Tuesday Seminar on Topology

Date, time & place Tuesday 17:00 - 18:30 056Room #056 (Graduate School of Math. Sci. Bldg.) KOHNO Toshitake, KAWAZUMI Nariya, KITAYAMA Takahiro, SAKASAI Takuya Tea: 16:30 - 17:00 Common Room

Seminar information archive

2020/07/21

18:00-19:00   Online
Pre-registration required. See our seminar webpage.
Dexie Lin (The University of Tokyo)
Monopole Floer homology for codimension-3 Riemannian foliation (ENGLISH)
[ Abstract ]
In this paper, we give a systematic study of Seiberg-Witten theory on closed oriented manifold with codimension-3 oriented Riemannian foliation. Under a certain topological condition, we construct the basic monopole Floer homologies for a transverse spinc structure with a bundle-like metric, generic perturbation and a complete local system. We will show that these homologies are independent of the bundle-like metric and generic perturbation. The major difference between the basic monopole Floer homologies and the ones on manifolds is the necessity to use the complete local system to construct the monopole Floer homologies.
[ Reference URL ]
https://zoom.us/webinar/register/WN_oS594Z6BRyaKNCvlm3yCoQ

2020/07/14

17:30-18:30   Online
Joint with Lie Groups and Representation Theory Seminar. Pre-registration required. See our seminar webpage.
Takayuki Okuda (Hiroshima University)
Kobayashi's properness criterion and totally geodesic submanifolds in locally symmetric spaces (JAPANESE)
[ Abstract ]
Let G be a Lie group and X a homogeneous G-space. A discrete subgroup of G acting on X properly is called a discontinuous group for X. We are interested in constructions and classifications of discontinuous groups for a given X.

It is well-known that if the isotropies of G on X are compact, any closed subgroup acts on X properly. However, the cases where the isotropies are non-compact, the same claim does not hold in general.

Let us consider the case where G is a linear reductive. In this situation, T. Kobayashi [Math. Ann. (1989)], [J. Lie Theory (1996)]
gave a criterion for the properness of the action on a homogeneous G-space X of closed subgroups in G.

In this talk, we consider homogeneous G-spaces of reductive types realized as families of totally geodesic submanifolds in non-compact Riemannian symmetric spaces. As a main result, we give a translation of Kobayashi's criterion within the framework of Riemannian geometry. In particular, for a torsion-free discrete subgroup of G, the criterion can be stated in terms of totally geodesic submanifolds in the Riemannian locally symmetric space corresponding to the subgroup in G.
[ Reference URL ]
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html

2020/07/07

17:00-18:00   Online
Pre-registration required. See our seminar webpage.
Yuta Nozaki (Hiroshima University)
Abelian quotients of the Y-filtration on the homology cylinders via the LMO functor (JAPANESE)
[ Abstract ]
We construct a series of homomorphisms on the Y-filtration on the homology cylinders via the mod $\mathbb{Z}$ reduction of the LMO functor. The restriction of our homomorphism to the lower central series of the Torelli group does not factor through Morita's refinement of the Johnson homomorphism. We use it to show that the abelianization of the Johnson kernel of a closed surface has torsion elements. This is the joint work with Masatoshi Sato and Masaaki Suzuki.
[ Reference URL ]
https://zoom.us/webinar/register/WN_oS594Z6BRyaKNCvlm3yCoQ

2020/06/30

17:00-18:00   Online
Pre-registration required. See our seminar webpage.
Daniel Matei (IMAR Bucharest)
Homology of right-angled Artin kernels (ENGLISH)
[ Abstract ]
The right-angled Artin groups A(G) are the finitely presented groups associated to a finite simplicial graph G=(V,E), which are generated by the vertices V satisfying commutator relations vw=wv for every edge vw in E. An Artin kernel
Nh(G) is defined by an epimorphism h of A(G) onto the integers. In this talk, we discuss the module structure over the Laurent polynomial ring of the homology groups of Nh(G).
[ Reference URL ]
https://zoom.us/webinar/register/WN_oS594Z6BRyaKNCvlm3yCoQ

2020/06/23

17:00-18:00   Online
Pre-registration required. See our seminar webpage.
Hokuto Konno (The University of Tokyo)
Gauge theory and the diffeomorphism and homeomorphism groups of 4-manifolds (JAPANESE)
[ Abstract ]
I will explain my recent collaboration with several groups that develops gauge theory for families
to extract difference between the diffeomorphism groups and the homeomorphism groups of 4-manifolds.
After Donaldson’s celebrated diagonalization theorem, gauge theory has given strong constraints on the topology of smooth 4-manifolds. Combining such constraints with Freedman’s theory, one may find many non-smoothable topological 4-manifolds.
Recently, a family version of this argument was started by T. Kato, N. Nakamura and myself, and soon later it was developed also by D. Baraglia and his collaborating work with myself. More precisely, considering gauge theory for smooth fiber bundles of 4-manifolds, they obtained some constraints on the topology of smooth 4-manifold bundles. Using such constraints, they detected non-smoothable topological fiber bundles of smooth 4-manifolds. The existence of such bundles implies that there is homotopical difference between the diffeomorphism and homeomorphism groups of the 4-manifolds given as the fibers.
If time permits, I will also mention my collaboration with Baraglia which shows that a K3 surface gives a counterexample to the Nielsen realization problem in dimension 4. This example reveals also that there is difference between the Nielsen realization problems asked in the smooth category and the topological category.
[ Reference URL ]
https://zoom.us/webinar/register/WN_oS594Z6BRyaKNCvlm3yCoQ

2020/01/28

17:00-18:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Nozomu Sekino (The University of Tokyo)
Existence problems for fibered links (JAPANESE)
[ Abstract ]
It is known that every connected orientable closed 3-manifold has a fibered knot. However, finding (and classifying) fibered links whose fiber surfaces are fixed homeomorphism type in a given 3-manifold is difficult in general. We give a criterion of a simple closed curve on a genus 2g Heegaard surface being a genus g fibered knot in terms of its Heegaard diagram. As an application, we can prove the non-existence of genus one fibered knots in some Seifert manifolds.
There is one generalization of fibered links, homologically fibered links. This requests that the complement of the "fiber surface" is a homologically product of a surface and an interval. We give a necessary and sufficient condition for a connected sums of lens spaces of having a homologically fibered link whose fiber surfaces are some fixed types as some algebraic equations.

2020/01/28

18:00-19:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Jun Watanabe (The University of Tokyo)
Fibred cusp b-pseudodifferential operators and its applications (JAPANESE)
[ Abstract ]
Melrose's b-calculus and its variants are important tools to study index problems on manifolds with singularities. In this talk, we introduce a new variant "fibred cusp b-calculus", which is a generalization of fibred cusp calculus of Mazzeo-Melrose and b-calculus of Melrose. We discuss the basic property of this calculus and give a relative index formula. As its application, we prove the index theorem for a Z/k manifold with boundary, which is a generalization of the mod k index theorem of Freed-Melrose.

2020/01/14

17:00-18:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Ryohei Chihara (The University of Tokyo)
SO(3)-invariant G2-geometry (JAPANESE)
[ Abstract ]
Berger's classification of holonomy groups of Riemannian manifolds includes exceptional cases of the Lie groups G2 and Spin(7). Many authors have studied G2- and Spin(7)-manifolds with torus symmetry. In this talk, we generalize the celebrated examples due to Bryant and Salamon and study G2-manifolds with SO(3)-symmetry. Such torsion-free G2-structures are described as a dynamical system of SU(3)-structures on an SO(3)-fibration over a 3-manifold. As a main result, we reduce this system into a constrained Hamiltonian dynamical system on the cotangent bundle over the space of all Riemannian metrics on the 3-manifold. The Hamiltonian function is very similar to that of the Hamiltonian formulation of general relativity.

2020/01/14

18:00-19:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Tsukasa Ishibashi (The University of Tokyo)
Algebraic entropy of sign-stable mutation loops (JAPANESE)
[ Abstract ]
Since its discovery, the cluster algebra has been developed with friutful connections with other branches of mathematics, unifying several combinatorial operations as well as their positivity notions. A mutation loop induces several dynamical systems via cluster transformations, and they form a group which can be seen as a combinatorial generalization of the mapping class groups of marked surfaces.
We introduce a new property of mutation loops called the sign stability, with a focus on an asymptotic behavior of the iteration of the tropicalized cluster X-transformation. A sign-stable mutation loop has a numerical invariant which we call the "cluster stretch factor", in analogy with the stretch factor of a pseudo-Anosov mapping class on a marked surface. We compute the algebraic entropies of the cluster A- and X-transformations induced by a sign-stable mutation loop, and conclude that these two coincide with the logarithm of the cluster stretch factor. This talk is based on a joint work with Shunsuke Kano.

2020/01/07

17:00-18:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Yasuhiko Asao (The University of Tokyo)
Magnitude homology of crushable spaces (JAPANESE)
[ Abstract ]
The magnitude homology and the blurred magnitude homology are novel notions of homology theory for general metric spaces coined by Leinster et al. They are expected to be dealt with in the context of Topological Data Analysis since its original idea is based on a kind of "persistence of points clouds". However, little property of them has been revealed. In this talk, we see that the blurred magnitude homology is trivial when a metric space is contractible by a distance decreasing homotopy. We use techniques from singular homology theory.

2020/01/07

18:00-19:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Tomohiro Asano (The University of Tokyo)
Intersection number estimate of rational Lagrangian immersions in cotangent bundles via microlocal sheaf theory (JAPANESE)
[ Abstract ]
Guillermou associated sheaves to exact Lagrangian submanifolds in cotangent bundles and proved topological properties of the Lagrangian submanifolds. In this talk, I will give an estimate on the displacement energy of rational Lagrangian immersions in cotangent bundles with intersection number estimates via microlocal sheaf theory. This result overlaps with results by Chekanov, Liu, and Akaho via Floer theory. This is joint work with Yuichi Ike.

2019/12/17

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Kei Irie (The University of Tokyo)
Symplectic homology of fiberwise convex sets and homology of loop spaces (JAPANESE)
[ Abstract ]
For any (compact) subset in the symplectic vector space, one can define its symplectic capacity by using symplectic homology, which is a version of Floer homology.
In general, it is very difficult to compute or estimate this capacity directly from its definition, since the definition of Floer homology involves counting solutions of nonlinear PDEs (so called Floer equations). In this talk, we consider the symplectic vector space as the cotangent bundle of the Euclidean space, and show a formula which computes symplectic homology and capacity of fiberwise convex sets from homology of loop spaces. We also explain two applications of this formula.

2019/12/10

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Takeyoshi Kogiso (Josai University)
q-Deformation of a continued fraction and its applications (JAPANESE)
[ Abstract ]
A kind of q-Deformation of continued fractions was introduced by Morier-Genoud and Ovsienko. The most important application of this q-deformation of regular (or negative) continued fraction expansion of rational number r/s is to calculate the Jones polynomial of the rational link of r/s. Moreover we can apply the q-deformation of this continued fraction to quadratic irrational number theory and combinatorics.

On the other hand, there exist another recipes for determing the Jones polynomials by using Lee-Schiffler's snake graph and by using Kogiso-Wakui's Conway-Coxeter frieze method. Therefore, another approach of the result due to Morier-Genoud and Ovsienko can be considered from the viewpoint of a Conway-Coxeter frieze and a snake graph. Furthermore, we can consider a cluster-variable transformation of continued fractions as a further generalization by using ancestoral triangles used in the Kogiso-Wakui.

2019/12/03

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Anton Zeitlin (Louisiana State University)
Homotopy Gerstenhaber algebras, Courant algebroids, and Field Equations (ENGLISH)
[ Abstract ]
I will talk about the underlying homotopical structures within field equations, which emerge in string theory as conformal invariance conditions for sigma models. I will show how these, often hidden, structures emerge from the homotopy Gerstenhaber algebra associated to vertex and Courant algebroids, thus making all such equations the natural objects within vertex algebra theory.

2019/11/26

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Marco De Renzi (Waseda University)
$2+1$-TQFTs from non-semisimple modular categories (ENGLISH)
[ Abstract ]
Non-semisimple constructions have substantially generalized the standard approach of Witten, Reshetikhin, and Turaev to quantum topology, producing powerful invariants and TQFTs with unprecedented properties. We will explain how to use the theory of modified traces to renormalize Lyubashenko’s closed 3-manifold invariants coming from finite twist non-degenerate unimodular ribbon categories. Under the additional assumption of factorizability, our renormalized invariants extend to $2+1$-TQFTs, unlike Lyubashenko’s original ones. This general framework encompasses important examples of non-semisimple modular categories which were left out of previous non-semisimple TQFT constructions.

Based on a joint work with Azat Gainutdinov, Nathan Geer, Bertrand Patureau, and Ingo Runkel.

2019/11/19

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Ramón Barral Lijó (Ritsumeikan University)
The smooth Gromov space and the realization problem (ENGLISH)
[ Abstract ]
The n-dimensional smooth Gromov space consists of the pointed isometry classes of complete Riemannian n-manifolds. In this talk we will present the definition and basic properties of this space as well as two different applications: The first addresses the following classical problem in foliation theory.

Realization problem: what kind of manifolds can be leaves of compact foliations?

Our joint work with Álvarez López has produced the following solution in the context of foliated spaces.

Theorem. Every Riemannian manifold of bounded geometry is a leaf in a compact foliated space X endowed with a metric tensor. Moreover, we can assume that X has trivial holonomy and is transversely Cantor.

The second application is the recent research by Abert and Biringer on the subject of unimodular random Riemannian manifolds.

2019/11/05

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Kiyonori Gomi (Tokyo Institute of Technology)
Magnitude homology of geodesic space (JAPANESE)
[ Abstract ]
Magnitude is an invariant which counts `effective number of points' on a metric space. Its categorification is magnitude homology. This notion is first formulated for metric spaces associated to simple graphs by Hepworth and Willerton, and then for any metric spaces by Leinster and Shulman. The definition of the magnitude homology is easy, but its calculation is rather difficult. For example, the magnitude homology of the circle with geodesic metric was known partially. In my talk, I will explain my result that fully determines the magnitude homology of any geodesic metric space subject to a certain non-branching assumption. In this result, the magnitude homology is described in terms of geodesics. Complete and connected Riemannian manifolds are examples of the geodesic metric spaces satisfying the assumption.

2019/10/29

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Chung-Jun Tsai (National Taiwan University)
Strong stability of minimal submanifolds (ENGLISH)
[ Abstract ]
It is well known that the distance function to a totally geodesic submanifold of a negatively curved ambient manifold is a convex function. One can identify a strong stability condition on minimal submanifolds that generalizes the above scenario. Besides a strong local uniqueness property, a strongly stable minimal submanifold is also Lipschitz stable under the mean curvature flow. We will also discuss some famous local (complete, non-compact) models. This is based on a joint work with Mu-Tao Wang.

2019/10/15

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Gwénaël Massuyeau (Université de Bourgogne)
Generalized Dehn twists on surfaces and surgeries in 3-manifolds (ENGLISH)
[ Abstract ]
(Joint work with Yusuke Kuno.) Given an oriented surface S and a simple closed curve C in S, the "Dehn twist" along C is the homeomorphism of S defined by "twisting" S around C by a full twist. If the curve C is not simple, this transformation of S does not make sense anymore, but one can consider two possible generalizations: one possibility is to use the homotopy intersection form of S to "simulate" the action of a Dehn twist on the (Malcev completion of) the fundamental group of S; another possibility is to view C as a curve on the top boundary of the cylinder S×[0,1], to push it arbitrarily into the interior so as to obtain, by surgery along the resulting knot, a new 3-manifold. In this talk, we will relate two those possible generalizations of a Dehn twist and we will give explicit formulas using a "symplectic expansion" of the fundamental group of S.

2019/10/08

17:30-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Masaki Tsukamoto (Kyushu University)
How can we generalize hyperbolic dynamics to group actions? (JAPANESE)
[ Abstract ]
Hyperbolicity is one of the most fundamental concepts in the study of dynamical systems. It provides rich (expansive and positive entropy) and yet controllable (stable and having some nice measures) dynamical systems. Then, can we generalize this to group actions?

A naive approach seems difficult. For example, suppose $Z^2$ smoothly acts on a finite dimensional compact manifold. Then it is easy to see that its entropy is zero. In other words, there is no rich $Z^2$-actions in the ordinary finite dimensional world. So we must go to infinite dimension. But what kind structure can we expect in the infinite dimensional world?

The purpose of this talk is to explain that mean dimension seems to play an important role in such a research direction. In particular, we explain the following principle :

If $Z^k$ acts on a space $X$ with some hyperbolicity, then we can control the mean dimension of the sub-action of any rank $(k-1)$ subgroup $G$ of $Z^k$.

This talk is based on the joint works with Tom Meyerovitch and Mao Shinoda.

2019/10/01

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Jun Murakami (Waseda University)
Quantized SL(2) representations of knot groups (JAPANESE)
[ Abstract ]
Let K be a knot and G be a group. The representation space of K for the group G means the space of homomorphisms from the knot group to G and is defined by using the group ring C[G], where C[G] is the ring of functions on G and has a commutative Hopf algebra structure. This construction can be generalized to any commutative Hopf algebras.

In this talk, we extend this construction to any braided Hopf algebras with braided commutativity. A typical example is BSL(2), which is the braided SL(2) introduced by S. Majid. Applying the above construction to BSL(2), we get the space of BSL(2) representations, which provides a quantization of SL(2) representations of a knot. This is joint work with Roloand van der Veen.

2019/07/16

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Kimihiko Motegi (Nihon University)
Seifert vs. slice genera of knots in twist families and a characterization of braid axes (JAPANESE)
[ Abstract ]
Twisting a knot $K$ in $S^3$ along a disjoint unknot $c$ produces a twist family of knots $\{K_n\}$ indexed by the integers. Comparing the behaviors of the Seifert genus $g(K_n)$ and the slice genus $g_4(K_n)$ under twistings, we prove that if $g(K_n) - g_4(K_n) < C$ for some constant $C$ for infinitely many integers $n > 0$ or $g(K_n) / g_4(K_n)$ limits to $1$, then the winding number of $K$ about $c$ equals either zero or the wrapping number. As a key application, if $\{K_n\}$ or the mirror twist family $\{\overline{K_n}\}$ contains infinitely many tight fibered knots, then the latter must occur. This leads to the characterization that $c$ is a braid axis of $K$ if and only if both $\{K_n\}$ and $\{\overline{K_n}\}$ each contain infinitely many tight fibered knots. We also give a necessary and sufficient condition for $\{K_n\}$ to contain infinitely many L-space knots, and apply the characterization to prove that satellite L-space knots have braided patterns, which answers a question of both Baker-Moore and Hom in the positive. This result also implies an absence of essential Conway spheres for satellite L-space knots, which gives a partial answer to a conjecture of Lidman-Moore.

2019/07/09

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Florent Schaffhauser (Université de Strasbourg)
Mod 2 cohomology of moduli stacks of real vector bundles (ENGLISH)
[ Abstract ]
The rational cohomology ring of the moduli stack of holomorphic vector bundles of fixed rank and degree over a compact Riemann surface was studied by Atiyah and Bott using tools of differential geometry and algebraic topology: they found generators of that ring and computed its Poincaré series. In joint work with Chiu-Chu Melissa Liu, we study in a similar way the mod 2 cohomology ring of the moduli stack of real vector bundles of fixed topological type over a compact Riemann surface with real structure. The goal of the talk is to explain the principle of that computation, emphasizing the analogies and differences between the real and complex cases, and discuss applications of the method. In particular, we provide explicit generators of mod 2 cohomology rings of moduli stacks of vector bundles over a real algebraic curve.

2019/07/02

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Shun Wakatsuki (The University of Tokyo)
Brane coproducts and their applications (JAPANESE)
[ Abstract ]
The loop coproduct is a coproduct on the homology of the free loop space of a Poincaré duality space (or more generally a Gorenstein space). In this talk, I will introduce two kinds of brane coproducts which are generalizations of the loop coproduct to the homology of a sphere space (i.e. the mapping space from a sphere). Their constructions are based on the finiteness of the dimensions of mapping spaces in some sense. As an application, I will show the vanishing of some cup products on sphere spaces by comparing these two brane coproducts. This gives a generalization of a result of Menichi for the case of free loop spaces.

2019/06/25

17:00-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Tian-Jun Li (University of Minnesota)
Geometry of symplectic log Calabi-Yau surfaces (ENGLISH)
[ Abstract ]
This is a survey on the geometry of symplectic log Calabi-Yau surfaces, which are the symplectic analogues of Looijenga pairs. We address the classification up to symplectic deformation, the relations between symplectic circular sequences and anti-canonical sequences, contact trichotomy, and symplectic fillings. This is a joint work with Cheuk Yu Mak.