## Tuesday Seminar on Topology

Seminar information archive ～04/13｜Next seminar｜Future seminars 04/14～

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**

### 2008/06/03

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

On the geometry of certain slices of the character variety of a knot group

**山口 祥司**(東京大学大学院数理科学研究科)On the geometry of certain slices of the character variety of a knot group

[ Abstract ]

joint work with Fumikazu Nagasato (Meijo University)

This talk is concerned with certain subsets in the character variety of a knot group.

These subsets are called '"slices", which are defined as a level set of a regular function associated to a meridian of a knot.

They are related to character varieties for branched covers along the knot.

Some investigations indicate that an equivariant theory for a knot is connected to a theory for branched covers via slices, for example, the equivariant signature of a knot and the equivariant Casson invariant.

In this talk, we will construct a map from slices into the character varieties for branched covers and investigate the properties.

In particular, we focus on slices called "trace-free", which are used to define the Casson-Lin invariant, and the relation to the character variety for two--fold branched cover.

joint work with Fumikazu Nagasato (Meijo University)

This talk is concerned with certain subsets in the character variety of a knot group.

These subsets are called '"slices", which are defined as a level set of a regular function associated to a meridian of a knot.

They are related to character varieties for branched covers along the knot.

Some investigations indicate that an equivariant theory for a knot is connected to a theory for branched covers via slices, for example, the equivariant signature of a knot and the equivariant Casson invariant.

In this talk, we will construct a map from slices into the character varieties for branched covers and investigate the properties.

In particular, we focus on slices called "trace-free", which are used to define the Casson-Lin invariant, and the relation to the character variety for two--fold branched cover.

### 2008/05/20

17:00-18:30 Room #056 (Graduate School of Math. Sci. Bldg.)

Turaev-Viro TQFT splitting.

**Jer\^ome Petit**(東京工業大学, JSPS)Turaev-Viro TQFT splitting.

[ Abstract ]

The Turaev-Viro invariant is a 3-manifolds invariant. It is obtained in this way :

1) we use a combinatorial description of 3-manifolds, in this case it is : triangulation / Pachner moves

2) we define a scalar thanks to a categorical data (spherical category) and a topological data (triangulation)

3)we verify that the scalar is invariant under Pachner moves, then we obtain a 3-manifolds invariant.

The Turaev-Viro invariant can also be extended to a TQFT. Roughly speaking a TQFT is a data which assigns a finite dimensional vector space to every closed surface and a linear application to every 3-manifold with boundary.

In this talk, we will give a decomposition of the Turaev-Viro TQFT. More precisely, we decompose it into blocks. These blocks are given by a group associated to the spherical category which was used to construct the Turaev-Viro invariant. We will show that these blocks define a HQFT (roughly speaking a TQFT with an "homotopical data"). This HQFT is obtained from an homotopical invariant, which is the homotopical version of the Turaev-Viro invariant. Moreover this invariant can be used to obtain the homological Turaev-Viro invariant defined by Yetter.

The Turaev-Viro invariant is a 3-manifolds invariant. It is obtained in this way :

1) we use a combinatorial description of 3-manifolds, in this case it is : triangulation / Pachner moves

2) we define a scalar thanks to a categorical data (spherical category) and a topological data (triangulation)

3)we verify that the scalar is invariant under Pachner moves, then we obtain a 3-manifolds invariant.

The Turaev-Viro invariant can also be extended to a TQFT. Roughly speaking a TQFT is a data which assigns a finite dimensional vector space to every closed surface and a linear application to every 3-manifold with boundary.

In this talk, we will give a decomposition of the Turaev-Viro TQFT. More precisely, we decompose it into blocks. These blocks are given by a group associated to the spherical category which was used to construct the Turaev-Viro invariant. We will show that these blocks define a HQFT (roughly speaking a TQFT with an "homotopical data"). This HQFT is obtained from an homotopical invariant, which is the homotopical version of the Turaev-Viro invariant. Moreover this invariant can be used to obtain the homological Turaev-Viro invariant defined by Yetter.

### 2008/05/13

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

The problem of maximum Thurston--Bennequin number for knots

**Tamas Kalman**(東京大学大学院数理科学研究科, JSPS)The problem of maximum Thurston--Bennequin number for knots

[ Abstract ]

Legendrian submanifolds of contact 3-manifolds are

one-dimensional, just like knots. This ``coincidence'' gives rise to an

interesting and expanding intersection of contact and symplectic geometry

on the one hand and classical knot theory on the other. As an

illustration, we will survey recent results on maximizing the

Thurston--Bennequin number (which is a measure of the twisting of the

contact structure along a Legendrian) within a smooth knot type. In

particular, we will show how Kauffman's state circles can be used to solve

the maximization problem for so-called +adequate (among them, alternating

and positive) knots and links.

Legendrian submanifolds of contact 3-manifolds are

one-dimensional, just like knots. This ``coincidence'' gives rise to an

interesting and expanding intersection of contact and symplectic geometry

on the one hand and classical knot theory on the other. As an

illustration, we will survey recent results on maximizing the

Thurston--Bennequin number (which is a measure of the twisting of the

contact structure along a Legendrian) within a smooth knot type. In

particular, we will show how Kauffman's state circles can be used to solve

the maximization problem for so-called +adequate (among them, alternating

and positive) knots and links.

### 2008/04/22

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

Special fibers of pencils of hypersurfaces

**Sergey Yuzvinsky**(University of Oregon)Special fibers of pencils of hypersurfaces

[ Abstract ]

We consider pencils of hypersurfaces of degree $d>1$ in the complex

$n$-dimensional projective space subject to the condition that the

generic fiber is irreducible. We study the set of completely reducible

fibers, i.e., the unions of hyperplanes. The first surprinsing result is

that the cardinality of thie set has very strict uniformed upper bound

(not depending on $d$ or $n$). The other one gives a characterization

of this set in terms of either topology of its complement or combinatorics

of hyperplanes. We also include into consideration more general special

fibers are iimportant for characteristic varieties of the hyperplane

complements.

We consider pencils of hypersurfaces of degree $d>1$ in the complex

$n$-dimensional projective space subject to the condition that the

generic fiber is irreducible. We study the set of completely reducible

fibers, i.e., the unions of hyperplanes. The first surprinsing result is

that the cardinality of thie set has very strict uniformed upper bound

(not depending on $d$ or $n$). The other one gives a characterization

of this set in terms of either topology of its complement or combinatorics

of hyperplanes. We also include into consideration more general special

fibers are iimportant for characteristic varieties of the hyperplane

complements.

### 2008/04/15

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

On the invariants of knots and 3-manifolds related to the restricted quantum group

**村上 順**(早稲田大学理工)On the invariants of knots and 3-manifolds related to the restricted quantum group

[ Abstract ]

I would like to talk about the colored Alexander invariant and the logarithmic

invariant of knots and links. They are constructed from the universal R-matrices

of the semi-resetricted and restricted quantum groups of sl(2) respectively,

and they are related to the hyperbolic volumes of the cone manifolds along

the knot. I also would like to explain an attempt to generalize these invariants to

a three manifold invariant which relates to the volume of the manifold actually.

I would like to talk about the colored Alexander invariant and the logarithmic

invariant of knots and links. They are constructed from the universal R-matrices

of the semi-resetricted and restricted quantum groups of sl(2) respectively,

and they are related to the hyperbolic volumes of the cone manifolds along

the knot. I also would like to explain an attempt to generalize these invariants to

a three manifold invariant which relates to the volume of the manifold actually.

### 2008/01/29

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

The rotation number function on groups of circle diffeomorphisms

A Diagrammatic Construction of Third Homology Classes of Knot Quandles

**松田 能文**(東京大学大学院数理科学研究科) 16:30-17:30The rotation number function on groups of circle diffeomorphisms

[ Abstract ]

ポアンカレは、円周の向きを保つ同相写像に対して、回転数の有理性と有限軌道の存在が

同値であることを示した。この講演では、この事実が円周の向きを保つ同相写像のなすあ

る種の群に対して一般化できることを説明する。特に、円周の向きを保つ実解析的微分同

相のなす非離散的な群に対して、回転数関数による像の有限性と有限軌道の存在が同値で

あることを示す。

ポアンカレは、円周の向きを保つ同相写像に対して、回転数の有理性と有限軌道の存在が

同値であることを示した。この講演では、この事実が円周の向きを保つ同相写像のなすあ

る種の群に対して一般化できることを説明する。特に、円周の向きを保つ実解析的微分同

相のなす非離散的な群に対して、回転数関数による像の有限性と有限軌道の存在が同値で

あることを示す。

**木村 康人**(東京大学大学院数理科学研究科) 17:30-18:30A Diagrammatic Construction of Third Homology Classes of Knot Quandles

[ Abstract ]

There exists a family of third (quandle / rack) homology classes,

called the shadow (fundamental / diagram) classes,

of the knot quandle, which are obtained from

the shadow colourings of knot diagrams.

We will show the construction of these homology classes,

and also show their relation to the shadow quandle cocycle

invariants of knots and that to other third homology classes.

There exists a family of third (quandle / rack) homology classes,

called the shadow (fundamental / diagram) classes,

of the knot quandle, which are obtained from

the shadow colourings of knot diagrams.

We will show the construction of these homology classes,

and also show their relation to the shadow quandle cocycle

invariants of knots and that to other third homology classes.

### 2008/01/15

16:30-17:30 Room #056 (Graduate School of Math. Sci. Bldg.)

Adiabatic limits of eta-invariants and the Meyer functions

**飯田 修一**(東京大学大学院数理科学研究科)Adiabatic limits of eta-invariants and the Meyer functions

[ Abstract ]

The Meyer function is the function defined on the hyperelliptic

mapping class group, which gives a signature formula for surface

bundles over surfaces.

In this talk, we introduce certain generalizations of the Meyer

function by using eta-invariants and we discuss the uniqueness of this

function and compute the values for Dehn twists.

The Meyer function is the function defined on the hyperelliptic

mapping class group, which gives a signature formula for surface

bundles over surfaces.

In this talk, we introduce certain generalizations of the Meyer

function by using eta-invariants and we discuss the uniqueness of this

function and compute the values for Dehn twists.

### 2007/12/18

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

Groupoid lifts of representations of mapping classes

**R.C. Penner**(USC and Aarhus University)Groupoid lifts of representations of mapping classes

[ Abstract ]

The "Ptolemy groupoid" is the fundamental path groupoid of the dual to the ideal cell decomposition of the decorated Teichmueller space of a punctured or bordered surface, and the "Torelli groupoid" is thesimilar discretization of the fundamental path groupoid of the quotient

by the Torelli subgroup of mapping classes that acts identically on the first integral homology of the surface. Mapping classes can be represented as appropriate elements of the Ptolemy groupoid and likewise for elements of the Torelli group in the Torelli groupoid.

A natural series of questions is to wonder which representations of mapping class groups, Torelli groups, and their subgroups can be lifted to the groupoid level. In a series of joint works with J. Andersen, A. Bene, N. Kawazumi, and S. Morita, we have given explicit lifts of a number of classical representations: The Johnson representations of the classical and higher Torelli groups

and the symplectic representation of the mapping class group all lift to the Torelli groupoid. Furthermore, the Nielsen representation of the mapping class group as automorphisms of a

free group lifts to the Ptolemy groupoid, and hence so too does any representation

of the mapping class group that factors through its action on the fundamental group of

the surface such as the Magnus representation. We shall survey these various groupoid lifts and discuss current and potential future applications.

The "Ptolemy groupoid" is the fundamental path groupoid of the dual to the ideal cell decomposition of the decorated Teichmueller space of a punctured or bordered surface, and the "Torelli groupoid" is thesimilar discretization of the fundamental path groupoid of the quotient

by the Torelli subgroup of mapping classes that acts identically on the first integral homology of the surface. Mapping classes can be represented as appropriate elements of the Ptolemy groupoid and likewise for elements of the Torelli group in the Torelli groupoid.

A natural series of questions is to wonder which representations of mapping class groups, Torelli groups, and their subgroups can be lifted to the groupoid level. In a series of joint works with J. Andersen, A. Bene, N. Kawazumi, and S. Morita, we have given explicit lifts of a number of classical representations: The Johnson representations of the classical and higher Torelli groups

and the symplectic representation of the mapping class group all lift to the Torelli groupoid. Furthermore, the Nielsen representation of the mapping class group as automorphisms of a

free group lifts to the Ptolemy groupoid, and hence so too does any representation

of the mapping class group that factors through its action on the fundamental group of

the surface such as the Magnus representation. We shall survey these various groupoid lifts and discuss current and potential future applications.

### 2007/12/11

16:30-18:40 Room #056 (Graduate School of Math. Sci. Bldg.)

A Singular Version of The Poincar\\'e-Hopf Theorem

Chen Ruan cohomology of cotangent orbifolds and Chas-Sullivan string topology

**Xavier G\'omez-Mont**(CIMAT, Mexico) 16:30-17:30A Singular Version of The Poincar\\'e-Hopf Theorem

[ Abstract ]

The Poincar\\'e-Hopf Theorem asserts that the Euler Characteristic of a compact manifold is the sum of the indices of any vector field on it with isolated singularities.

A hypersurface in real or complex number space may be considered as the limit of the smooth hypersurfaces obtained from nearby regular values. The singularity contains “hidden” topology, which is unfolded by a smooth regeneration. At the singularity one has an algebraic invariant, the Jacobi Algebra, which is obtained by considering analytic functions modulo the partial derivatives. It contains topological information of the singularity.

One may consider vector fields tangent to a hypersurface with isolated singularities, and define topologically an index, which coincides with the sum of the Poincar\\'e-Hopf indices of a regeneration of it tangent to a nearby smooth hypersurface.

I will explain how to compute the index of a vector field X tangent to an isolated hypersurface singularity V using Homological Algebra, as the Euler Characteristic of the homology of the complex obtained by contracting differential forms on V with the vector field X. The formula contains several terms, but the higher order terms may be translated from the invariants of the singular point to invariants in the Jacobi Algebra, making this translation a local version of the Poincar\\'e-Hopf Theorem.

I will also explain how some of these ideas can be extended to complete intersections.

The Poincar\\'e-Hopf Theorem asserts that the Euler Characteristic of a compact manifold is the sum of the indices of any vector field on it with isolated singularities.

A hypersurface in real or complex number space may be considered as the limit of the smooth hypersurfaces obtained from nearby regular values. The singularity contains “hidden” topology, which is unfolded by a smooth regeneration. At the singularity one has an algebraic invariant, the Jacobi Algebra, which is obtained by considering analytic functions modulo the partial derivatives. It contains topological information of the singularity.

One may consider vector fields tangent to a hypersurface with isolated singularities, and define topologically an index, which coincides with the sum of the Poincar\\'e-Hopf indices of a regeneration of it tangent to a nearby smooth hypersurface.

I will explain how to compute the index of a vector field X tangent to an isolated hypersurface singularity V using Homological Algebra, as the Euler Characteristic of the homology of the complex obtained by contracting differential forms on V with the vector field X. The formula contains several terms, but the higher order terms may be translated from the invariants of the singular point to invariants in the Jacobi Algebra, making this translation a local version of the Poincar\\'e-Hopf Theorem.

I will also explain how some of these ideas can be extended to complete intersections.

**Miguel A. Xicotencatl**(CINVESTAV, Mexico) 17:40-18:40Chen Ruan cohomology of cotangent orbifolds and Chas-Sullivan string topology

[ Abstract ]

(Joint with: A. Gonzalez, E. Lupercio, C. Segovia, and B. Uribe)

At the end of 90's, two theories of topology were invented roughly at the same time and attracted considerable interest in the mathematical community. One is the Chas-Sullivan's loop product on the homology of loop space and the second one is Chen-Ruan's stringy cohomology of orbifold. It was an observation of Chen that inertia orbifold (which carries Chen-Ruan cohomology) is the space of constant loops of an orbifold. Therefore, two theories should interact. In this work we show that for an interesting family of orbifolds, the virtual orbifold cohomology, turns out to be a subalgebra of the homology of the loop orbifold, and is isomorphic, as algebras, to the Chen-Ruan orbifold cohomology of its cotangent orbifold.

(Joint with: A. Gonzalez, E. Lupercio, C. Segovia, and B. Uribe)

At the end of 90's, two theories of topology were invented roughly at the same time and attracted considerable interest in the mathematical community. One is the Chas-Sullivan's loop product on the homology of loop space and the second one is Chen-Ruan's stringy cohomology of orbifold. It was an observation of Chen that inertia orbifold (which carries Chen-Ruan cohomology) is the space of constant loops of an orbifold. Therefore, two theories should interact. In this work we show that for an interesting family of orbifolds, the virtual orbifold cohomology, turns out to be a subalgebra of the homology of the loop orbifold, and is isomorphic, as algebras, to the Chen-Ruan orbifold cohomology of its cotangent orbifold.

### 2007/12/04

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

Morse theory for abelian hyperkahler quotients

**今野 宏**(東京大学大学院数理科学研究科)Morse theory for abelian hyperkahler quotients

[ Abstract ]

In 1980's Kirwan computed Betti numbers of symplectic quotients by using Morse theory. In this talk, we develop this method to hyperkahler quotients by abelian Lie groups. In this method, many computations are much more simplified in the case of hyperkahler quotients than the case of symplectic quotients. As a result we compute not only the Betti numbers, but also the cohomology rings of abelian hyperkahler quotients.

In 1980's Kirwan computed Betti numbers of symplectic quotients by using Morse theory. In this talk, we develop this method to hyperkahler quotients by abelian Lie groups. In this method, many computations are much more simplified in the case of hyperkahler quotients than the case of symplectic quotients. As a result we compute not only the Betti numbers, but also the cohomology rings of abelian hyperkahler quotients.

### 2007/11/27

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

A quandle cocycle invariant for handlebody-links

**石井 敦**(京都大学数理解析研究所)A quandle cocycle invariant for handlebody-links

[ Abstract ]

[joint work with Masahide Iwakiri (Osaka City University)]

A handlebody-link is a disjoint union of circles and a

finite trivalent graph embedded in a closed 3-manifold.

We consider it up to isotopies and IH-moves.

Then it represents an ambient isotopy class of

handlebodies embedded in the closed 3-manifold.

In this talk, I explain how a quandle cocycle invariant

is defined for handlebody-links.

[joint work with Masahide Iwakiri (Osaka City University)]

A handlebody-link is a disjoint union of circles and a

finite trivalent graph embedded in a closed 3-manifold.

We consider it up to isotopies and IH-moves.

Then it represents an ambient isotopy class of

handlebodies embedded in the closed 3-manifold.

In this talk, I explain how a quandle cocycle invariant

is defined for handlebody-links.

### 2007/11/20

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

A certain slice of the character variety of a knot group

and the knot contact homology

**長郷 文和**(東京工業大学大学院理工学研究科)A certain slice of the character variety of a knot group

and the knot contact homology

[ Abstract ]

For a knot $K$ in 3-sphere, we can consider representations of

the knot group $G_K$ into $SL(2,\\mathbb{C})$.

Their characters construct an algebraic set.

This is so-called the $SL(2,\\mathbb{C})$-character variety of

$G_K$ and denoted by $X(G_K)$.

In this talk, we introduce a slice (a subset) $S_0(K)$ of $X(G_K)$.

In fact, this slice is closely related to the A-polynomial

and the abelian knot contact homology.

For example, the A-polynomial $A_K(m,l)$ of a knot $K$ is

a two-variable polynomial knot invariant defined by using

the character variety $X(G_K)$.

Then we can show that for any {\\it small knot} $K$, the number of

irreducible components of $S_0(K)$ gives an upper bound of

the maximal degree of the A-polynomial $A_K(m,l)$ in terms of

the variable $l$.

Moreover, for any 2-bridge knot $K$, we can show that

the coordinate ring of $S_0(K)$ is exactly the degree 0

abelian knot contact homology $HC_0^{ab}(K)$.

We will mainly explain these facts.

For a knot $K$ in 3-sphere, we can consider representations of

the knot group $G_K$ into $SL(2,\\mathbb{C})$.

Their characters construct an algebraic set.

This is so-called the $SL(2,\\mathbb{C})$-character variety of

$G_K$ and denoted by $X(G_K)$.

In this talk, we introduce a slice (a subset) $S_0(K)$ of $X(G_K)$.

In fact, this slice is closely related to the A-polynomial

and the abelian knot contact homology.

For example, the A-polynomial $A_K(m,l)$ of a knot $K$ is

a two-variable polynomial knot invariant defined by using

the character variety $X(G_K)$.

Then we can show that for any {\\it small knot} $K$, the number of

irreducible components of $S_0(K)$ gives an upper bound of

the maximal degree of the A-polynomial $A_K(m,l)$ in terms of

the variable $l$.

Moreover, for any 2-bridge knot $K$, we can show that

the coordinate ring of $S_0(K)$ is exactly the degree 0

abelian knot contact homology $HC_0^{ab}(K)$.

We will mainly explain these facts.

### 2007/11/06

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

Thustion's inequality and open book foliations

**児玉 大樹**(東京大学大学院数理科学研究科)Thustion's inequality and open book foliations

[ Abstract ]

We will study codimension 1 foliations on 3-manifolds.

Thurston's inequality, which implies convexity of the foliation in

some sense, folds for Reebless foliations [Th]. We will discuss

whether the inequality holds or not for open book foliations.

[Th] W. Thurston: Norm on the homology of 3-manifolds, Memoirs of the

AMS, 339 (1986), 99--130.

We will study codimension 1 foliations on 3-manifolds.

Thurston's inequality, which implies convexity of the foliation in

some sense, folds for Reebless foliations [Th]. We will discuss

whether the inequality holds or not for open book foliations.

[Th] W. Thurston: Norm on the homology of 3-manifolds, Memoirs of the

AMS, 339 (1986), 99--130.

### 2007/10/30

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

$L_{\\infty}$ action on Lagrangian filtered $A_{\\infty}$ algebras.

**太田 啓史**(名大多元数理)$L_{\\infty}$ action on Lagrangian filtered $A_{\\infty}$ algebras.

[ Abstract ]

I will discuss $L_{\\infty}$ actions on Lagrangian filtered

$A_{\\infty}$ algebras by cycles of the ambient symplectic

manifold. In the course of the construction,

I like to remark that the stable map compactification is not

sufficient in some case when we consider ones from genus zero

bordered Riemann surface. Also, if I have time, I like to discuss

some relation to (absolute) Gromov-Witten invariant and other

applications.

(This talk is based on my joint work with K.Fukaya, Y-G Oh and K. Ono.)

I will discuss $L_{\\infty}$ actions on Lagrangian filtered

$A_{\\infty}$ algebras by cycles of the ambient symplectic

manifold. In the course of the construction,

I like to remark that the stable map compactification is not

sufficient in some case when we consider ones from genus zero

bordered Riemann surface. Also, if I have time, I like to discuss

some relation to (absolute) Gromov-Witten invariant and other

applications.

(This talk is based on my joint work with K.Fukaya, Y-G Oh and K. Ono.)

### 2007/10/23

16:30-18:00 Room #002 (Graduate School of Math. Sci. Bldg.)

Spaces of subspheres and their applications

**Jun O'Hara**(首都大学東京)Spaces of subspheres and their applications

[ Abstract ]

The set of q-dimensional subspheres in S^n is a Grassmann manifold which has natural pseudo-Riemannian structure, and in some cases, symplectic structure as well. Both of them are conformally invariant.

I will explain some examples of their applications to geometric aspects of knots and links.

The set of q-dimensional subspheres in S^n is a Grassmann manifold which has natural pseudo-Riemannian structure, and in some cases, symplectic structure as well. Both of them are conformally invariant.

I will explain some examples of their applications to geometric aspects of knots and links.

### 2007/10/16

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

Toric Sasaki-Einstein manifolds

**二木 昭人**(東京工業大学大学院理工学研究科)Toric Sasaki-Einstein manifolds

[ Abstract ]

A compact toric Sasaki manifold admits a Sasaki-Einstein metric if and only if it is obtained by the Delzant construction from a toric diagram of a constant height. As an application we see that the canonical line bundle of a toric Fano manifold admits a complete Ricci-flat K\\"ahler metric.

A compact toric Sasaki manifold admits a Sasaki-Einstein metric if and only if it is obtained by the Delzant construction from a toric diagram of a constant height. As an application we see that the canonical line bundle of a toric Fano manifold admits a complete Ricci-flat K\\"ahler metric.

### 2007/10/09

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

Classification of codimension-one locally free actions of the affine group of the real line.

**浅岡 正幸**(京都大学大学院理学研究科)Classification of codimension-one locally free actions of the affine group of the real line.

[ Abstract ]

By GA, we denote the group of affine and orientation-preserving transformations

of the real line. In this talk, I will report on classification of locally free action of

GA on closed three manifolds, which I obtained recently. In 1979, E.Ghys proved

that if such an action preserves a volume, then it is smoothly conjugate to a homogeneous action. However, it was unknown whether non-homogeneous action exists. As a consequence of the classification, we will see that the unit tangent bundle of a closed surface of higher genus admits a finite-parameter family of

non-homogeneous actions.

By GA, we denote the group of affine and orientation-preserving transformations

of the real line. In this talk, I will report on classification of locally free action of

GA on closed three manifolds, which I obtained recently. In 1979, E.Ghys proved

that if such an action preserves a volume, then it is smoothly conjugate to a homogeneous action. However, it was unknown whether non-homogeneous action exists. As a consequence of the classification, we will see that the unit tangent bundle of a closed surface of higher genus admits a finite-parameter family of

non-homogeneous actions.

### 2007/07/17

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

Orbifold Cohomology of Wreath Product Orbifolds and

Cohomological HyperKahler Resolution Conjecture

**松村 朝雄**(東京大学大学院数理科学研究科)Orbifold Cohomology of Wreath Product Orbifolds and

Cohomological HyperKahler Resolution Conjecture

[ Abstract ]

Chen-Ruan orbifold cohomology ring was introduced in 2000 as

the degree zero genus zero orbifold Gromov-Witten invariants with

three marked points. We will review its construction in the case of

global quotient orbifolds, following Fantechi-Gottsche and

Jarvis-Kaufmann-Kimura. We will describe the orbifold cohomology of

wreath product orbifolds and explain its application to Ruan's

cohomological hyperKahler resolution conjecture.

Chen-Ruan orbifold cohomology ring was introduced in 2000 as

the degree zero genus zero orbifold Gromov-Witten invariants with

three marked points. We will review its construction in the case of

global quotient orbifolds, following Fantechi-Gottsche and

Jarvis-Kaufmann-Kimura. We will describe the orbifold cohomology of

wreath product orbifolds and explain its application to Ruan's

cohomological hyperKahler resolution conjecture.

### 2007/07/10

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

Combable functions, quasimorphisms, and the central limit theorem

(joint with Koji Fujiwara)

**Danny C. Calegari**(California Institute of Technology)Combable functions, quasimorphisms, and the central limit theorem

(joint with Koji Fujiwara)

[ Abstract ]

Quasimorphisms on groups are dual to stable commutator length,

and detect extremal phenomena in topology and dynamics. In typical groups

(even in a free group) stable commutator length is very difficult to

calculate, because the space of quasimorphisms is too large to study

directly without adding more structure.

In this talk, we show that a large class of quasimorphisms - the so-called

"counting quasimorphisms" on word-hyperbolic groups - can be effectively

described using simple machines called finite state automata. From this,

and from the ergodic theory of finite directed graphs, one can deduce a

number of properties about the statistical distribution of the values of a

counting quasimorphism on elements of the group.

Quasimorphisms on groups are dual to stable commutator length,

and detect extremal phenomena in topology and dynamics. In typical groups

(even in a free group) stable commutator length is very difficult to

calculate, because the space of quasimorphisms is too large to study

directly without adding more structure.

In this talk, we show that a large class of quasimorphisms - the so-called

"counting quasimorphisms" on word-hyperbolic groups - can be effectively

described using simple machines called finite state automata. From this,

and from the ergodic theory of finite directed graphs, one can deduce a

number of properties about the statistical distribution of the values of a

counting quasimorphism on elements of the group.

### 2007/07/03

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

Two invariants of pseudo--Anosov mapping classes: hyperbolic volume vs dilatation

(joint work with Mitsuhiko Takasawa)

**金 英子**(東京工業大学情報理工学研究科)Two invariants of pseudo--Anosov mapping classes: hyperbolic volume vs dilatation

(joint work with Mitsuhiko Takasawa)

[ Abstract ]

We concern two invariants of pseudo-Anosov mapping classes.

One is the dilatation of pseudo--Anosov maps and the other is the volume

of mapping tori. To study how two invariants are related, fixing a surface

we represent a mapping class by using the standard generator set and compute

these two for all pseudo--Anosov mapping classes with up to some word length.

In the talk, we observe two properties:

(1) The ratio of the topological entropy (i.e. logarithm of the dilatation) to

the volume is bounded from below by some positive constant which only

depends on the surface.

(2) The conjugacy class having the minimal dilatation reaches the minimal volume.

On the observation (1), in case of the mapping class group of a once--punctured

torus, we give a concrete lower bound of the ratio.

We concern two invariants of pseudo-Anosov mapping classes.

One is the dilatation of pseudo--Anosov maps and the other is the volume

of mapping tori. To study how two invariants are related, fixing a surface

we represent a mapping class by using the standard generator set and compute

these two for all pseudo--Anosov mapping classes with up to some word length.

In the talk, we observe two properties:

(1) The ratio of the topological entropy (i.e. logarithm of the dilatation) to

the volume is bounded from below by some positive constant which only

depends on the surface.

(2) The conjugacy class having the minimal dilatation reaches the minimal volume.

On the observation (1), in case of the mapping class group of a once--punctured

torus, we give a concrete lower bound of the ratio.

### 2007/06/12

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

The Kodaira dimension of symplectic 4-manifolds

**Tian-Jun Li**(University of Minnesota)The Kodaira dimension of symplectic 4-manifolds

[ Abstract ]

Various results and questions about symplectic4-manifolds can be

formulated in terms of the notion of the Kodaira dimension. In particular,

we will discuss the classification and the geography problems. It is interesting

to understand how it behaves undersome basic constructions.Time permitting

we will discuss the symplectic birational aspect of this notion and speculate

how to extend it to higher dimensional manifolds.

Various results and questions about symplectic4-manifolds can be

formulated in terms of the notion of the Kodaira dimension. In particular,

we will discuss the classification and the geography problems. It is interesting

to understand how it behaves undersome basic constructions.Time permitting

we will discuss the symplectic birational aspect of this notion and speculate

how to extend it to higher dimensional manifolds.

### 2007/06/05

17:00-18:30 Room #117 (Graduate School of Math. Sci. Bldg.)

Symplectic mapping classes and fillings

**Emmanuel Giroux**(ENS Lyon)Symplectic mapping classes and fillings

[ Abstract ]

We will describe a joint work in progress with Paul Biran in

which contact geometry is combined with properties of Lagrangian manifolds

in subcritical Stein domains to obtain nontrivaility results for symplectic

mapping classes.

We will describe a joint work in progress with Paul Biran in

which contact geometry is combined with properties of Lagrangian manifolds

in subcritical Stein domains to obtain nontrivaility results for symplectic

mapping classes.

### 2007/05/15

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

Kontsevich's characteristic classes for higher dimensional homology sphere bundles

**渡邉 忠之**(京都大学数理解析研究所)Kontsevich's characteristic classes for higher dimensional homology sphere bundles

[ Abstract ]

As an analogue of the perturbative Chern-Simons theory, Maxim Kontsevich

constructed universal characteristic classes of smooth fiber bundles with fiber

diffeomorphic to a singularly framed odd dimensional homology sphere.

In this talk, I will give a sketch proof of our result on non-triviality of the

Kontsevich classes for 7-dimensional homology sphere bundles.

As an analogue of the perturbative Chern-Simons theory, Maxim Kontsevich

constructed universal characteristic classes of smooth fiber bundles with fiber

diffeomorphic to a singularly framed odd dimensional homology sphere.

In this talk, I will give a sketch proof of our result on non-triviality of the

Kontsevich classes for 7-dimensional homology sphere bundles.

### 2007/05/08

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

On the vanishing of the Rohlin invariant

**森山 哲裕**(東京大学大学院数理科学研究科)On the vanishing of the Rohlin invariant

[ Abstract ]

The vanishing of the Rohlin invariant of an amphichiral integral

homology $3$-sphere $M$ (i.e. $M \\cong -M$) is a natural consequence

of some elementary properties of the Casson invariant. In this talk, we

give a new direct (and more elementary) proof of this vanishing

property. The main idea comes from the definition of the degree 1

part of the Kontsevich-Kuperberg-Thurston invariant, and we progress

by constructing some $7$-dimensional manifolds in which $M$ is embedded.

The vanishing of the Rohlin invariant of an amphichiral integral

homology $3$-sphere $M$ (i.e. $M \\cong -M$) is a natural consequence

of some elementary properties of the Casson invariant. In this talk, we

give a new direct (and more elementary) proof of this vanishing

property. The main idea comes from the definition of the degree 1

part of the Kontsevich-Kuperberg-Thurston invariant, and we progress

by constructing some $7$-dimensional manifolds in which $M$ is embedded.

### 2007/04/24

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

Realization of twisted $K$-theory and

finite-dimensional approximation of Fredholm operators

**五味 清紀**(東京大学大学院数理科学研究科)Realization of twisted $K$-theory and

finite-dimensional approximation of Fredholm operators

[ Abstract ]

A problem in twisted $K$-theory is to realize twisted $K$-groups generally by means of finite-dimensional geometric objects, like vector bundles. I would like to talk about an approach toward the problem by means of Mikio Furuta's generalized vector bundles. By using a twisted version of the generalized vector bundle and a finite-dimensional approximation of Fredholm operators, I construct a group into which there exists a natural injection from the twisted $K$-group twisted by any third integral cohomology class.

A problem in twisted $K$-theory is to realize twisted $K$-groups generally by means of finite-dimensional geometric objects, like vector bundles. I would like to talk about an approach toward the problem by means of Mikio Furuta's generalized vector bundles. By using a twisted version of the generalized vector bundle and a finite-dimensional approximation of Fredholm operators, I construct a group into which there exists a natural injection from the twisted $K$-group twisted by any third integral cohomology class.