## Seminar information archive

#### Lectures

10:45-11:45   Room #002 (Graduate School of Math. Sci. Bldg.)
Pascal Chossat (CNRS / University of Nice)
Pattern formation in the hyperbolic plane (ENGLISH)
[ Abstract ]
Initially motivated by a model for the visual perception of textures by the cortex, the problem of pattern formation in the hyperbolic plane, or equivalently the Poincaré disc D, shows some similar but mostly quite different features from the same problem posed on the Euclidean plane. The hyperbolic structure induces a large variety of possible periodic patterns and even the bifurcation of "hyperbolic" traveling waves. We call these patterns "H-planforms". I shall show how H-planforms are determined by the means of equivariant bifurcation theory and Helgason-Fourier analysis in D. However the question of their observability is still open. The talk will be illustrated with pictures of H-planforms that have been computed using non trivial algorithms based on harmonic analysis in D.

#### GCOE lecture series

13:30-14:30   Room #123 (Graduate School of Math. Sci. Bldg.)
Haim Brezis (Rutgers University / Technion)
How Poincare became my hero (ENGLISH)
[ Abstract ]
I recently discovered little-known texts of Poincare which include fundamental results on PDEs together with prophetic insights into their future impact on various branches of modern mathematics.

#### GCOE lecture series

14:50-17:30   Room #123 (Graduate School of Math. Sci. Bldg.)
Haim Brezis (Rutgers University / Technion)
Can you hear the degree of a map from the circle into itself? An intriguing story which is not yet finished (ENGLISH)
[ Abstract ]
A few years ago - following a suggestion by I. M. Gelfand - I discovered an intriguing connection between the topological degree of a map from the circle into itself and its Fourier coefficients. This relation is easily justified when the map is smooth. However, the situation turns out to be much more delicate if one assumes only continuity, or even Holder continuity. I will present recent developments and open problems. The initial motivation for this direction of research came from the analysis of the Ginzburg-Landau model.

### 2012/11/27

#### Tuesday Seminar on Topology

16:30-18:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Hiraku Nozawa (JSPS-IHES fellow)
On a finite aspect of characteristic classes of foliations (JAPANESE)
[ Abstract ]
Characteristic classes of foliations are not bounded due to Thurston.
In this talk, we will explain finiteness of characteristic classes for
foliations with certain transverse structures (e.g. transverse
conformally flat structure) and its relation to unboundedness and
rigidity of foliations.
(This talk is based on a joint work with Jesús Antonio
Álvarez López at University of Santiago de Compostela,
which is available as arXiv:1205.3375.)

#### Lie Groups and Representation Theory

16:30-18:00   Room #126 (Graduate School of Math. Sci. Bldg.)
Hiroshi Konno (the University of Tokyo)
Convergence of Kahler to real polarizations on flag manifolds (JAPANESE)
[ Abstract ]
In this talk we will discuss geometric quantization of a flag manifold. In particular, we construct a family of complex structures on a flag manifold that converge 'at the quantum level' to the real polarization coming from the Gelfand-Cetlin integrable system.
Our construction is based on a toric degeneration of flag varieties and a deformation of K¥"ahler structure on toric varieties by symplectic potentials.
This is a joint work with Mark Hamilton.

### 2012/11/26

#### Algebraic Geometry Seminar

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Toshiyuki Katsura (Hosei University)
A configuration of rational curves on the superspecial K3 surface (JAPANESE)

#### Seminar on Geometric Complex Analysis

10:30-12:00   Room #126 (Graduate School of Math. Sci. Bldg.)
Shin Nayatani (Nagoya University)
Quaternionic CR geometry (JAPANESE)

### 2012/11/22

#### Lectures

13:30-14:15   Room #002 (Graduate School of Math. Sci. Bldg.)
Danielle Hilhorst (CNRS / Univ. Paris-Sud)
A multiple scale pattern formation cascade in reaction-diffusion systems of activator-inhibitor type (ENGLISH)
[ Abstract ]
A family of singular limits of reaction-diffusion systems of activator-inhibitor type in which stable stationary sharp-interface patterns may form is investigated. For concreteness, the analysis is performed for the FitzHugh-Nagumo model on a suitably rescaled bounded domain in $\\R^N$, with $N \\geq 2$. It is proved that when the system is sufficiently close to the limit the dynamics starting from the appropriate smooth initial data breaks down into five distinct stages on well-separated time scales, each of which can be approximated by a suitable reduced problem. The analysis allows to follow fully the progressive refinement of spatiotemporal patterns forming in the systems under consideration and provides a framework for understanding the pattern formation scenarios in a large class of physical, chemical, and biological systems modeled by the class of reaction-diffusion equations, which we consider. This is joint work with Marie Henry and Cyrill Muratov.
[ Reference URL ]
http://www.ms.u-tokyo.ac.jp/gcoe/index_007.html

#### Lectures

14:25-15:10   Room #002 (Graduate School of Math. Sci. Bldg.)
Thanh Nam Ngyuen (University of Paris-Sud)
Formal asymptotic limit of a diffuse interface tumor-growth model (ENGLISH)
[ Abstract ]
We consider a diffuse interface tumor-growth model, which has the form of a phase-field system. We discuss the singular limit of this problem. More precisely, we formally prove that as the reaction coefficient tends to zero, the solution converges to the solution of a free boundary problem.

This is a joint work with Danielle Hilhorst, Johannes Kampmann and Kristoffer G. van der Zee.
[ Reference URL ]
http://www.ms.u-tokyo.ac.jp/gcoe/index_007.html

#### Lectures

15:30-16:15   Room #002 (Graduate School of Math. Sci. Bldg.)
Peter Gordon (Akron University)
Gelfand type problem for two phase porous media (ENGLISH)
[ Abstract ]
In this talk I will introduce a generalization of well known Gelfand problem arising in a Frank-Kamenetskii theory of thermal explosion. This generalization is a natural extension of the Gelfand problem to two phase materials, where, in contrast to classical Gelfand problem which utilizes single temperature approach, the state of the system is described by two different temperatures. As a result the problem is modeled by a system of two coupled nonlinear heat equations. The new ingredient in such a generalized Gelfand problem is a presence of inter-phase heat exchange which can be viewed as a strength of coupling for the system.

I will show that similar to classical Gelfand problem the thermal explosion (blow up of solution) for generalized Gelfand problem occurs exclusively due to the absence of stationary temperature distribution, that is non-existence of solution of corresponding elliptic problem. I also will show that the presence of inter-phase heat exchange delays a thermal explosion. Moreover, in the limit of infinite heat exchange between phases the problem of thermal explosion in two phase porous media reduces to classical Gelfand problem with re-normalized constants. The latter result partially justifies a single temperature approach to two phase systems often used in a physical literature.

This is a joint work with Vitaly Moroz (Swansea University).
[ Reference URL ]
http://www.ms.u-tokyo.ac.jp/gcoe/index_007.html

#### Lectures

16:25-17:10   Room #002 (Graduate School of Math. Sci. Bldg.)
Cyrill Muratov (New Jersey Institute of Technology)
On the shape of charged drops: an isoperimetric problem with a competing non-local term (ENGLISH)
[ Abstract ]
In this talk I will give an overview of my recent work with H. Knuepfer on the analysis of a class of geometric problems in the calculus of variations. I will discuss the basic questions of existence and non-existence of energy minimizers for the isoperimetric problem with a competing non-local term. A complete answer will be given for the case of slowly decaying kernels in two space dimensions, and qualitative properties of the minimizers will be established for general Riesz kernels.
[ Reference URL ]
http://www.ms.u-tokyo.ac.jp/gcoe/index_007.html

### 2012/11/21

#### PDE Real Analysis Seminar

10:30-11:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Giovanni Pisante (Seconda Università degli Studi di Napoli)
Shape Optimization And Asymptotic For The Twisted Dirichlet Eigenvalue (ENGLISH)
[ Abstract ]
Aim of the talk is to discuss some recent results obtained with G. Croce and A. Henrot on a generalization of the functional defining the first twisted eigenvalue.
We look at the set functional defined by minimizing a Rayleigh quotient involving Lebesgue norms with different exponents p and q among functions satisfying a zero boundary condition as well as a zero mean condition of order q.
First under suitable conditions on p and q, that ensure the existence of a minimizing function, we investigate the validity of an isoperimetric type inequality of the Reyleigh-Faber-Krahn type.
Then we study the limit of the functional for p and q tending to 1 and to infinity and discuss the relation with the limits of the second eigenvalues of the p-laplacian operator.

#### Classical Analysis

16:00-17:30   Room #128 (Graduate School of Math. Sci. Bldg.)
Philip Boalch (ENS-DMA & CNRS Paris)
Beyond the fundamental group (ENGLISH)
[ Abstract ]
Moduli spaces of representations of the fundamental group of a Riemann surface have been studied from numerous points of view and appear in many parts of mathematics and theoretical physics. They form an interesting class of symplectic manifolds, they often have Kahler or hyperkahler metrics (in which case they are diffeomorphic to spaces of Higgs bundles, i.e. Hitchin integrable systems), and they admit nonlinear actions of braid groups and mapping class groups with fascinating dynamical properties. The aim of this talk is to describe some aspects of this story and sketch their extension to the context of the "wild fundamental group", which naturally appears when one considers {\\em meromorphic} connections on Riemann surfaces. In particular some new examples of hyperkahler manifolds appear in this way, some of which are familiar from classical work on the Painleve equations.

### 2012/11/20

#### Tuesday Seminar on Topology

16:30-18:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Kentaro Nagao (Nagoya University)
3 dimensional hyperbolic geometry and cluster algebras (JAPANESE)
[ Abstract ]
The cluster algebra was discovered by Fomin-Zelevinsky in 2000.
Recently, the structures of cluster algebras are recovered in
many areas including the theory of quantum groups, low
dimensional topology, discrete integrable systems, Donaldson-Thomas
theory, and string theory and there is dynamic development in the
research on these subjects. In this talk I introduce a relation between
3 dimensional hyperbolic geometry and cluster algebras motivated
by some duality in string theory.

#### Lie Groups and Representation Theory

16:30-17:30   Room #126 (Graduate School of Math. Sci. Bldg.)
Ali Baklouti (Sfax University)
On the geometry of discontinuous subgroups acting on some homogeneous spaces (ENGLISH)
[ Abstract ]
Let $G$ be a Lie group, $H$ a closed subgroup of $G$ and \\Gamma$a discontinuous subgroup for the homogeneous space$G/H$. I first introduce the deformation space${\\mathcal{T}}^{K_o}(\\Gamma, G, H)$of the action of$\\Gamma$on$G/H$in the sense of Kobayashi and some of its refined versions, namely the Clifford--Klein space of deformations of the form${\\mathcal{X}}=\\Gamma \\backslash G/H$. The deformation space${\\mathcal{T}}^{G_o}(\\Gamma, G,H)$of marked$(G,H)$-structures on${\\mathcal{X}}$in the sense of Goldman is also introduced. As an important motivation, I will explain the connection between the spaces${\\mathcal{T}}^{K_o}(\\Gamma, G, H)$and${\\mathcal{T}}^{G_o}(\\Gamma, G, H)$and study some of their topological features, namely the rigidity in the sense of Selberg--Weil--Kobayashi and the stability in the sense of Kobayashi--Nasrin. The latter appears to be of major interest to write down the connection explicitly. ### 2012/11/19 #### Lectures 16:45-17:45 Room #126 (Graduate School of Math. Sci. Bldg.) Hendrik Weber (University of Warwick) Invariant measure of the stochastic Allen-Cahn equation: the regime of small noise and large system size (ENGLISH) [ Abstract ] We study the invariant measure of the one-dimensional stochastic Allen-Cahn equation for a small noise strength and a large but finite system. We endow the system with inhomogeneous Dirichlet boundary conditions that enforce at least one transition from -1 to 1. We are interested in the competition between the energy'' that should be minimized due to the small noise strength and the entropy'' that is induced by the large system size. Our methods handle system sizes that are exponential with respect to the inverse noise strength, up to the critical'' exponential size predicted by the heuristics. We capture the competition between energy and entropy through upper and lower bounds on the probability of extra transitions between$\\pm 1\$. These bounds are sharp on the exponential scale and imply in particular that the probability of having one and only one transition from -1 to +1 is exponentially close to one. In addition, we show that the position of the transition layer is uniformly distributed over the system on scales larger than the logarithm of the inverse noise strength.
Our arguments rely on local large deviation bounds, the strong Markov property, the symmetry of the potential, and measure-preserving reflections.
This is a joint work with Felix Otto and Maria Westdickenberg.

#### Algebraic Geometry Seminar

15:30-17:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Yukinobu Toda (IPMU)
Stability conditions and birational geometry (JAPANESE)
[ Abstract ]
I propose a conjecture which claims that MMP for a smooth projective variety is realized as a variation of Bridgeland moduli spaces of semistable objects in the derived category of coherent sheaves. I will discuss the surface case and extremal contractions for 3-folds. In the former case, the conjecture is completely solved. In the latter case, I will construct the perverse t-structure associated to the extremal contraction, and construct a candidate of the desired stability condition as a double tilting of the perverse heart.

#### GCOE Seminars

15:30-17:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Alfred Ramani (Ecole Polytechnique)
Linearisable mappings (ENGLISH)
[ Abstract ]
We present a series of results on linearisable second-order mappings.
Three distinct families of such mappings do exist: projective, mappings of Gambier type and mappings which we have dubbed "of third kind".
Our starting point are the linearisable mappings belonging to the QRT family. We show how they can be linearised and how in some cases their explicit solution can be constructed. We discuss also the growth property of these mappings, a property intimately related to linearisability.
In the second part of the talk we address the question of the extension of these mappings to a non-autonomous form.
We show that the QRT invariant can also be extended (to a quantity which depends explicitly on the independent variable). Using this non-autonomous form we show that it is possible to construct the explicit solution of all third-kind mappings. We discuss also the relation of mappings of the third kind to Gambier-type mappings. We show that a large subclass of third-kind mappings can be considered as the discrete derivative of Gambier-type ones.

#### Seminar on Geometric Complex Analysis

10:30-12:00   Room #126 (Graduate School of Math. Sci. Bldg.)
Yohei Komori (Waseda University)
On a degenerate family of Riemann surfaces of genus two over an elliptic curve (JAPANESE)
[ Abstract ]
We construct a degenerate family of Riemann surfaces of genus two constructed as double branched covering surfaces of a fixed torus. We determine its singular fibers and holomorphic sections.

### 2012/11/16

#### Colloquium

16:30-17:30   Room #002 (Graduate School of Math. Sci. Bldg.)
Akito FUTAKI (University of Tokyo)
Integral invariants in complex differential geometry (JAPANESE)

#### GCOE Seminars

16:30-18:00   Room #122 (Graduate School of Math. Sci. Bldg.)
Dietmar Bisch (Vanderbilt University)
Subfactors with small Jones index (ENGLISH)

### 2012/11/14

#### Number Theory Seminar

18:00-19:00   Room #002 (Graduate School of Math. Sci. Bldg.)
Pierre Berthelot (Université de Rennes 1)
De Rham-Witt complexes with coefficients and rigid cohomology
(ENGLISH)
[ Abstract ]
For a smooth scheme over a perfect field of characteristic p, we will explain a generalization of the classical comparison theorem between crystalline cohomology and de Rham-Witt cohomology to the case of cohomologies with coefficients in a p-torsion free crystal. This provides in particular a description of the rigid cohomology of a proper singular scheme in terms of a de Rham-Witt complex built from a closed immersion into a smooth scheme.

#### Geometry Colloquium

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Kotaro Kawai (Tohoku University)
Construction of coassociative submanifolds (JAPANESE)
[ Abstract ]
The notion of coassociative submanifolds is defined as the special class of the minimal submanifolds in G_2 manifolds. In this talk, we introduce the method to construct coassociative submanifolds by using the symmetry of the Lie group action. As an application, we give explicit examples in the 7-dimensional Euclidean space and in the anti-self-dual bundle over the 4-sphere.

### 2012/11/13

#### Tuesday Seminar on Topology

16:30-18:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Takahiro Kitayama (RIMS, Kyoto University,JSPS PD)
The virtual fibering theorem and sutured manifold hierarchies (JAPANESE)
[ Abstract ]
In 2007 Agol showed that every irreducible 3-manifold whose fundamental
group is nontrivial and virtually residually finite rationally solvable
(RFRS) is virtually fibered. In the proof he used the theory of
least-weight taut normal surfaces introduced and developed by Oertel and
Tollefson-Wang. We give another proof using complexities of sutured
manifolds. This is a joint work with Stefan Friedl (University of
Cologne).

#### Lie Groups and Representation Theory

16:30-18:00   Room #126 (Graduate School of Math. Sci. Bldg.)
Oskar Hamlet (Chalmers University)
Tight maps, a classification (ENGLISH)
[ Abstract ]
Tight maps/homomorphisms were introduced during the study of rigidity properties of surface groups in Hermitian Lie groups. In this talk I'll discuss the properties of tight maps, their connection to rigidity theory and my work classifying them.