## Diff.-Equations and Singularities Seminar

European Online Seminar on Functional Equations

## Scope of the Seminar

The seminar covers all topics related to Singularities of Functional Equations such as differential equations, difference equations and integral differential equations. We aim to bring together researchers from different fields in mathematics like analysis, differential and difference algebra, dynamical systems theory, differential topology and geometry who are interested in this topic. The idea of the seminar is not to have talks on the latest technical advances, but to provide monthly introductory and survey talks accessible to people with different backgrounds to foster interaction between the different fields.

## Format of the Seminar

The seminar takes place monthly, every first Friday at 5:30 pm CET. It is held online via Zoom and for organisational reasons we kindly ask you to register, if you are interested to attend the seminar. A few days before a scheduled talk you will receive an email with a link to the Zoom meeting.

A talk will take about 45-60 minutes and if the speaker agrees, it will be recorded and the video will be published on the website. After each talk we will have some time for discussions. This will give us the opportunity to speak about details of the talk and to exchange ideas which hopefully lead to new research collaborations.

## Overview of Upcoming and Past Talks

### Academic Year 2023/24

6. October: Alexey Ovchinnikov, City University of New York

3. November: No talk

1. December: No talk

2. February: Tsvetana Stoyanova, Sofia University, Bulgaria

1. March: Charlotte Hardouin, Université de Toulouse

### Academic Year 2022/23

7. October: Galina FILIPUK, University of Warsaw, Poland

4. November: José Manuel AROCA, University of Valladolid, Spain

2. December: David BLÁZQUEZ SANZ, Universidad Nacional de Colombia, Medellín

3. February: Guy CASALE, University of Rennes, France

3. March: Jacques Arthur Weil will talk in the second half of the academic year 2023

5. May: Carla Valencia NEGRETE, Ibero-American University, Mexico

2. June: Fuensanta AROCA, National Autonomous University of Mexico

## Next Talk

### TBA, 5:30 pm CET

Speaker: TBA

Title: TBA

Abstract: TBA

## Previous Talks

Speaker: Charlotte Hardouin, Université de Toulouse

Title: Differential transcendence for solutions of linear difference equations - genus zero and one case

A function of the complex variable is called differentially transcendental if it doesn't satisfy a polynomial differential equation. One of the famous example is the Gamma function proved to be differentially transcendental by Hölder. It happens that Hölder's proof relies heavily on the fact that Gamma satisfies a linear difference equation $\gamma(z+1)=z\Gamma(z)$ over $\mathbb{C}(z)$. With Adamczewski and Dreyfus, we proved that any formal power series, that is a solution of a linear difference equation over $\mathbb{C}(z)$ and not of exponential type, is either rational or differentially transcendental. Recently, with de Shalit and Roques, we descried all the differentially algebraic solutions of difference equations over elliptic curves. In this talk, I will show how these results can be obtained by the combination of parametrized Galois theory, the study of integrable difference modules over the projective line or an elliptic curve and the description of all differentially algebraic solutions of rank one equations.

Speaker: Tsvetana Stoyanova, Sofia University, Bulgaria

Title: Non-integrability of the Painleve equations as Hamiltonian systems

In 2000 Morales-Ruiz posed the problem of the non-integrability of the rational Painleve equations (when they have rational particular solutions) from the point of view of the Hamiltonian dynamics. In this talk I will review some results on the topic and their connection with the Morales-Ramis-Simo theory.

Speaker: Fuensanta AROCA, National Autonomous University of Mexico

Title: Newton polygon, polytope and Gröbner fan

Speaker: Carla Valencia NEGRETE, Ibero-American University, Mexico

Title: Small parameters in gaseous boundary layers with total constant energy

One approach to work on singular problems in hydrodynamics is to see them as small parameter problems. In particular, the formal study of incompressible and compressible boundary layers can be stated through the application of diffeomorphisms over a rectangular domain. On this occasion, we will talk about how a particular change of variables gives us simplified limit formulas for a gaseous boundary layer with total constant energy, and the questions that remain.

Speaker: Guy CASALE, University of Rennes, France

Title: Algebraic relations between solutions of a differential equation

In 2004, K Nishioka proved that if y_1,..., y_n are solutions of P_I : y''= 6 y2 + x such that

tr.deg._{C(x)}(y_1,y'_1,..., y_n,y'_n) < 2n

then there exists i<j such that y_i = y_j. In this talk I will explain how the Galois Groupoid of the differential equation can be used to prove (half of) this theorem. When a second order equation has a transitive, simple, infinite dimensional Galois Groupoid then if y_1,..., y_n are solutions such that tr.deg._{C(x)}(y_1,y'_1,..., y_n,y'_n) < 2n,

there exist i such that tr.deg._{C(x)}(y_i,y'_i) <2, or

there exist i<j such that tr.deg._{C(x)}(y_i,y'_j, y_i,y'_j) =2.

Speaker: David BLÁZQUEZ SANZ, Universidad Nacional de Colombia, Medellín

Title: Ax-Schanuel and Ax-Lindenmann type results from a differential Galois theoretic perspective

based on joint work with Guy Casale, James Freitag and Joel Nagloo

Ax-Schanuel and Ax-Lindenmann type results specify the kind of algebraic relations that are preserved by some transcendental functions. For instance, if there is some algebraic relation x and y and also between j(x) and j(y), being j the modular function, then there x and y are related by a Moebius transformation with rational coefficients and j(x) and j(y) satisfy a modular polynomial. In this talk we will review recent research on the topic and speak about the link of this kind of results and the differential Galois theory for principal connections.

In this talk I will present an overview of the use of Newton's polygon in valued fields and differential valued fields to obtain formal solutions of algebraic equations and algebraic differential equations.

Speaker: Galina FILIPUK, University of Warsaw, Poland

Title: On the Hamiltonians of the Painleve and quasi-Painleve equations

In this talk I shall present various recent results joint with different collaborators, A. Dzhamay, A. Stokes, A. Ligeza and T. Kecker, which can be obtained for the Painleve and quasi-Painleve equations using the geometric approach. I shall briefly explain the analogue of the polynomial Hamiltonian structure for the Takasaki equation and general quasi-Painleve equations.

Speaker: Farid Tari, University of São Paulo, Brazil

Title: Invariants of implicit differential equations

Implicit differential equations (IDEs) occur naturally in differential geometry of surfaces. For example, on a regular surface in the Euclidean or Minkowski 3-space the pairs of foliations given by the asymptotic and principal curves are solutions of IDEs that are written in the form

a(x,y) dy² + b(x,y) dxdy + c(x,y) dx²=0.

Such IDEs are often called Binary Differential Equations (BDEs).

I will present in this talk some old and new results on local invariants of BDEs and IDEs. An invariant of an IDE/BDE, introduced in [Bruce-Tari, 1998] and called the multiplicity of the IDE/BDE at its singular point, counts the maximum number of stable singularities that can appear in a local deformation of the equation. Umbilic points on a generic surface are stable, that is, they persist under small deformations of the surface and the local configuration of the lines of principal curvature also remain unchanged under the deformation. However, the BDE of these lines is not stable when deformed within the set of all BDEs. In [Fernandes-Tari, to appear] we introduce an invariant of an analytic surface M in the Euclidean/Minkowski 3-space at its umbilic points, and call it the multiplicity of the umbilic point. The multiplicity of an umbilic counts the maximum number of stable umbilic points that can appear under small deformations of the surface at a non-stable umbilic point.

Speaker: Daniel Panazzolo, University of Upper Alsace, France

Title: Resolution of singularities for differential operators on surfaces

We consider analytic differential operators of order n defined on two-dimensional manifolds. Namely, linear operators locally of the form

Σ fij (∂/∂x)^i (∂/∂y)^j

with fij analytic functions. After introducing the notion of elementary singular point for such operators, we discuss a theorem of resolution of singularities, generalizing the classical result of Bendixson-Seidenberg for vector fields in dimension two.

The Theorem this talk is concerned with was published by J. Denef and L. Lipshitz in the Mathematische Annalen (1984). It states that there exists an algorithm which solves the following problem: given a finite system of polynomial ODE with coefficients in Q[x], decide whether this system admits for solution a tuple of formal power series in K[[x]] where K is the field of the complex numbers (some other fields are considered in the paper). The proof is complicated. It is supposed to be algorithmic but, as far as I know, has never been implemented. In this talk, I will present its main ingredients.

Speaker: Rafael Sendra, University of Alcalá, Madrid, Spain

Title: Solving algebraic differential equations with the aid of algebraic geometry

In this talk we intend to give an overview of some of the existing methods for solving algebraic differential equations through the use of algebraic geometry techniques. More specifically, the idea is to associate an algebraic variety to the differential equation (or equations) and, using properties of this variety, infer information that allows the differential equation to be solved or simplified to be able to approach it with other techniques. In this sense, the main tool to be used will be the existence of parameterizations of the associated variety.

Following the spirit of the seminar, the talk will focus on the ideas, without trying to cover the more technical aspects; all with the help of examples.

## Organizers

Sebastian Falkensteiner (MPI Leipzig, Germany)

Jorge Mozo Fernández (University of Valladolid, Spain)

Alberto Lastra (University of Alcalá, Madrid, Spain)

Alexey Remizov (MIPT, Moscow, Russia)

Werner Seiler (University of Kassel, Germany)

Matthias Seiß (University of Kassel, Germany)

## Scientific Board

Moulay Barkatou (University of Limoges, France)

José Cano (University of Valladolid, Spain)

Herwig Hauser (University of Vienna, Austria)

Joris van der Hoeven (École Polytechnique, France)

Daniel Robertz (RWTH Aachen University, Germany)

Rafael Sendra (University of Alcalá, Madrid, Spain)

Bernd Sturmfels (MPI Leipzig, Germany)

Speaker: Alexey Ovchinnikov, City University of New York

Title: Parameter estimation in ODE models via data interpolation, differential algebra, and polynomial system solving

We will discuss an approach for parameter estimation in ODE models that avoids optimization. It relies on differential algebra, output data interpolation for derivation estimation, and on polynomial system solving. This approach provides a framework for a robust implementation.