for realizing a conformal manifold of dimension n as a submanifold of a Ricci-flat Lorentz metric on a manifold of dimension n+2, which is now called the ambient space. Using this correspondence, one can construct many examples of conformal invariants and conformally invariant operators. However, if n is even, their construction of the ambient space is obstructed at the jet of order n/2 and thereby the application of the ambient space was limited. In this talk, I'll recall basic ideas of the ambient space and then explain how to avoid the difficulty and go beyond the obstruction. This is a joint work with Robin Graham.

### 2009/04/21

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Some algebraic aspects of KZ systems

**Ivan Marin**(Univ. Paris VII)Some algebraic aspects of KZ systems

[ Abstract ]

Knizhnik-Zamolodchikov (KZ) systems enables one

to construct representations of (generalized)

braid groups. While this geometric construction is

now very well understood, it still brings to

attention, or helps constructing, new algebraic objects.

In this talk, we will present some of them, including an

infinitesimal version of Iwahori-Hecke algebras and a

generalization of the Krammer representations of the usual

braid groups.

Knizhnik-Zamolodchikov (KZ) systems enables one

to construct representations of (generalized)

braid groups. While this geometric construction is

now very well understood, it still brings to

attention, or helps constructing, new algebraic objects.

In this talk, we will present some of them, including an

infinitesimal version of Iwahori-Hecke algebras and a

generalization of the Krammer representations of the usual

braid groups.

### 2009/03/05

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Extending surface automorphisms over 4-space

**Shicheng Wang**(Peking University)Extending surface automorphisms over 4-space

[ Abstract ]

Let $e: M^p\\to R^{p+2}$ be a co-dimensional 2 smooth embedding

from a closed orientable manifold to the Euclidean space and $E_e$ be the subgroup of ${\\cal M}_M$, the mapping class group

of $M$, whose elements extend over $R^{p+2}$ as self-diffeomorphisms. Then there is a spin structure

on $M$ derived from the embedding which is preserved by each $\\tau \\in E_e$.

Some applications: (1) the index $[{\\cal M}_{F_g}:E_e]\\geq 2^{2g-1}+2^{g-1}$ for any embedding $e:F_g\\to R^4$, where $F_g$

is the surface of genus $g$. (2) $[{\\cal M}_{T^p}:E_e]\\geq 2^p-1$ for any unknotted embedding

$e:T^p\\to R^{p+2}$, where $T^p$ is the $p$-dimensional torus. Those two lower bounds are known to be sharp.

This is a joint work of Ding-Liu-Wang-Yao.

Let $e: M^p\\to R^{p+2}$ be a co-dimensional 2 smooth embedding

from a closed orientable manifold to the Euclidean space and $E_e$ be the subgroup of ${\\cal M}_M$, the mapping class group

of $M$, whose elements extend over $R^{p+2}$ as self-diffeomorphisms. Then there is a spin structure

on $M$ derived from the embedding which is preserved by each $\\tau \\in E_e$.

Some applications: (1) the index $[{\\cal M}_{F_g}:E_e]\\geq 2^{2g-1}+2^{g-1}$ for any embedding $e:F_g\\to R^4$, where $F_g$

is the surface of genus $g$. (2) $[{\\cal M}_{T^p}:E_e]\\geq 2^p-1$ for any unknotted embedding

$e:T^p\\to R^{p+2}$, where $T^p$ is the $p$-dimensional torus. Those two lower bounds are known to be sharp.

This is a joint work of Ding-Liu-Wang-Yao.

### 2009/01/27

17:00-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Lagrangian Floer homology and quasi homomorphism

from the group of Hamiltonian diffeomorphism

**深谷 賢治**(京都大学大学院理学研究科)Lagrangian Floer homology and quasi homomorphism

from the group of Hamiltonian diffeomorphism

[ Abstract ]

Entov-Polterovich constructed quasi homomorphism

from the group of Hamiltonian diffeomorphisms using

spectral invariant due to Oh etc.

In this talk I will explain a way to study this

quasi homomorphism by using Lagrangian Floer homology.

I will also explain its generalization to use quantum

cohomology with bulk deformation.

When applied to the case of toric manifold, it

gives an example where (infinitely) many quasi homomorphism

exists.

(Joint work with Oh-Ohta-Ono).

Entov-Polterovich constructed quasi homomorphism

from the group of Hamiltonian diffeomorphisms using

spectral invariant due to Oh etc.

In this talk I will explain a way to study this

quasi homomorphism by using Lagrangian Floer homology.

I will also explain its generalization to use quantum

cohomology with bulk deformation.

When applied to the case of toric manifold, it

gives an example where (infinitely) many quasi homomorphism

exists.

(Joint work with Oh-Ohta-Ono).

### 2009/01/20

16:30-17:30 Room #002 (Graduate School of Math. Sci. Bldg.)

Five dimensional $K$-contact manifolds of rank 2

**野澤 啓**(東京大学大学院数理科学研究科)Five dimensional $K$-contact manifolds of rank 2

[ Abstract ]

A $K$-contact manifold is an odd dimensional manifold $M$ with a contact form $\\alpha$ whose Reeb flow preserves a Riemannian metric on $M$. For examples, the underlying manifold with the underlying contact form of a Sasakian manifold is $K$-contact. In this talk, we will state our results on classification up to surgeries, the existence of compatible Sasakian metrics and a sufficient condition to be toric for closed $5$-dimensional $K$-contact manifolds with a $T^2$ action given by the closure of the Reeb flow, which are obtained by the application of Morse theory to the contact moment map for the $T^2$ action.

A $K$-contact manifold is an odd dimensional manifold $M$ with a contact form $\\alpha$ whose Reeb flow preserves a Riemannian metric on $M$. For examples, the underlying manifold with the underlying contact form of a Sasakian manifold is $K$-contact. In this talk, we will state our results on classification up to surgeries, the existence of compatible Sasakian metrics and a sufficient condition to be toric for closed $5$-dimensional $K$-contact manifolds with a $T^2$ action given by the closure of the Reeb flow, which are obtained by the application of Morse theory to the contact moment map for the $T^2$ action.

### 2009/01/20

17:30-18:30 Room #002 (Graduate School of Math. Sci. Bldg.)

Surface links which are coverings of a trivial torus knot (JAPANESE)

**中村 伊南沙**(東京大学大学院数理科学研究科)Surface links which are coverings of a trivial torus knot (JAPANESE)

[ Abstract ]

We consider surface links which are in the form of coverings of a

trivial torus knot, which we will call torus-covering-links.

By definition, torus-covering-links include

spun $T^2$-knots, turned spun $T^2$-knots, and symmetry-spun tori.

We see some properties of torus-covering-links.

We consider surface links which are in the form of coverings of a

trivial torus knot, which we will call torus-covering-links.

By definition, torus-covering-links include

spun $T^2$-knots, turned spun $T^2$-knots, and symmetry-spun tori.

We see some properties of torus-covering-links.

### 2009/01/13

16:30-17:30 Room #002 (Graduate School of Math. Sci. Bldg.)

Compactification of the homeomorphism group of a graph

**山下 温**(東京大学大学院数理科学研究科)Compactification of the homeomorphism group of a graph

[ Abstract ]

Topological properties of homeomorphism groups, especially of finite-dimensional manifolds,

have been of interest in the area of infinite-dimensional manifold topology.

For a locally finite graph $\\Gamma$ with countably many components,

the homeomorphism group $\\mathcal{H}(\\Gamma)$

and its identity component $\\mathcal{H}_+(\\Gamma)$ are topological groups

with respect to the compact-open topology. I will define natural compactifications

$\\overline{\\mathcal{H}}(\\Gamma)$ and

$\\overline{\\mathcal{H}}_+(\\Gamma)$ of these groups and describe the

topological type of the pair $(\\overline{\\mathcal{H}}_+(\\Gamma), \\mathcal{H}_+(\\Gamma))$

using the data of $\\Gamma$. I will also discuss the topological structure of

$\\overline{\\mathcal{H}}(\\Gamma)$ where $\\Gamma$ is the circle.

Topological properties of homeomorphism groups, especially of finite-dimensional manifolds,

have been of interest in the area of infinite-dimensional manifold topology.

For a locally finite graph $\\Gamma$ with countably many components,

the homeomorphism group $\\mathcal{H}(\\Gamma)$

and its identity component $\\mathcal{H}_+(\\Gamma)$ are topological groups

with respect to the compact-open topology. I will define natural compactifications

$\\overline{\\mathcal{H}}(\\Gamma)$ and

$\\overline{\\mathcal{H}}_+(\\Gamma)$ of these groups and describe the

topological type of the pair $(\\overline{\\mathcal{H}}_+(\\Gamma), \\mathcal{H}_+(\\Gamma))$

using the data of $\\Gamma$. I will also discuss the topological structure of

$\\overline{\\mathcal{H}}(\\Gamma)$ where $\\Gamma$ is the circle.

### 2008/12/09

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Tits alternative in $Diff^1(S^1)$

**Bertrand Deroin**(CNRS, Orsay, Universit\'e Paris-Sud 11)Tits alternative in $Diff^1(S^1)$

[ Abstract ]

The following form of Tits alternative for subgroups of

homeomorphisms of the circle has been proved by Margulis: or the group

preserve a probability measure on the circle, or it contains a free

subgroup on two generators. We will prove that if the group acts by diffeomorphisms of

class $C^1$ and does not preserve a probability measure on the circle, then

in fact it contains a subgroup topologically conjugated to a Schottky group.

This is a joint work with V. Kleptsyn and A. Navas.

The following form of Tits alternative for subgroups of

homeomorphisms of the circle has been proved by Margulis: or the group

preserve a probability measure on the circle, or it contains a free

subgroup on two generators. We will prove that if the group acts by diffeomorphisms of

class $C^1$ and does not preserve a probability measure on the circle, then

in fact it contains a subgroup topologically conjugated to a Schottky group.

This is a joint work with V. Kleptsyn and A. Navas.

### 2008/12/02

17:00-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Vanishing and Rigidity

**金井 雅彦**(名古屋大学多元数理科学研究科)Vanishing and Rigidity

[ Abstract ]

The aim of my talk is to reveal an unforeseen link between

the classical vanishing theorems of Matsushima and Weil, on the one hand,

andrigidity of the Weyl chamber flow, a dynamical system arising from a higher-rank

noncompact Lie group, on the other. The connection is established via

"transverse extension theorems": Roughly speaking, they claim that a tangential 1- form of the

orbit foliation of the Weyl chamber flow that is tangentially closed

(and satisfies a certain mild additional condition) can be extended to a closed 1- form on the

whole space in a canonical manner. In particular, infinitesimal

rigidity of the orbit foliation of the Weyl chamber flow is proved as an application.

The aim of my talk is to reveal an unforeseen link between

the classical vanishing theorems of Matsushima and Weil, on the one hand,

andrigidity of the Weyl chamber flow, a dynamical system arising from a higher-rank

noncompact Lie group, on the other. The connection is established via

"transverse extension theorems": Roughly speaking, they claim that a tangential 1- form of the

orbit foliation of the Weyl chamber flow that is tangentially closed

(and satisfies a certain mild additional condition) can be extended to a closed 1- form on the

whole space in a canonical manner. In particular, infinitesimal

rigidity of the orbit foliation of the Weyl chamber flow is proved as an application.

### 2008/11/25

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Circle-valued Morse theory for knots and links

**Andrei Pajitnov**(Univ. de Nantes)Circle-valued Morse theory for knots and links

[ Abstract ]

We will discuss several recent developments in

this theory. In the first part of the talk we prove that the Morse-Novikov number of a knot is less than or equal to twice the tunnel number of the knot, and present consequences of this result. In the second part we report on our joint project with Hiroshi Goda on the half-transversal Morse-Novikov theory for 3-manifolds.

We will discuss several recent developments in

this theory. In the first part of the talk we prove that the Morse-Novikov number of a knot is less than or equal to twice the tunnel number of the knot, and present consequences of this result. In the second part we report on our joint project with Hiroshi Goda on the half-transversal Morse-Novikov theory for 3-manifolds.

### 2008/11/18

17:00-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

複素力学系のくりこみと剛性

**宍倉 光広**(京都大学大学院理学研究科)複素力学系のくりこみと剛性

[ Abstract ]

無限に分岐が集積しているような

ある種の力学系においては、相空間やパラメータ空間の

微細構造が取り上げる個々の力学系の族に寄らない

普遍的構造をもったりすることが知られており、

ある意味で剛性の問題とつながっている。この現象の説明には、

ある部分集合への再帰写像の構成から得られる、あるクラスの

力学系全体の空間で定義されるメタ力学系としての

くりこみ作用素の考え方が重要である。これに関して、

講演者と稲生啓行による中立的不動点をもつ複素力学系の

近放物型くりこみの話を中心に解説する。

無限に分岐が集積しているような

ある種の力学系においては、相空間やパラメータ空間の

微細構造が取り上げる個々の力学系の族に寄らない

普遍的構造をもったりすることが知られており、

ある意味で剛性の問題とつながっている。この現象の説明には、

ある部分集合への再帰写像の構成から得られる、あるクラスの

力学系全体の空間で定義されるメタ力学系としての

くりこみ作用素の考え方が重要である。これに関して、

講演者と稲生啓行による中立的不動点をもつ複素力学系の

近放物型くりこみの話を中心に解説する。

### 2008/11/11

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

The second rational homology group of the moduli space of curves

with level structures

**Thomas Andrew Putman**(MIT)The second rational homology group of the moduli space of curves

with level structures

[ Abstract ]

Let $\\Gamma$ be a finite-index subgroup of the mapping class

group of a closed genus $g$ surface that contains the Torelli group. For

instance, $\\Gamma$ can be the level $L$ subgroup or the spin mapping class

group. We show that $H_2(\\Gamma;\\Q) \\cong \\Q$ for $g \\geq 5$. A corollary

of this is that the rational Picard groups of the associated finite covers

of the moduli space of curves are equal to $\\Q$. We also prove analogous

results for surface with punctures and boundary components.

Let $\\Gamma$ be a finite-index subgroup of the mapping class

group of a closed genus $g$ surface that contains the Torelli group. For

instance, $\\Gamma$ can be the level $L$ subgroup or the spin mapping class

group. We show that $H_2(\\Gamma;\\Q) \\cong \\Q$ for $g \\geq 5$. A corollary

of this is that the rational Picard groups of the associated finite covers

of the moduli space of curves are equal to $\\Q$. We also prove analogous

results for surface with punctures and boundary components.

### 2008/11/04

16:30-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Lefschetz SL(2)-action and cohomology of Kaehler manifolds

**Misha Verbitsky**(ITEP, Moscow)Lefschetz SL(2)-action and cohomology of Kaehler manifolds

[ Abstract ]

Let M be compact Kaehler manifold. It is well

known that any Kaehler form generates a Lefschetz SL(2)-triple

acting on cohomology of M. This action can be used to compute

cohomology of M. If M is a hyperkaehler manifold, of real

dimension 4n, then the subalgebra of its cohomology generated by

the second cohomology is isomorphic to a polynomial algebra,

up to the middle degree.

Let M be compact Kaehler manifold. It is well

known that any Kaehler form generates a Lefschetz SL(2)-triple

acting on cohomology of M. This action can be used to compute

cohomology of M. If M is a hyperkaehler manifold, of real

dimension 4n, then the subalgebra of its cohomology generated by

the second cohomology is isomorphic to a polynomial algebra,

up to the middle degree.