2007年4月 -- 7月
[English]
[過去のプログラム]
16:30 -- 18:00 数理科学研究科棟(東京大学駒場キャンパス)
Tea: 16:00 -- 16:30 コモンルーム
Last updated July 9, 2007
世話係
河野俊丈
河澄響矢
4月17日 -- 056号室, 16:30 -- 18:00
小林 俊行 (東京大学大学院数理科学研究科)
閉じた局所対称空間の存在問題について
(Existence Problem of Compact Locally Symmetric Spaces)
Abstract:
The local to global study of geometries was a major trend of 20th century geometry, with remarkable developments achieved particularly in Riemannian geometry. In contrast, in areas such as Lorentz geometry, familiar to us as the space-time of relativity theory, and more generally in pseudo-Riemannian geometry, as well as in various other kinds of geometry (symplectic, complex geometry, ...), surprising little is known about global properties of the geometry even if we impose a locally homogeneous structure.
In this talk, I will give a survey on the recent developments on the question about how the local geometric structure affects the global nature of non-Riemannian manifolds with emphasis on the existence problem of compact models of locally symmetric spaces.
4月24日 -- 056号室, 16:30 -- 18:00
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.
5月8日 -- 056号室, 16:30 -- 18:00
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.
5月15日 -- 056号室, 16:30 -- 18:00
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.
6月5日 -- 117号室, 17:00 -- 18:30
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.
6月12日 -- 056号室, 16:30 -- 18:00
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 under some basic constructions.Time permitting
we will discuss the symplectic birational aspect of this notion and speculate
how to extend it to higher dimensional manifolds.
7月3日 -- 056号室, 16:30 -- 18:00
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.
7月10日 -- 056号室, 16:30 -- 18:00
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.
7月17日 -- 056号室, 16:30 -- 18:00
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.