Seminar information archive

Seminar information archive ~04/19Today's seminar 04/20 | Future seminars 04/21~

FMSP Lectures

13:00-15:05   Room #002 (Graduate School of Math. Sci. Bldg.)
Chung-jun Tsai (National Taiwan University)
Topic on minimal submanifolds (6/6) (ENGLISH)
[ Abstract ]
The main theme of these lectures will be theory about minimal submanifolds, which are higher dimensional generalizations of geodesics. A naive motivation is that one tries to understand the geometry from its special submanifolds (minimal, etc.).

For minimal submanifolds, the equations are no longer ODEs, but elliptic PDEs. This increases the difficulties. The study are very good examples for the application of methods from PDEs and calculus of variations. We will try to explain some important results in this theory, which stimulate many of the researches today.

Here are some specific materials we plan to cover: Simon’s work based on the second variational formula, Sacks - Uhlenback theorem on the existence of minimal 2-spheres, the theory of stable minimal hypersurfaces by Schoen-Simon-Yau.
[ Reference URL ]
http://fmsp.ms.u-tokyo.ac.jp/FMSPLectures_Tsai.pdf

Information Mathematics Seminar

16:50-18:35   Room #122 (Graduate School of Math. Sci. Bldg.)
Yasunari Suzuki (NTT Secure Platform Laboratories)
Foundation of quanutm computing (Japanese)
[ Abstract ]
Explanation of quanutm computing.

2019/10/30

Algebraic Geometry Seminar

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Andrew Macpherson (IPMU)
A Tannakian perspective on rigid analytic geometry (English)
[ Abstract ]
Raynaud's conception of analytic geometry contends that the category of analytic spaces over a non-Archimedean field is a (suitably "geometric") localisation of the category of formal schemes over the ring of integers at a class of modifications "along the central fibre". Unfortunately, as with all existing presentations of non-Archimedean geometry, this viewpoint is confounded by a proliferation of technical difficulties if one does not impose absolute finiteness conditions on the formal schemes under consideration.

I will argue that by combining Raynaud's idea with a Tannakian perspective which prioritises the module category, we can obtain a reasonable framework for rigid analytic geometry with no absolute finiteness hypotheses whatsoever, but which has descent for finitely presented modules.

Lie Groups and Representation Theory

16:30-18:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Quentin Labriet (Reims University)
On holographic transform (English)
[ Abstract ]
In representation theory, decomposing the restriction of a given representation $¥pi$ of a Lie group $G$ to an appropriate subgroup $G'$ is an important issue referred to as a branching law. In this context, one can define symmetry breaking operators, as $G'$-intertwining operators between the restriction $¥pi¥vert_{G'}$ and its irreducible
components. Going in the opposite direction gives rise to holographic operators and the notion of holographic transform.

I will illustrate this construction by two examples :

- the diagonal case where one considers the restriction problem for $¥pi$ being an outer product of two holomorphic discrete series representations, $G=SL(2,R)¥times SL(2,R)$ and $G'=SL(2,R)$.

- the conformal case for the restriction of a scalar valued holomorphic discrete series representation $¥pi$ of $G=SO(2,n)$ to $G'=SO(2,n-1)$.

I will then explain different methods for an explicit construction of such holographic operators in these cases, and present some of my results and open problems in this direction.

2019/10/29

Tuesday Seminar on Topology

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.

2019/10/28

Seminar on Geometric Complex Analysis

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Junjiro Noguchi (Univ. of Tokyo)
On Kiyoshi Oka's unpublished papers 1943 (Japanese)

2019/10/25

Colloquium

15:30-16:30   Room #123 (Graduate School of Math. Sci. Bldg.)
Yves Benoist ( CNRS, Paris-Sud)
Arithmeticity of discrete subgroups (英語)
[ Abstract ]
By a theorem of Borel and Harish-Chandra,
an arithmetic group in a semisimple Lie group is a lattice.
Conversely, by a celebrated theorem of Margulis,
in a higher rank semisimple Lie group G
any irreducible lattice is an arithmetic group.

The aim of this lecture is to survey an
arithmeticity criterium for discrete subgroups
which are not assumed to be lattices.
This criterium, obtained with Miquel,
generalizes works of Selberg and Hee Oh
and solves a conjecture of Margulis. It says:
a discrete irreducible Zariski-dense subgroup
of G that intersects cocompactly at least one
horospherical subgroup of G is an arithmetic group.

2019/10/24

FMSP Lectures

13:00-15:05   Room #002 (Graduate School of Math. Sci. Bldg.)
Chung-jun Tsai (National Taiwan University)
Topic on minimal submanifolds (5/6) (ENGLISH)
[ Abstract ]
The main theme of these lectures will be theory about minimal submanifolds, which are higher dimensional generalizations of geodesics. A naive motivation is that one tries to understand the geometry from its special submanifolds (minimal, etc.).

For minimal submanifolds, the equations are no longer ODEs, but elliptic PDEs. This increases the difficulties. The study are very good examples for the application of methods from PDEs and calculus of variations. We will try to explain some important results in this theory, which stimulate many of the researches today.

Here are some specific materials we plan to cover: Simon’s work based on the second variational formula, Sacks - Uhlenback theorem on the existence of minimal 2-spheres, the theory of stable minimal hypersurfaces by Schoen-Simon-Yau.

Logic

13:30-15:00   Room #156 (Graduate School of Math. Sci. Bldg.)
Yuya Okawa (Chiba University)
Generalizations of Bennet's results about partially conservative sentences (JAPANESE)

Applied Analysis

16:00-17:30   Room #128 (Graduate School of Math. Sci. Bldg.)

2019/10/23

Lie Groups and Representation Theory

16:30-18:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Clemens Weiske (Aarhus University)
Symmetry breaking and unitary branching laws for finite-multiplicity pairs of rank one (English)
[ Abstract ]
Let (G,G’) be a real reductive finite multiplicity pair of rank one, i.e. a rank one real reductive group G with reductive subgroup G’, such that the space of symmetry breaking operators (SBOs) between all (smooth admissible) irreducible representations is finite dimensional.

We give a classification of SBOs between spherical principal series representations of G and G’, essentially generalizing the results on (O(1,n+1),O(1,n)) of Kobayashi—Speh (2015). Moreover we show how to decompose unitary representations occurring in (not necessarily) spherical principal series representations of G in terms of unitary G’ representations, by making use of the knowledge gathered in the classification of the SBOs and the structure of the open P’orbit in G/P as a homogenous G’-space, where P’ is a minimal parabolic in G’ and P is a minimal parabolic in G. This includes the construction of discrete spectra in the restriction of complementary series representations and unitarizable composition factors.

2019/10/21

Seminar on Geometric Complex Analysis

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Yoshihiko Matsumoto (Osaka Univ.)
Canonical almost complex structures on ACH Einstein manifolds
[ Abstract ]
Einstein ACH (asymptotically complex hyperbolic) manifolds are seen as a device that establishes a correspondence between CR geometry on the boundary and Riemannian geometry in “the bulk.” This talk concerns an idea of enriching the geometric structure of the bulk by adding some almost complex structure compatible with the metric. I will introduce an energy functional of almost complex structures and discuss an existence result of critical points when the given ACH Einstein metric is a small perturbation of the Cheng-Yau complete K?hler-Einstein metric on a bounded strictly pseudoconvex domain. The renormalized Chern-Gauss-Bonnet formula is also planned to be discussed.

2019/10/17

FMSP Lectures

13:00-15:05   Room #002 (Graduate School of Math. Sci. Bldg.)
Chung-jun Tsai (National Taiwan University)
Topic on minimal submanifolds (4/6) (ENGLISH)
[ Abstract ]
The main theme of these lectures will be theory about minimal submanifolds, which are higher dimensional generalizations of geodesics. A naive motivation is that one tries to understand the geometry from its special submanifolds (minimal, etc.).

For minimal submanifolds, the equations are no longer ODEs, but elliptic PDEs. This increases the difficulties. The study are very good examples for the application of methods from PDEs and calculus of variations. We will try to explain some important results in this theory, which stimulate many of the researches today.

Here are some specific materials we plan to cover: Simon’s work based on the second variational formula, Sacks - Uhlenback theorem on the existence of minimal 2-spheres, the theory of stable minimal hypersurfaces by Schoen-Simon-Yau.
[ Reference URL ]
http://fmsp.ms.u-tokyo.ac.jp/FMSPLectures_Tsai.pdf

Information Mathematics Seminar

16:50-18:35   Room #122 (Graduate School of Math. Sci. Bldg.)
Hiroshi Fujiwara (BroadBand Tower, Inc.)
5G Data Center in CASE+AI Era (Japanese)
[ Abstract ]
Explanation of the recent situation of Data Center, and of its relation to
Connected、Autonomous、Sharing & Services、Electrification.

Mathematical Biology Seminar

14:00-16:00   Room #052 (Graduate School of Math. Sci. Bldg.)
Merlin C. Koehnke (Institute of Environmental Systems Research, School of Mathematics/Computer Science, Osnabrueck University) 14:00-15:00
Complex spatiotemporal dynamics in a simple predator-prey model (ENGLISH)
[ Abstract ]
A simple reaction-diffusion predator-prey model with Holling type IV functional response
and logistic growth in the prey is considered. The functional response can be interpreted as
a group defense mechanism, i.e., the predation rate decreases with resource density when the
prey density is high enough [1]. Such a mechanism has been described in diverse biological
interactions [2,3]. For instance, high densities of filamentous algae can decrease filtering
rates of filter feeders [4].
The model will be described and linked to plankton dynamics. Nonspatial considerations reveal that the zooplankton may go extinct or coexistence (stationary or oscillatory) between
zoo- and phytoplankton may emerge depending on the choice of parameters. However,
including space, the dynamics are more complex. In particular, spatiotemporal irregular
oscillations can rescue the predator from extinction. These oscillations can be characterized
as spatiotemporal chaos. The results provide a simple mechanism not only for the emergence
of inhomogeneous plankton distributions [5] but also for the occurrence of chaos in plankton communities [6]. Possible underlying mechanisms for this phenomenon will be discussed.
References
[1] Freedman, H. I., Wolkowicz, G. S. (1986). Predator-prey systems with group defence: the
paradox of enrichment revisited. Bulletin of Mathematical Biology, 48(5-6), 493–508.
[2] Tener, J. S.. Muskoxen in Canada: a biological and taxonomic review. Vol. 2. Dept. of Northern
Affairs and National Resources, Canadian Wildlife Service, 1965.
[3] Holmes, J. C. (1972). Modification of intermediate host behaviour by parasites. Behavioural
aspects of parasite transmission.
[4] Davidowicz, P., Gliwicz, Z. M., Gulati, R. D. (1988). Can Daphnia prevent a blue-green algal
bloom in hypertrophic lakes? A laboratory test. Limnologica. Jena, 19(1), 21–26.
[5] Abbott, M., 1993. Phytoplankton patchiness: ecological implicationsand observation methods.
In: Levin, S.A., Powell, T.M., Steele, J.H.(Eds.), Patch Dynamics. Lecture Notes in Biomathematics, vol. 96. Springer-Verlag, Berlin, pp. 37–49.
[6] Beninc`a, E. et al. (2008). Chaos in a long-term experiment with a plankton community. Nature,
451(7180), 822.
Horst Malchow (Institute of Environmental Systems Research, School of Mathematics/Computer Science, Osnabrueck University) 15:00-16:00
Functional response of competing populations to environmental variability (ENGLISH)
[ Abstract ]
The possible control of competitive invasion by infection of the invader and multiplicative
noise is studied. The basic model is the Lotka-Volterra competition system with emergent
carrying capacities. Several stationary solutions of the non-infected and infected system are
identi ed as well as parameter ranges of bistability. The latter are used for the numerical
study of di usive invasion phenomena. The Fickian di usivities, the infection but in particular the white and colored multiplicative noise are the control parameters. It is shown
that not only competition, possible infection and mobilities are important drivers of the
invasive dynamics but also the noise and especially its color and the functional response of
populations to the emergence of noise.
The variability of the environment can additionally be modelled by applying Fokker-Planck
instead of Fickian di usion. An interesting feature of Fokker-Planck di usion is that for spatially varying di usion coecients the stationary solution is not a homogeneous distribution.
Instead, the densities accumulate in regions of low di usivity and tend to lower levels for
areas of high di usivity. Thus, the stationary distribution of the Fokker-Planck di usion can
be interpreted as a re
ection of di erent levels of habitat quality [1-5]. The latter recalls the
seminal papers on environmental density, cf. [6-7]. Appropriate examples will be presented.
References
[1] Bengfort, M., Malchow, H., Hilker, F.M. (2016). The Fokker-Planck law of diffusion and
pattern formation in heterogeneous media. Journal of Mathematical Biology 73(3), 683-704.
[2] Siekmann, I., Malchow, H. (2016). Fighting enemies and noise: Competition of residents
and invaders in a stochastically fluctuating environment. Mathematical Modelling of Natural
Phenomena 11(5), 120-140.
[3] Siekmann, I., Bengfort, M., Malchow, H. (2017). Coexistence of competitors mediated by
nonlinear noise. European Physical Journal Special Topics 226(9), 2157-2170.
[4] Kohnke, M.C., Malchow, H. (2017). Impact of parameter variability and environmental noise
on the Klausmeier model of vegetation pattern formation. Mathematics 5, 69 (19 pages).
[5] Bengfort, M., Siekmann, I., Malchow, H. (2018). Invasive competition with Fokker-Planck
di usion and noise. Ecological Complexity 34, 134-13.
[6] Morisita, M. (1971). Measuring of habitat value by the \environmental density" method. In:
Spatial patterns and statistical distributions (Patil, C.D., Pielou, E.C., Waters, W.E., eds.),
Statistical Ecology, vol. 1, pp. 379-401. Pennsylvania State University Press, University Park.
[7] N. Shigesada, N., Kawasaki, K., Teramoto, E. (1979). Spatial segregation of interacting species.
Journal of Theoretical Biology 79, 83-99.

2019/10/16

Operator Algebra Seminars

16:45-18:15   Room #126 (Graduate School of Math. Sci. Bldg.)
Shinobu Hikami (OIST)
Developments in conformal bootstrap analysis

Number Theory Seminar

17:30-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Liang Xiao (BICMR, Peking University)
On slopes of modular forms (ENGLISH)
[ Abstract ]
In this talk, I will survey some recent progress towards understanding the slopes of modular forms, with or without level structures. This has direct application to the conjecture of Breuil-Buzzard-Emerton on the slopes of Kisin's crystabelline deformation spaces. In particular, we obtain certain refined version of the spectral halo conjecture, where we may identify explicitly the slopes at the boundary when given a reducible non-split generic residual local Galois representation. This is a joint work in progress with Ruochuan Liu, Nha Truong, and Bin Zhao.

Algebraic Geometry Seminar

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Yusuke Sato (University of Tokyo/ IPMU)
Multidimensional continued fraction for Gorenstein cyclic quotient singularity
[ Abstract ]
Let G be a finite cyclic subgroup of GL(n,C). Then Cn/G is a cyclic quotient singularity. In the case n = 2, Cn/G possess the unique minimal resolution, and it is obtained by Hirzubruch-Jung continued fraction. In this talk, we show a sufficient condition of existence of crepant desingularization for Gorenstein abelian quotient singularities in all dimensions by using Ashikaga’s continuous fractions. Moreover, as a corollary, we prove that all three dimensional Gorenstein abelian quotient singularities possess a crepant desingularization.

2019/10/15

Tuesday Seminar on Topology

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.

2019/10/10

Information Mathematics Seminar

16:50-18:35   Room #122 (Graduate School of Math. Sci. Bldg.)
Katsuyuki Takashima (Mitsubishi Electric Co./Kyushu Univ.)
Post-Quantum Cryptography from Isogenies (Japanese)
[ Abstract ]
Explanation of the isogeny-based post-quantum cryptography

FMSP Lectures

13:00-15:05   Room #002 (Graduate School of Math. Sci. Bldg.)
Chung-jun Tsai (National Taiwan University)
Topic on minimal submanifolds (3/6) (ENGLISH)
[ Abstract ]
The main theme of these lectures will be theory about minimal submanifolds, which are higher dimensional generalizations of geodesics. A naive motivation is that one tries to understand the geometry from its special submanifolds (minimal, etc.).

For minimal submanifolds, the equations are no longer ODEs, but elliptic PDEs. This increases the difficulties. The study are very good examples for the application of methods from PDEs and calculus of variations. We will try to explain some important results in this theory, which stimulate many of the researches today.

Here are some specific materials we plan to cover: Simon’s work based on the second variational formula, Sacks - Uhlenback theorem on the existence of minimal 2-spheres, the theory of stable minimal hypersurfaces by Schoen-Simon-Yau.
[ Reference URL ]
http://fmsp.ms.u-tokyo.ac.jp/FMSPLectures_Tsai.pdf

Discrete mathematical modelling seminar

17:00-18:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Boris Konopelchenko (INFN, sezione di Lecce, Lecce, Italy)
Universal parabolic regularization of gradient catastrophes for the Burgers-Hopf equation and singularities of the plane into plane mappings of parabolic type (English)
[ Abstract ]
Two intimately connected topics, namely, regularization of gradient catastrophes of all orders for the Burgers-Hopf equation via the Jordan chain and the singularities of the plane into plane mappings
associated with two-component hydrodynamic type systems of parabolic type are discussed.
It is shown that the regularization of all gradient catastrophes (generic and higher orders) for the Burgers-Hopf equation is achieved by the step by step embedding of the Burgers-Hopf equation into multi-component parabolic systems of quasilinear PDEs with the most degenerate Jordan blocks. Infinite parabolic Jordan chain provides us with the complete regularization. This chain contains Burgers and KdV equations as particular reductions.
It is demonstrated that the singularities of the plane into planes mappings associated with the two-component system of quasilinear PDEs of parabolic type are quite different from those in hyperbolic and elliptic cases. Impediments arising in the application of the original Whitney's approach to such case are discussed. It is shown that flex is the lowest singularity while higher singularities are given by ( k+1,k+2) curves which are of cusp type for k=2n+1, n=1,2,...,. Regularization of these singularities is discussed.

Presentation is based on two publications:

1. B. Konopelchenko and G. Ortenzi, Parabolic regularization of the gradient catastrophes for the Burgers-Hopf equation and Jordan chain, J. Phys. A: Math. Theor., 51 (2018) 275201.

2. B.G. Konopelchenko and G. Ortenzi, On the plane into plane mappings of hydrodynamic type. Parabolic case. Rev. Math. Phys.,32 (2020) 2020006. Online access. arXiv:1904.00901.

2019/10/09

Operator Algebra Seminars

16:45-18:15   Room #126 (Graduate School of Math. Sci. Bldg.)
Yosuke Kubota (Riken)
Relative K-homology group of $C^*$-algebras and almost flat vector bundle on manifolds with boundary

Number Theory Seminar

17:00-18:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Yuanqing Cai (Kyoto University)
Twisted doubling integrals for classical groups (ENGLISH)
[ Abstract ]
In the 1980s, Piatetski-Shapiro and Rallis discovered a family of Rankin-Selberg integrals for the classical groups that did not rely on Whittaker models. This is the so-called doubling method. It grew out of Rallis' work on the inner products of theta lifts -- the Rallis inner product formula.
In this talk, we present a family of Rankin-Selberg integrals (the twisted doubling method, in joint work with Friedberg, Ginzburg, and Kaplan) for the tensor product L-function of a pair of automorphic cuspidal representations, one of a classical group, the other of a general linear group. This can be viewed as a generalization of the doubling integrals of Piatetski-Shapiro and Rallis. Time permitting, we will discuss the twisted doubling integrals for Brylinski-Deligne covers of classical groups.

2019/10/08

Tuesday Seminar on Topology

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.

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