#### RG Analysis and Partial Differential Equations

# Graduate seminar on Advanced topics in PDE

## Organizers

## Schedule

##### April 14 - Organizational meeting

The organizational meeting takes place in person.

##### April 21 - Lorenzo Pompili (Bonn)

##### Title: On the stability of the KP-II line soliton via a Bäcklund transformation

##### Abstract:

The use of techniques coming from completely integrable equations and the inverse scattering machinery has drawn increasing attention in recent years in order to study conserved quantities, long time dynamics, multisolitons and low regularity well-posedness of several 1-dimensional integrable, dispersive PDEs in L²-based Sobolev spaces. Applications of these techniques to higher dimensional settings have essentially not been developed yet. In this talk, I will discuss the modulational stability of the line soliton solution of the KP-II equation on R², w ith the aim of finding a complementary approach to the one of earlier results by T. Mizumachi: the rough idea is to use a nonlinear symmetry of the equation arising from the integrable structure, in order to subtract the soliton from the original solution, and reduce the whole problem to the stability of the zero solution. We obtain a large family of perturbations of the line soliton solution which remain uniformly close in time to a modulated soliton. The method looks robust and opens the way to the study of the stability of the KP-II multisolitons.

##### April 28 - Lars Becker (Bonn)

##### Title: On Fourier extension from the circle

##### Abstract:

This talk is about the conjecture that constant functions are maximizers for the endpoint Tomas-Stein Fourier extension inequality for the circle. The analogous conjecture for the two sphere was proven in an elegant paper by Foschi, and based on his proof, a program for proving the conjecture for the circle was formulated by Carneiro, Foschi, Oliveira e Silva and Thiele. We will discuss this program and recent progress on it. In particular, we will show the conjectured sharp inequality for functions with spatial support in a small neighbourhood of two antipodal points on the circle.

##### May 5 - Giovanni Covi (Bonn) - *Joint session with the Research group in PDE and Inverse Problems*

##### Title: A reduction of the fractional Calderon problem to the local Calderon problem

##### Abstract:

We relate the anisotropic variable coefficient fractional and local Calderon problems by means of the Caffarelli-Silvestre extension. In particular, we prove that the Dirichlet-to-Neumann data for the former determine the Dirichlet-to-Neumann data for the latter in dimensions two and higher. As a consequence, any uniqueness result for the local problem also implies a uniqueness result for the nonlocal problem. Finally, we discuss obstructions in the reversing of this procedure.

##### May 12 - *No Talk*

##### May 19 -Mingming Cao (ICMAT)

##### Title: The Dirichlet boundary value problem for elliptic systems

##### Abstract:

We will talk about well-posedness of the Dirichlet boundary value problem for elliptic systems in the upper half-space. Considering different kinds of boundary data, we will establish Rubio de Francia extrapolation on general Banach function and modular spaces, which can be viewed as the final version of extrapolation. We also give applications of extrapolation to singular integral operators.

##### May 26 -Lars Niedorf (Kiel)

##### Title: Spectral multipliers for sub-Laplacians on Métivier groups

##### Abstract:

Let G be a two-step stratified Lie group and L be a homogeneous sub-Laplacian on G. We consider the operators F(L) defined via functional calculus. Due to a celebrated theorem of Christ and Mauceri/Meda, the operator F(L) is of weak type (1,1) and bounded on L^{p}for $1 < p < \infty$ whenever F satisfies a scale-invariant smoothness condition of order s > Q/2. Here Q = d

_{1}+ 2d

_{2}denotes the homogeneous dimension of G, with d

_{1}and d

_{2}being the dimensions of the first and second layer of the stratification of the Lie algebra of G, respectively. Müller/Stein and Hebisch discovered that for the case of G being a Heisenberg (-type) group, this threshold can be pushed down to s > d/2, with d = d

_{1}+ d

_{2}being the topological dimension of G. This result has since then been extended to various other settings. In this talk, I present one such spectral multiplier theorem in the setting of Métivier groups, which only requires s > d(1/p-1/2) as a regularity condition for boundedness on L

^{p}. The proof relies on restriction type estimates, where the multiplier is additionally truncated along the spectrum of a sub-Laplacian on the second layer of the stratification of the Lie algebra.

##### June 2 - *No Talk* (due to the PDE conference in Bonn)

##### June 9- Gong Chen (Georgia Tech) *Hybrid mode*

##### Title: Dynamics of multi-solitons to Klein-Gordon equations

##### Abstract:

I will report my recent joint work with Jacek Jendrej on muti-solitons to the Klein-Gordon equations including their asymptotic stability and classification.

##### June 16 - Andreas Seeger (University of Wisconsin-Madison)

##### Title:

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##### June 23 - Fred Lin (Bonn)

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##### June 30, 14.15 - Geoffrey Lacour (Université Clermont Auvergne)

##### Title:

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##### June 30, 15.15 - Valentina Ciccone (Bonn)

##### Title:

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##### July 7 - David Wallauch (Univeristy of Vienna)

##### Title:

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##### July 14 - Alexander Volberg (Michigan State University and HCM, Bonn)

##### Title:

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## News

Prof. Catharina Stroppel joined the North Rhine-Westphalia Academy for Sciences and Arts

Hirzebruch lecture by Rahul Pandharipande on Monday, May 15, 2023

Lucas Mann recevices the Hausdorff memorial award 2021/22

Bachelor Prize 2021/22 of the BMG awarded

Prof. Daniel Huybrechts receives the Compositio Prize for the periode 2017-2019

Prof. Catharina Stroppel receives Gottfried Wilhelm Leibniz Prize 2023

Rajula Srivastava receives Association for Women in Mathematics Dissertation Prize

Prof. Ana Caraiani wins a New Horizons in Mathematics Prizes 2023

Pius XI Medal awarded to Professor Peter Scholze

Prof. Valentin Blomer und Prof. Georg Oberdieck erhalten ERC grants

Grants for Mathematics students from Ukraine

Prof. Jessica Fintzen is awarded a Whitehead Prize of the London Mathematical Society

Prof. Peter Scholze elected as Foreign Member of the Royal Society

Bonner Mathematik belegt bei Shanghai Ranking den 1. Platz in Deutschland und weltweit den 13. Platz

Prof. Daniel Huybrechts erhält gemeinsam mit Debarre, Macri und Voisin ERC Synergy Grant