### Prony’s method: an old trick for new problems

Tomas Sauer
In 1795, French mathematician Gaspard de Prony invented an ingenious trick to solve a recovery problem, aiming at reconstructing functions from their values at given points, which arose from a specific application in physical chemistry. His technique became later useful in many different areas, such as signal processing, and it relates to the concept of sparsity that gained a lot of well-deserved attention recently. Prony’s contribution, therefore, has developed into a very modern mathematical concept.

### Spaces of Riemannian metrics

Mauricio Bustamante & Jan-Bernhard Kordaß
Riemannian metrics endow smooth manifolds such as surfaces with intrinsic geometric properties, for example with curvature. They also allow us to measure quantities like distances, angles and volumes. These are the notions we use to characterize the “shape” of a manifold. The space of Riemannian metrics is a mathematical object that encodes the many possible ways in which we can geometrically deform the shape of a manifold.

### The codimension

Antonio Lerario
In this snapshot we discuss the notion of codimension, which is, in a sense, “dual” to the notion of dimension and is useful when studying the relative position of one object insider another one.

### A Well-Posedness Result for Viscous Compressible Fluids with Only Bounded Density

Raphaël Danchin, Francesco Fanelli & Marius Paicu
We are concerned with the existence and uniqueness of solutions with only bounded density for the barotropic compressible Navier-Stokes equations. Assuming that the initial velocity has slightly sub-critical regularity and that the initial density is a small perturbation (in the $L^\infty$ norm) of a positive constant, we prove the existence of local-in-time solutions. In the case where the density takes two constant values across a smooth interface (or, more generally, has striated regularity with respect...

### A McKay Correspondence for Reflection Groups

Ragnar-Olaf Buchweitz, Eleonore Faber & Colin Ingalls
We construct a noncommutative desingularization of the discriminant of a finite reflection group $G$ as a quotient of the skew group ring $A=S*G$. If $G$ is generated by order two reflections, then this quotient identifies with the endomorphism ring of the reflection arrangement $\mathcal{A}(G)$ viewed as a module over the coordinate ring $S^G/(\Delta)$ of the discriminant of $G$. This yields, in particular, a correspondence between the nontrivial irreducible representations of $G$ to certain maximal Cohen--Macaulay...

### Affine Space Fibrations

Rajendra V. Gurjar, Kayo Masuda & Masayoshi Miyanishi
We discuss various aspects of affine space fibrations. Our interest will be focused in the singular fibers, the generic fiber and the propagation of properties of a given smooth special fiber to nearby fibers.

### Criteria for Algebraicity of Analytic Functions

Jacek Bochnak, Janusz Gwoździewicz & Wojciech Kucharz
We consider functions defined on an open subset of a nonsingular, either real or complex, algebraic set. We give criteria for an analytic function to be a Nash (resp. regular, resp. polynomial) function. Our criteria depend only on the behavior of such a function along irreducible nonsingular algebraic curves passing trough a given point. In the proofs we use results on algebraicity of formal power series, which are also established in this paper.

### Herbrand’s Theorem as Higher Order Recursion

Bahareh Afshari, Stefan Hetzl & Graham E. Leigh
We provide a means to compute Herbrand disjunctions directly from sequent calculus proofs with cuts. Our approach associates to a first-order classical proof $\pi \vdash \exists v F$, where $F$ is quantifier free, an acyclic higher order recursion scheme $\mathscr H$ whose language is finite and yields a Herbrand disjunction for $\exists v F$. More generally, we show that the language of $\mathscr H$ contains the Herbrand disjunction implicit in any cut-free proof obtained from...

### Z2-Thurston Norm and Complexity of 3-Manifolds, II

William Jaco, J. Hyam Rubinstein, Jonathan Spreer & Stephan Tillmann
In this sequel to earlier papers by three of the authors, we obtain a new bound on the complexity of a closed 3-manifold, as well as a characterisation of manifolds realising our complexity bounds. As an application, we obtain the first infinite families of minimal triangulations of Seifert fibred spaces modelled on Thurston's geometry $\widetilde{\text{SL}_2(\mathbb{R})}.$

### Wer ist Alexander Grothendieck?

Winfried Scharlau
The Oberwolfach Lecture "Wer ist Alexander Grothendieck" (in German) was held by Prof. Dr. Winfried Scharlau as a public lecture on occasion of the annual meeting of the Gesellschaft für Mathematische Forschung e.V. 2006 in Oberwolfach. For more works on A. Grothendieck by W. Scharlau see: http://www.scharlau-online.de/ag_1.html

### Categorical Linearly Ordered Structures

Rod Downey, Alexander Melnikov & Keng Meng Ng
We prove that for every computable limit ordinal $\alpha$ there exists a computable linear ordering $\mathcal{A}$ which is $\Delta^0_\alpha$-categorical and $\alpha$ is smallest such, but nonetheless for every isomorphic computable copy $\mathcal{B}$ of $\mathcal{A}$ there exists a $\beta< \alpha$ such that $\mathcal{A} \cong_{\Delta^0_\beta} \mathcal{B}$. This answers a question left open in the earlier work of Downey, Igusa, and Melnikov. We also show that such examples can be found among ordered abelian groups and real-closed fields.

### Geometry behind one of the Painlevé III differential equations

Claus Hertling
The Painlevé equations are second order differential equations, which were first studied more than 100 years ago. Nowadays they arise in many areas in mathematics and mathematical physics. This snapshot discusses the solutions of one of the Painlevé equations and presents old results on the asymptotics at two singular points and new results on the global behavior.

### Data assimilation: mathematics for merging models and data

Matthias Morzfeld & Sebastian Reich
When you describe a physical process, for example, the weather on Earth, or an engineered system, such as a self-driving car, you typically have two sources of information. The first is a mathematical model, and the second is information obtained by collecting data. To make the best predictions for the weather, or most effectively operate the self-driving car, you want to use both sources of information. Data assimilation describes the mathematical, numerical and computational framework...

### On the Invariants of the Cohomology of Complements of Coxeter Arrangements

J. Matthew Douglass, Götz Pfeiffer & Gerhard Röhrle
We refine Brieskorn's study of the cohomology of the complement of the reflection arrangement of a finite Coxeter group W. As a result we complete the verification of a conjecture by Felder and Veselov that gives an explicit basis of the space of W-invariants in this cohomology ring.

### Solving quadratic equations in many variables

Jean-Pierre Tignol
Fields are number systems in which every linear equation has a solution, such as the set of all rational numbers $\mathbb{Q}$ or the set of all real numbers $\mathbb{R}$. All fields have the same properties in relation with systems of linear equations, but quadratic equations behave differently from field to field. Is there a field in which every quadratic equation in five variables has a solution, but some quadratic equation in four variables has no...

### Mathematics plays a key role in scientific computing

Chi-Wang Shu
I attended a very interesting workshop at the research center MFO in Oberwolfach on “Recent Developments in the Numerics of Nonlinear Hyperbolic Conservation Laws”. The title sounds a bit technical, but in plain language we could say: The theme is to survey recent research concerning how mathematics is used to study numerical algorithms involving a special class of equations. These equations arise from computer simulations to solve application problems including those in aerospace engineering, automobile...

### Topological recursion

Piotr Sułkowski
In this snapshot we present the concept of topological recursion – a new, surprisingly powerful formalism at the border of mathematics and physics, which has been actively developed within the last decade. After introducing necessary ingredients – expectation values, random matrices, quantum theories, recursion relations, and topology – we explain how they get combined together in one unifying picture.

### Computing with symmetries

Colva M. Roney-Dougal
Group theory is the study of symmetry, and has many applications both within and outside mathematics. In this snapshot, we give a brief introduction to symmetries, and how to compute with them.

### Mathematik in Oberwolfach

Reinhold Remmert
Greeting on the occasion of the dedication ceremony of the enlargement of the library building, 05 May 2007.

### Fast Solvers for Highly Oscillatory Problems

Alex Barnett
Waves of diverse types surround us. Sound, light and other waves, such as microwaves, are crucial for speech, mobile phones, and other communication technologies. Elastic waves propagating through the Earth bounce through the Earth’s crust and enable us to “see” thousands of kilometres in depth. These propagating waves are highly oscillatory in time and space, and may scatter off obstacles or get “trapped” in cavities. Simulating these phenomena on computers is extremely important. However, the...

### Computing Congruence Quotients of Zariski Dense Subgroups

Alla Detinko, Dane Flannery & Alexander Hulpke
We obtain a computational realization of the strong approximation theorem. That is, we develop algorithms to compute all congruence quotients modulo rational primes of a finitely generated Zariski dense group $H \leq \mathrm{SL}(n, \mathbb{Z})$ for $n \geq 2$. More generally, we are able to compute all congruence quotients of a finitely generated Zariski dense subgroup of $\mathrm{SL}(n, \mathbb{Q})$ for $n > 2$.

### The Algebraic Statistics of an Oberwolfach Workshop

Anna Seigal
Algebraic Statistics builds on the idea that statistical models can be understood via polynomials. Many statistical models are parameterized by polynomials in the model parameters; others are described implicitly by polynomial equalities and inequalities. We explore the connection between algebra and statistics for some small statistical models.

### Deformation Classification of Real Non-Singular Cubic Threefolds with a Marked Line

Sergey Finashin & Viatcheslav Kharlamov
We prove that the space of pairs $(X,l)$ formed by a real non-singular cubic hypersurface $X\subset P^4$ with a real line $l\subset X$ has 18 connected components and give for them several quite explicit interpretations. The first one relates these components to the orbits of the monodromy action on the set of connected components of the Fano surface $F_\mathbb{R}(X)$ formed by real lines on $X$. For another interpretation we associate with each of the 18...

### The Martin Boundary of Relatively Hyperbolic Groups with Virtually Abelian Parabolic Subgroups

Matthieu Dussaule, Ilya Gekhtman, Victor Gerasimov & Leonid Potyagailo
Given a probability measure on a finitely generated group, its Martin boundary is a way to compactify the group using the Green's function of the corresponding random walk. We give a complete topological characterization of the Martin boundary of finitely supported random walks on relatively hyperbolic groups with virtually abelian parabolic subgroups. In particular, in the case of nonuniform lattices in the real hyperbolic space ${\mathcal H}^n$, we show that the Martin boundary coincides with...

### Exceptional Legendrian Torus Knots

Hansjörg Geiges & Sinem Onaran
We present classification results for exceptional Legendrian realisations of torus knots. These are the first results of that kind for non-trivial topological knot types. Enumeration results of Ding-Li-Zhang concerning tight contact structures on certain Seifert fibred manifolds with boundary allow us to place upper bounds on the number of tight contact structures on the complements of torus knots; the classification of exceptional realisations of these torus knots is then achieved by exhibiting suffciently many realisations...

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