Tuesday Seminar on Topology
[Japanese] [Past Programs]
16:00 -- 17:30 Graduate School of Mathematical Sciences,
The University of Tokyo (with live streaming on Zoom)
or
16:00 -- 17:00 Online seminar on Zoom.
Last updated March 21, 2026
Information :
Hokuto Konno
Yuichi Ike
Takuya Sakasai
Pre-registration is required.
Once you register, you can attend all our seminars until the end of March, 2026.
The zoom meeting will be opened 15 minutes before start.
Making audio and video recordings is prohibited.
April 7, 16:00-17:30 -- Room 056 with live streaming on Zoom
Tatsumasa Suzuki (The University of Tokyo)
Price twist and pochette surgery constructing non-simply connected closed 4-manifolds
Abstract: A cut-and-paste operation along an embedded real projective plane in a 4-manifold is called a Price twist.
A Price twist on the 4-sphere produces, up to diffeomorphism, at most three 4-manifolds:
the 4-sphere itself, a homotopy 4-sphere, and a non-simply connected closed 4-manifold.
In general, the classification of diffeomorphism types of non-simply connected closed 4-manifolds is still far from being well understood. In this talk,
we focus on Price twists on the 4-sphere associated with embeddings of the real projective plane of Kinoshita type that yield non-simply connected 4-manifolds.
We present several properties of these manifolds and results on the classification of their diffeomorphism types.
We also explain pochette surgery, introduced by Zjuñici Iwase and Yukio Matsumoto,
which is closely related to the results of this work.
This talk is based on joint work with Tsukasa Isoshima (Keio University).
April 14, 16:00-17:30 -- Room 056 with live streaming on Zoom
Yukihiro Okamoto (Tokyo Metropolitan University)
Non-contractible loops of Legendrian tori from families of knots
Abstract: The unit cotangent bundle of the Euclidean space R3 has a canonical contact structure.
In this talk, we discuss loops of Legendrian tori in this 5-dimensional contact manifold.
In particular,
we focus on loops arising as families of the unit conormal bundles of knots in R3,
and I will explain a topological method to compute the monodromy on the Legendrian contact homology in degree 0 induced by those loops.
As an application, we get examples of non-contractible loops of Legendrian tori which are contractible in the space of smoothly embedded tori.
This is joint work with Marián Poppr.
April 21, 16:00-17:00 -- Online on Zoom
Masaki Taniguchi (Kyoto University)
Exotic diffeomorphisms on a contractible 4-manifold surviving two stabilization
Abstract: Wall's stabilization principle suggests that exotic phenomena in dimension four in the orientable category disappear after taking connected sums with sufficiently many S2xS2.
Since most known exotic pairs of closed 4-manifolds become diffeomorphic after one stabilization,
a natural question was: is a single S2xS2 enough?
Recently, Jianfeng Lin constructed an exotic diffeomorphism on a closed 4-manifold-a diffeomorphism topologically isotopic to the identity but not smoothly isotopic-that survives one stabilization.
In this talk, we provide a relative exotic diffeomorphism on a compact contractible 4-manifold that survives two stabilizations.
This gives the first exotic phenomenon in the orientable category that survives two stabilizations.
This is joint work with Sungkyung Kang and Junghwan Park.
May 12, 16:00-17:00 -- Online on Zoom
Sanghoon Kwak (Seoul National University)
Mapping class group of Infinite graph: 'Big' Out(Fn)
Abstract: Algom-Kfir and Bestvina introduced the mapping class groups of locally finite,
infinite graphs in 2021. Since Out(Fn) can be realized as the mapping group of a finite graph,
their construction may be viewed as a "big" version of Out(Fn). In this talk,
we survey the algebraic and coarse geometric properties of these groups and discuss a relationship with mapping class groups of infinite-type surfaces ("big mapping class groups").
This talk is based on joint work with Ryan Dickmann, George Domat, and Hannah Hoganson, in various collaborations.