












Onsdag 23 november 2022 kl 13:1514:15 Evgeniy Lokharu, Lunds universitet
Titel: Fine properties of steady water waves
Sammanfattning: In this talk we will discuss some recent results on two dimensional steady water waves. We will explain how the Benjamin and Lighthill conjecture can be significantly refined and will prove a new bound for the amplitude of an arbitrary Stokes wave in terms of the nondimensional Bernoulli constant. Our result, in particular, implies the inequality a ≤ Cc^2/g, where a is the amplitude, c is the speed of the wave, and g is the gravitational constant. This fact is valid for arbitrary Stokes waves irrespectively of the amplitude with an absolute constant C. Another observation is that any extreme Stokes wave over a sufficiently deep stream has necessarily a small amplitude, provided the nondimensional mass flux is much smaller than the depth.

Onsdag 16 november 2022 kl 13:1514:15 Michael Felsberg, Institutionen för systemteknik, Linköpings universitet
Titel: Steerable 3D Spherical Neurons
Sammanfattning:Emerging from lowlevel vision theory, steerable filters found their counterpart in prior work on steerable convolutional neural networks equivariant to rigid transformations. In our work, we propose a steerable feedforward learningbased approach that consists of neurons with spherical decision surfaces and operates on point clouds. Such spherical neurons are obtained by conformal embedding of Euclidean space and have recently been revisited in the context of learning representations of point sets. Focusing on 3D geometry, we exploit the isometry property of spherical neurons and derive a 3D steerability constraint. After training spherical neurons to classify point clouds in a canonical orientation, we use a tetrahedron basis to quadruplicate the neurons and construct rotationequivariant spherical filter banks. We then apply the derived constraint to interpolate the filter bank outputs and, thus, obtain a rotationinvariant network. Finally, we use a synthetic point set and realworld 3D skeleton data to verify our theoretical findings. The code is available here.

Onsdag 9 november 2022 kl 13:1514:15 Mark Allen, Brigham Young University, Provo, USA och Institut MittagLeffler, Djursholm
Title: Isoperimetric and FaberKrahn Inequalities
Abstract: In this talk we review the isoperimetric inequality and how it leads to the FaberKrahn inequality. We will then discuss how to establish quantitative forms of these inequalities. This entails measuring how close a set is to a ball if the perimeter (or first eigenvalue of the Laplacaian) is close to that of the ball. We will conclude by showing how quantitative inequalities can be useful.

Onsdag 26 oktober 2022 kl 13:1514:15 Erik Darpö, Matematiska institutionen, Linköpings universitet
Titel: Quivers, path algebras and representations
Sammanfattning: In this talk, we will have a look at quivers and their representations: what they are, what they are good for, and how they are related to (noncommutative) rings/algebras and their modules. We will speak about one of the early, seminal results in the representation theory of algebras, Gabriel's theorem, which classifies quivers with only finitely many isomorphism classes of indecomposable representations in terms of Dynkin diagrams.

Fredag 21 oktober 2022 kl 13:1514:15 János Barát, Alfréd Rényi Institute of Mathematics, Budapest, Ungern INSTÄLLT

Titel: Constructions
Sammanfattning: I will show a few episodes of my mathematical carrier in Combinatorics, where constructions played an important role. My intention is to emphasize the differences in these stories. I try to cover the following scenarios:

 There is a longstanding conjecture, but nobody can prove it. Can we find examples to show the statement is actually sharp?
 There is a statement of the nonexistence of an object. This claim appears in the most renowned book of the subject. You start a collaboration and your new colleague gives you the manuscript of the proof, but you find a small inaccuracy. Can this change the entire picture?
 There is a difficult problem. You try to warm up with a simpler one of the same flavor. After a while you convince yourself that you grabbed the bottleneck of the question. Can a construction inspire the entire proof?
Possibly some more, if time permits.
Onsdag 28 september 2022 kl 13:1514:15 JörgUwe Löbus, Matematiska institutionen, Linköpings universitet

Titel: Knudsen Type Group and Boltzmann Type Equation  Boundary Shape and Boundary Conditions
Sammanfattning: We consider certain Boltzmann type equations on a bounded physical and a bounded velocity space under the presence of both, reflective as well as diffusive boundary conditions. We introduce conditions on the shape of the physical space and on the relation between the reflective and the diffusive part in the boundary conditions such that the associated Knudsen type semigroup can be extended to time $t\in\mathbb{R}$. Furthermore, we provide conditions under which there exists a unique global solution to a Boltzmann type equation for time $t\ge 0$ or for time $t\in [\tau_0,\infty)$ for some $\tau_0<0$ which is independent of the initial value at time $0$. Depending on the collision kernel, $\tau_0$ can be arbitrarily small.

The corresponding paper is available under here.

Onsdag 21 september 2022 kl 13:1514:15 Jonathan Nilsson, Matematiska institutionen, Linköpings universitet
Titel: Old and new families of Lie algebra modules
Sammanfattning: Representation theory for Lie algebras is a rich subject with connections throughout several areas of mathematics and physics. An important aspect in understanding the category of representations of a Lie algebra is to obtain a classification of its simple objects, but unfortunately such a classification seems intractable for all but the smallest Lie algebras. Nevertheless, by imposing some restrictions one can construct and classify several classes of simple modules, such as finitedimensional modules, Whittaker modules and GelfandZetlin modules. In the first part of this talk I will give an overview of several wellknown families of Lie algebra modules. Then I will present my own contributions to this field, discussing modules which are free over the enveloping algebra of a Cartansubalgebra, as well as some new families of modules for the Lie algebra of vector fields on affine varieties. The talk is partly based on joint work with Yuly Billig and Vyacheslav Futorny.
Onsdag 24 augusti 2022 kl 13:1514:15 Klara Stokes, Umeå universitet
Titel: Incidence geometries with trialities from coset geometries and embeddings of graphs on surfaces

Sammanfattning: Triality is a classical notion in geometry that arose in the context of the Lie groups of type D_4. Another notion of triality appears in the context of reflexible maps. In this talk I will show how to construct incidence geometries with trialities for other groups, using techniques such as gain/voltage graphs, embeddings of graphs on surfaces (maps) and as coset geometries. I will focus on the case in which the incidence geometry has a triality but no dualities. This is joint work with Dimitri Leemans.
Onsdagen 8 juni 2022, Juha Lehrbäck, Jyväskylä University
Titel: Weakly porous sets and Muckenhoupt $A_1$ distance weights
Sammanfattning: Let $E\subset\mathbb{R}^n$ and let $w(x)=d(x,E)^{\alpha}$, where $\alpha>0$.
If $E$ is porous, then it is known that $w$ is a Muckenhoupt $A_1$ weight if and only if the Assouad dimension of $E$ is strictly less than $n\alpha$. Since $E$ is porous if and only if the Assouad dimension of $E$ is strictly less than $n$, it follows that for all porous sets there exists some $\alpha>0$ such that $w$ is an $A_1$ weight.
In this talk, I extend this result to nonporous sets and show that $d(x,E)^{\alpha}$ is an $A_1$ weight, for some $\alpha>0$, if and only if $E$ is \emph{weakly porous}. During the talk, I will introduce the relevant concepts and for instance give examples of some nonporous sets which are weakly porous.
This talk is based on an ongoing joint work with Carlos Mudarra and Antti Vähäkangas.
Onsdag 1 juni, Ezio Venturino, Università di Torino
Titel: Fighting alien species invasions through mathematical modeling
Sammanfattning: Recently, we have investigated the phenomenon of native populations replacement by exotic ones, focusing on accidentally or intentionally introduced
species in Italy and UK. Two major cases are of interest. American grey squirrels are gradually outcompeting the indigenous red ones both in Italy and in UK. In the latter case, the situation is worsened by the presence of a virus, carried by the aliens, for which it is harmless, but which ultimately is found lethal for the native species. In Italy the European hare is slowly forcing the extinction of the native mountain hare. The main difference in this case is that the contacts among the two occur just on the boundary region of the territories occupied by the two species. This allows us to formulate a nonclassical interaction model, based on the concept of herd behavior. The latter is extended to other possible situations, showing the onset of some new features in the phase space, revealing quite different results from the classical ones.
OBS! Lokal: C3
Torsdag 19 maj 10.1511.15 Hans Nguyen, University of Nottingham
Titel: Homological methods in random noncommutative geometry
Sammanfattning: Noncommutative geometry is a generalisation of ordinary (commutative) geometry, where also noncommutative algebras are regarded as algebras of functions of spaces. In this talk, we will focus on the socalled fuzzy spectral triples, which are a finite version of the celebrated spectral triples of Alain Connes that constitute the cornerstone of noncommutative Riemannian (spin) geometry. Since fuzzy spectral triples are fully classified, they form a good setting for studying random noncommutative geometry. In particular, their finite nature allows for a rigorous formulation and study of the path integral over the space of geometries, leading to toymodels for quantum gravity. The new aspect of our work is that we take into account the noncommutative analogue of diffeomorphism symmetries in the formulation of such path integrals, which we will implement through suitable homological methods.
Lokal: Hopningspunkten
Onsdag 18 maj 2022, Signe Lundqvist, Umeå universitet
Titel: When is a rod configuration infinitesimally rigid?
Sammanfattning: The mathematical theory of structural rigidity has a long history. In the nineteenth century, Cauchy studied rigidity of polyhedra, and Maxwell studied graph frameworks. The rigidity theory of graph frameworks has since been studied extensively. PollaczekGeiringer, and later Laman, proved a combinatorial characterization of the minimally rigid graphs in the plane. Combinatorial rigidity theory is also concerned with geometric realizations of other combinatorial structures. A rod configuration is a realisation of a hypergraph as points and straight lines in the plane, where the lines behave as rigid bodies. In this talk, we will discuss approaches for determining whether a given rod configuration is infinitesimally rigid. This is based on joint work with Klara Stokes and LarsDaniel Öhman.
Onsdag 11 maj 2022, Magnus Herberthson, Matematiska institutionen, Linköpings universitet
Titel: Characterising the second order moments of diffusion tensor distributions
Sammanfattning: In magnetic resonance imaging (MRI), it is possible to probe tissue on a subresolution level. The signal obtain by MRI is then formed by a family of diffusion tensors, with may have certain statistical properties. In this talk we are interested in characterising the second order moment of this family, which is a forth order tensor with certain symmetries.
Onsdag 4 maj 2022, Anders Björn, Matematiska institutionen, Linköpings universitet
Titel: $p$harmonic Green functions on metric measure spaces and growth rate exponents
Sammanfattning: On (unweighted) ${\bf R}^n$, the dimension $n$ determines (together with $p$) the Sobolev exponent $p^*=np/(np)$. Also the growth rate for the $p$harmonic Green function $u(x)=x^{(pn)/(p1)}$ is given by $n$ (and $p$).
In this talk I will consider similar facts on a complete metric space equipped with a doubling measure supporting a Poincaré inequality. I will explain how one defines Sobolev spaces (socalled Newtonian spaces based on upper gradients) and $p$harmonic functions on metric spaces. I will then spend some time on how to define, and what should be meant, by $p$harmonic Green functions and discuss their local integrability properties.
I will also try to give a few other examples where different growth rates coming directly from the measure appear naturally and are sharp. For unweighted ${\bf R}^n$ all these growth rates coincide (and are $n$), but this is not so in general.
Onsdag 30 mars 2022, Axel Hultman, Matematiska institutionen, Linköpings universitet
Titel: Quickstep posets and inversion arrangements
Sammanfattning: Given a graph with a fixed vertex $v$, I shall demonstrate a construction of a partially ordered set whose elements are subsets of the neighbourhood of $v$. The motivation comes from the combinatorics of certain hyperplane arrangements known as inversion arrangements and their somewhat mysterious interplay with the geometry of Schubert varieties. An attempt at an overview for nonexperts will be provided.
Onsdag 16 mars 2022, Daniel Fox, Universidad Politécnica de Madrid, Spanien
Titel: Partial associativity and quantitative nonassociativity
Sammanfattning: Taking seriously an apparently naive analogy between the multiplication of a not necessarily associative algebra and a covariant derivative operator leads to notions of partial and quantitative associativity modeled on various forms of curvature of a connection. Such notions identify classes of nonassociative algebras for which classification appears tractable and which include a diversity of interesting examples, including semisimple Lie algebras and their tensor products, semisimple Euclidean Jordan algebras, Griess algebras of vertex operator algebras (such as the Griess algebra of the monster finite simple group), and the algebra of metric curvature tensors with the HamiltonHuisken product. These notions will be surveyed, focusing on the notions of projective associativity for commutative algberas and sectional nonassociativity for metrized commutative algebras,
Onsdag 9 mars 2022, Tomas Sjödin, Matematiska institutionen, Linköpings universitet
Titel: On Laplacian growth in the plane and a conjecture of Gustafsson and Lin.
Sammanfattning: The Laplacian growth process of HeleShaw flow in the plane in the forward direction has been extensively studied for several decades now, where both classical as well as weak solutions to the moving boundary problem has been considered.
One major issue is that classical solutions will almost always break down in finite time. In their recent book Laplacian growth on branched Riemann surfaces Gustafsson and Lin considers the possibility, in the configuration of a simply connected initial domain and a point source, of extending the classical solutions to exist for all time by lifting them to a branched Riemann surface. Unfortunately, they got stuck on a technical problem and had to develop the theory based on the validity of a conjecture. Roughly speaking the problem in their construction is that one inevitably more or less at certain points in time will get to a situation that the boundary infinitesimally moves with infinite speed at some points, and the approach of classical solutions breaks down. The conjecture was then that the solution still exists and is simply connected for some small time interval. This conjecture was answered in the positive in the recent article On a conjecture of Gustafsson and Lin concerning Laplacian growth (TS and S.J. Gardiner). In this talk we will discuss the law of motion for the HeleShaw process, how it can be interpreted in terms of the Riemann map and finally discuss the conjecture of Gustafsson and Lin.
Länk till publikationen
Onsdag 23 februari 2022, Tatiana Tchemisova, University of Aveiro, Portugal
Anordnas i samarbete med Seminarier i optimeringslära
Titel: On phenomenon of Immobility in study of convex Optimization problems
Sammanfattning: We are concerned with convex problems of infinite Optimization, namely problems of convex SemiInfinite Programming (SIP) and linear problems of Semidefinite Programming (SDP).
SemiInfinite Programming deals with extremal problems that consist in minimization of an objective function of finitely many variables in a set described by an infinite system of constraints. SIP models appear in different fields of modern science and engineering where it is necessary to simulate a behavior of complex processes whose models contain at least one inequality constraint for each value of some parameter (for example, time) varying in a given compact domain.
In Semidefinite Programming, an objective function is minimized under the condition that some matrix valued function is positive semidefinite. When the objective function is linear and the matrix valued function is an affine combination of some symmetric matrices, we get a convex problem. There are many applications of SDP models to combinatorial optimization, control theory, approximation theory, etc.
Optimality conditions for Optimization problems are of special interest both from theoretical and practical points of view. A special attention is devoted to the results that do not need additional conditions on the constraints, so called constraint qualifications (CQ).
In the talk, we present the results on optimality and strict duality for convex SemiInfinite Programming problems which are obtained based on a new concept of immobile indices of constraints. We show how this concept can be applied to problems of linear SDP . The main result consists in new CQfree optimality conditions for the considered classes of Optimization problems.
Onsdag 19 januari 2022, Sebastián Reyes Carocca, Universidad de la Frontera, Chile
Titel: Loci of Riemann Surfaces with Automorphisms
Sammanfattning: TBA
Onsdag 24 november 2021, Evgeniy Lokharu, Matematiska institutionen, Linköpings universitet
Titel: On extreme steady water waves with vorticity
Sammanfattning: Extreme steady waves are exact solutions to Euler equations in two dimensions that posses surface singularities, where the relative velocity field vanishes. Already in 1880s Sir George Stokes made a remarkable for that time conjecture about extreme waves: the surface profile at singular points has to form a sharp corner of 120 degrees. This conjecture was very influential and had determined the research direction in the field for many years. In this talk we will discuss the history behind the problem and some recent new results obtained by the authors.
Onsdag 11 mars 2020, Juha Lerhbäck, University of Jyväskylä, Finland
Titel: Quasiadditivity properties of variational capacity and HardySobolev inequalities
Sammanfattning: Capacities are outer measures and hence subadditive, but they are practically never additive. A capacity is called quasiadditive, if it satisfies a converse for the subadditivity (with a multiplicative constant) with respect to a suitable cover of the underlying set. In this talk I consider this property for the variational capacity in an open set of the Euclidean space, with respect to a Whitney cover of the open set. In particular, I will characterize a generalized (q,p)version of the quasiadditivity using corresponding HardySobolev inequalities.
This talk is based on joint work with Juha Kinnunen and Antti Vähäkangas.
Onsdag 4 mars 2020, Sergey Nazarov, Matematiska institutionen, Linköpings universitet, och St Petersburg, Ryssland
Titel: The Neumann Laplacian: abnormal transmission acoustic waves through narrow canals
Onsdag 26 februari 2020, Sergei Silvestrov, Mälardalens högskola, Västerås
Titel: Homalgebra structures
Sammanfattning: In this colloquium lecture an introductory overview and open problems about Homalgebra structures will be given with emphasize on homalgebra generalizations of Lie algebras and associative algebras.
These interesting and rich algebraic structures appear for example when discretizing the differential calculus as well as in constructions of differential calculus on noncommutative spaces. In 1990’th quantum deformations of algebras, qdeformed oscillator algebras, qdeformations of Witt and Virasoro algebras and related families of algebras defined by generators and parameter commutation relations have been constructed in connection to quantum deformations and discretized models of mechanics and quantum mechanics, qdeformations of vertex operators, qdeformed conformal quantum field theory, qdeformed integrable systems, qdeformed superstrings and central extensions. Also, various quantum nary extensions of Nambu mechanics and related nary extensions of differential structures and of Lie algebras Jacobi identities have been considered. It was noticed that many of quantum algebras and qdeformed Lie algebras obey certain qdeformed versions of Jacobi identity generalizing Lie algebras Jacobi identity. Motivated by these works Hartwig, Larsson and Silvestrov in 2003 developed a general method of obtaining such deformations and generalized Jacobi identities based on general twisted derivations. This development, as well generalizations of supersymmetry, lead to development of more general algebraic structures such as quasiLie and HomLie algebras, Homassociative and HomLie admissible algebras, HomJordan algebras, HomPoisson algebras, HomYangBaxter equations, Hombialgebras, HomHopf algebras, and other homalgebra structures, as well as HomNambu and HomNambu Lie algebras some related nary Homalgebra generalizations of Nambu algebras, associative algebras and Lie algebras and their constructions.
Onsdag 12 februari 2020, Ahmed AlShujary, Matematiska institutionen (MAI), Linköpings universitet
Titel: KählerPoisson Algebras
Sammanfattning: : In this talk, we introduce KählerPoisson algebras and study their basic properties. The motivation comes from differential geometry, where one can show that the Riemannian geometry of an almost Kähler manifold can be formulated in terms of the Poisson algebra of smooth functions on the manifold. It turns out that one can identify an algebraic condition in the Poisson algebra (together with a metric) implying that most geometric objects can be given a purely algebraic formulation. This leads to the definition of a KählerPoisson algebra, which consists of a Poisson algebra and a metric fulfilling an algebraic condition. We show that every KählerPoisson algebra admits a unique LeviCivita connection on its module of inner derivations and, furthermore, that the corresponding curvature operator has all the classical symmetries. Moreover, we present a construction procedure which allows one to associate a KählerPoisson algebra to a large class of Poisson algebras. From a more algebraic perspective, we introduce basic notions, such as morphisms and subalgebras, as well as direct sums and tensor products. Finally, we initiate a study of the moduli space of KählerPoisson algebras; i.e for a given Poisson algebra, one considers classes of metrics giving rise to nonisomorphic KählerPoisson algebras. As it turns out, even the simple case of a Poisson algebra generated by two variables gives rise to a nontrivial classification problem.
Onsdag 29 januari 2020, Evgeniy Lokharu, Matematiska institutionen (MAI), Linköpings universitet
Titel: Nonexistence of subcritical solitary waves
Sammanfattning: We prove the nonexistence of twodimensional solitary gravity water waves with subcritical wave speeds and an arbitrary distribution of vorticity. This is a longstanding open problem, and even in the irrotational case there are only partial results relying on sign conditions or smallness assumptions. As a corollary, we obtain a relatively complete classification of solitary waves: they must be supercritical, symmetric, and monotonically decreasing on either side of a central crest. The proof introduces a new function related to the socalled flow force which has several surprising properties. In addition to solitary waves, our nonexistence result applies to "halfsolitary" waves (e.g. bores) which decay in only one direction.
This is a joint work with Vladimir Kozlov (MAI) and Miles H. Wheeler (University of Bath, UK).
Onsdag 22 januari 2020, Elina Rönnberg, Matematiska institutionen (MAI), Linköpings universitet
Seminariet arrangeras ihop med Tvärvetenskapliga seminarier på MAI.
Titel: Efficient use of hardware resources in avionic systems
Sammanfattning: A key ingredient when designing an avionic system – i. e. the electronic system of an aircraft – is to make sure that it always can be trusted. In modern integrated modular avionic systems, different aircraft functions share hardware resources on a common avionic platform. For such architectures, it is necessary to create a spatial and temporal partitioning of the system to prevent faults from propagating between different functions. One way to establish a temporal partitioning is through preruntime scheduling.
While the avionic systems are growing more and more complex, so is the challenge of scheduling them. Scheduling of the system has an important role when a new avionic system is developed. Typically, functions are added to the system over a period of several years and a scheduling tool is used both to determine if the platform can host the new functionality and, in case this is possible, to create a new schedule.
In this talk, I will discuss a design case from Saab Aeronautics and present an optimisationbased scheduling tool that we have developed. From an optimisation point of view, the problem can be described as a rich multiprocessor scheduling problem that also includes a communication network to be scheduled. Results are presented for practically relevant largescale instances with up to 60 000 tasks.
Onsdag 15 januari 2020, Andrew Ross Winters, Matematiska institutionen (MAI), Linköpings universitet
Titel: My numerical scheme crashed, now what?
Sammanfattning: Numerical methods to approximate the solution of partial differential equations are a powerful tool to model problems we otherwise could not. But can they tell us something even when they fail?
We present why numerical methods can break, and how to fix them. To do so, we motivate the discussion from a physical perspective, which we then translate and inspect with the language of mathematics.