### Inhalt des Dokuments

# Forschungsseminar

Name | Datum | Uhrzeit | Ort | Titel |
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TBA | 14.11.18 | 16:30 | MA 621 | TBA |

Abstract: TBA |
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Akiyoshi Tsuchiya | 24.10.18 | 16:30 | MA 621 | TBA |

Abstract: TBA |
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Leon Zhang | 29.08.18 | 16:30 | MA 621 | Constructing an enveloping membrane for a min-convex hull |

Abstract: Joswig, Sturmfels, and Yu (2007) describe a correspondence between a min-convex hull in the affine building of SL_d(K) and a tropical convex hull in tropical projective space TP^{d-1}, using a membrane containing the min-convex hull. We describe an algorithm for constructing such a membrane, which gives a bound on the number of points needed to span the tropical convex hull. |
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Charles Wang | 22.08.18 | 16:30 | MA 621 | Cluster Algebras and Grassmannians |

Abstract: We will study the interactions of combinatorics and geometry via cluster algebras. We will begin with the simple case of the Grassmannians Gr(2,n), where this connection is completely determined by the type A_n associahedron, corresponding to triangulations of an n+3-gon. Next, we will discuss the case for general Gr(k,n) and the Lagrangian Grassmannians LG(n), where will focus on combinatorial objects called plabic graphs. We will discuss how the cluster structure in these cases gives a method to understand the Newton-Okounkov bodies of these varieties, and give a connection to Khovanskii bases and toric degenerations. Time permitting, we will also the study relation to the tropical Grassmannians and positivity. |
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Stephan Tillmann | 18.07.18 | 16:30 | MA 621 | What is the Thurston norm? |

Abstract: Thurston’s norm on (co)homology of a 3-manifold reveals interesting topological properties of the 3-manifold. At the centre of its study is its unit ball, which is a finite polytope. Associated to some of its top dimensional faces are special triangulations of the manifold defined by Agol, called veering triangulations. After a general introduction to these topics, I will outline algorithms developed in collaboration with Daryl Cooper and Will Worden to compute the unit ball, and ask the audience for input on interesting properties of the polytopes that one might want to address. |
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Marek Kaluba | 11.07.18 | 16:30 | MA 621 | Non-commutative positivity and property (T) |

Abstract: The underlying theme of the talk is the reduction of the (hard analytic) ques- tion of Kazhdan’s property (T) to an (algebraic, computable) problem of non- commutative optimisation. In this formulation, property (T) is equivalent to the positivity of the element ∆ 2 − λ ∆ in (full) C ∗ -group algebra (where ∆ is the group Laplacian). This problem will serve as a motivation to explore the topic of non-commutative positivity. I will present certain results on positivity for free ∗ -algebras and group al- gebras. It turns out that, due to much richer representation theory of non- commutative objects, the Positivestellensätze in the non-commutative world are much stronger than in the commutative world: the positive cone is in many cases equal to the cone of sums of (hermitian) squares. Moreover one of the characterisations of cones in the “algebraic topology” of ∗ -algebra allows turning a numerical approximation of the the sum of squares decomposition (of ∆ 2 − λ ∆ ) into a mathematical proof of the positivity. I will finish by presenting the very concrete case of the application of the theory to S Aut( F n ), the (special) automorphism group of the free group on five generators. Using Gramm method we formulated The successful execution of the described programme was obstructed by the sheer size of the optimisation problem. To overcome these difficulties we exploited the symmetry of ∆ 2 − λ ∆ and used the theory of representations of finite groups to significantly reduce the complexity of the optimisation problem, wich might be of interest of its own. |
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Ngoc Tran (UT Austin/Hausdorff Center for Mathematics, Bonn) | 04.07.18 | 16:30 | MA 621 | A computational approach to tropical semigroups |

Abstract: In many semigroups it is difficult to construct and verify identities. For the semigroups of Tropical uppertriangular matrices, we show that each word has a signature, which is a sequence of polytopes, and two words form an identity if and only if their signatures agree. This leads to fast algorithms for constructing semigroup identities. Our algorithms allow us to disprove conjectures, prove new theorems on the structure of the semigroup, and form new conjectures at the intersection of probability, combinatorics and semigroup theory. In this talk, we will focus on explaining these conjectures from the viewpoint of polyhedral computations. Joint work with Marianne Johnson |
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Timur Sadykov | 28.06.18 | 14:15 | MA 316 | Amoebas of multivariate hypergeometric polynomials |

Abstract: Polynomial instances of hypergeometric functions in one and
several variables are very diverse. They comprise the classical
Chebyshev polynomials of the first and the second kind, the
Gegenbauer, Hermite, Jacobi, Laguerre and Legendre polynomials as
well as their numerous multivariate analogues. |
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Christian Stump | 20.06.18 | 16:30 | MA 621 | Reflection groups and singularity theory |

Abstract: I aim to motivate, as simple as possible and limited by my own understanding, Arnold's problem "Find applications of the complex reflection groups in singularity theory". I start with discussing the connection between the discriminant of a general polynomial in one variable and the realization of the braid group using the braid arrangement. I then intend to give a detailed introduction to reflection groups in rational / real / complex vector spaces, their reflection arrangements, and their discriminants, and in particular discuss the connection to Kleinian singularities associated with symmetry groups of platonic solids and binary polyhedral groups. |
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Kalina Mincheva (Yale University) | 13.06.18 | 16:30 | MA 621 | The Picard group of a tropical toric scheme |

Abstract: Through the process of tropicalization one obtains from an algebraic variety $X$ a combinatorial object called the tropicalization of $X$, $trop(X)$, that retains a lot of information about the original variety. Following the work of J. Giansiracusa and N. Giansiracusa, one can endow $trop(X)$ with more structure, to obtain a tropical scheme. Loosely speaking, we consider more equations than the ones needed to determine the tropical variety. We are interested what information about the original variety $X$ is preserved by the tropical scheme $X_\mathbb{T}$ (but possibly not by the tropical variety). In particular, we study the relation between the Picard group of $X$ and $X_\mathbb{T}$. We solve the problem in the case when $X$ is a toric variety. |
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Jonathan Spreer | 06.06.2018 | 16:30 | MA 621 | Separation-type combinatorial invariants for triangulations of manifolds |

Abstract: In my talk I will propose and discuss a set of combinatorial invariants of simplicial complexes which, in some sense, can be seen as a refinement of the f-vector. The invariants are very elementary and defined by counting connected components and/or homological features of induced subcomplexes. |
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Alperen Erguer | 30.05.18 | 16:30 | MA 406 | Multihomogenous Nonnegative Polynomials and Sums of Squares |

Abstract: It is due to Blekherman that we now know there are significantly more nonnegative forms than sums of squares for any fixed degree \(2d\) and large number of variables \(n\). What if we ask the same comparison for polynomials with a prescribed Newton polytope? I will present some general ideas from (modern) convex geometry that handles the case of multihomogeneous polynomials. These results refine the estimates of Blekherman and happens to have some implications in quantum information theory. I will also try to indicate missing ingredient(s) for handling general Newton polytopes, which might be of interest to the lovers of toric geometry and/or Lie algebras. |
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Travis Scrimshaw | 29.05.18 | 17:00 | MA 621 | On the Combinatorics of Crystals |

Abstract: In algebra, a crystal base or canonical base is a base of a representation, such that generators of a quantum group or semisimple Lie algebra have a particularly simple action on it. Crystal bases were introduced by Kashiwara (1990) and Lusztig (1990) (under the name of canonical bases). |
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Grigoris Paouris (Texas A&M University) | 24.05.18 | 16:15 | MA 313 | The "Small ball" property in high dimensional measures |

Abstract: I will discuss some methods/techniques to prove "small ball probabilities" and I will review some connections with other problems in Asymptotic Geometric Analysis. The emphasis will be on how high-dimensional geometry and convexity is crucial to understand the concentration behavior in the "small ball regime". |
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Grigoris Paouris (Texas A&M University) | 23.05.18 | 16:30 | MA 313 | Almost Euclidean section of high-dimensional convex bodies |

Abstract: The celebrated theorem of Dvoretzky states that for any n and \(\varepsilon \in (0,1)\) there exists a function \(k(n,ε)\) with the property that any n-dimensional symmetric convex body has a k-dimensional section at a distance at most \((1+\varepsilon)\) from the Euclidean ball and moreover \(k(n,\varepsilon)\) tends to infinity as \(n \to \infty\) for fixed \(\varepsilon\). The asymptotic behavior of this function remains an open problem 64 years after Grothendieck proposed it. I will discuss some recent developments on the problem. In particular I will present how the notion of superconcentration is central in the latest improvements. Based on joint work with P. Valettas. |
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Chris O'Neill (UC Davis) | 09.05.18 | 16:30 | MA 621 | Computing the delta set of an affine semigroup: a status report |

Abstract: An affine semigroup \(S\) is a subset of \(\mathbb Z_{\ge 0}^k\) that is closed under vector addition, and a factorization of \(a \in S\) is an expression of \(a\) as a sum of generators of \(S\). The delta set of \(a\) is a set of positive integers determined by the 'missing factorization lengths' of \(a\), and the delta set of \(S\) is the union of the delta sets of its elements. Although the delta set of any affine semigroup is finite, its definition as an infinite union makes explicit computation difficult. In this talk, we explore algebraic and geometric properties of the delta set, and survey the history of its computation for affine semigroups. The results presented here span the last 20 years, ranging from a first algorithm for a small class of semigroups that is impractical for even basic examples, to recent joint work with Garcia-Sanchez and Webb expressing the delta set of any affine semigroup in terms of Groebner bases, and include results from numerous undergraduate research projects. |
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Vijaylaxmi Trivedi (Tata Institute of Fundamental Research, Mumbai) | 02.05.18 | 16:30 | MA 406 | Hilbert-Kunz functions and HK density functions |

Abstract: We give a brief survey of Hilbert-Kunz function and its leading coefficient \(e_{HK}\) called HK multiplicity, for a commutative Noetherian ring. |
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Victor Magron (CNRS Grenoble) | 25.04.18 | 16:30 | MA 621 | On exact polynomial optimization |

Abstract: To compute certificates of nonnegativity, an approach based on sums of
squares (SOS) decompositions has been popularized by Lasserre and Parillo. |
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Ben Smith (Queen Mary University of London) | 18.04.18 | 16:30 | MA 621 | Matching fields and lattice points of simplicies |

Abstract: An (n,d)-matching field is a collection of matchings such that there is a unique matching for each d-subset of [n]. They naturally arise as minimal matchings of a weighted complete bipartite graph, and can be thought of as the tropical analogue to matroids. They have relationships to multiple combiatorial objects, in particular triangulations of the product of two simplices. In this talk, we will use this relationship to introduce a construction that associates to the matching field a collection of bipartite graphs and a bijection to the lattice points of a scaled simplex. We will then use this construction to prove two outstanding conjectures of Sturmfels and Zelevinsky on matching fields. |
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Michael Joswig (TU Berlin) | 11.04.18 | 16:30 | MA 621 | Coxeter groups, BN-pairs, and buildings |

Abstract: Buildings form a fundamental geometric concept which generalize the Coxeter complex of a Coxeter group. The theory was mostly developed by Tits (and Borel) in the 1960s and 1970s. While its origins are in Lie theory one particularly striking application was its contribution to the classification of the finite simple groups (completed around 1980). In this talk (of historical survey type) I will define the three notions in the title and look into one family of examples. In the end I will briefly try to sketch the impact on group theory in general. |
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Sascha Timme | 04.04.18 | 16:30 | MA 621 | Fast Computation of Amoebas in low Dimensions |

Abstract: The concept of an amoeba was introduced by Gelfand, Kapranov and Zelevinsky
in 1994 to study the relationship between the zero locus of a polynomial and
its Newton polytope. Amoebas have a lot of fascinating structural properties.
In particular, there exists for each amoeba a tropical hypersurface, called
the spine, which is a deformation retract of the amoeba. |
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Adam Kurpisz | 28.03.18 | 16:30 | MA 621 | On the hierarchies of relaxations for 0/1 optimization problems |

Abstract: Constructing efficient algorithms for combinatorial optimization problems is a task that requires a lot of knowledge, experience and creativity. The goal of this talk is to present the automatizable hierarchies of algorithms (Lasserre/Sum-of-Squares (SoS), Sherali Adams (SA)) and the recent development in this field, the Sum-of-Nonnegative-Circuit (SoNC) algorithm. |
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Francisco Criado | 07.02.18 | 16:30 | MA 621 | Packing algorithms applied to linear programming |

Abstract: The Yamnistsky-Levin algorithm (1982) is a variation of the ellipsoid method that uses a simplex instead of an ellipsoid. Unlike the ellipsoid method, it does not require square root computations, and it still runs in weakly polynomial time. However, its running time is still not practical. We introduce a variation of this method that examines a subset of cutting planes, and uses the Multiplicative Weights framework developed by Plotkin, Shmoys and Tardos for packing linear programs. We will show how our algorithm improves the original YL algorithm for a simplification of the problem, and what questions remain open to give a complete proof of convergence. |
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Romanos Malikiosis | 24.01.18 | 16:30 | MA 406 | Fuglede's spectral set conjecture on cyclic groups |

Abstract: Fuglede's conjecture (1974) states that a bounded measurable subset in \(\mathbb{R}^d\) accepts an orthogonal basis of exponential functions (i.e. it is spectral) if and only if it tiles the space with a discrete set of translations. This conjecture turned out to be false by Tao's counterexample in 2003. Using Tao's ideas, counterexamples in finite Abelian groups such as \(\mathbb{Z}_N^d\) can be lifted to counterexamples in \(\mathbb{R}^d\), thus shifting the interest on this conjecture to this setting in recent years. This has been successful for \(d\geq3\), but the conjecture is still open for \(d=1,2\). |
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Linda Kleist | 20.12.17 | 16:30 | MA 621 | Rainbow cycles in flip graphs |

Abstract: The vertices of flip graph are combinatorial or geometric objects, and its edges link two of these objects whenever they can be obtained from one another by an elementary operation called a flip. With each edge, we associate the type of a flip (thought of as a color) and seek for r-rainbow cycles, i.e., cycles where every flip type occurs exactly r times. |
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Franz Schuster (TU Wien) | 06.12.17 | 16:30 | MA 406 | Affine vs. Euclidean Sobolev inequalities |

Abstract: In this talk we explain how every even, zonal measure on the Euclidean unit sphere gives rise to a sharp Sobolev inequality for functions of bounded variation which directly implies the classical Euclidean Sobolev inequality. The strongest member of this large family of inequalities is shown to be the only affine invariant one among them – the affine Zhang-Sobolev inequality. We discuss in some detail the geometry behind these analytic inequalities and also relate our new Sobolev inequalities to the sharp Gromov-Gagliardo-Nirenberg Sobolev inequality for general norms and discuss further improvements of special cases. |
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Gennadiy Averkov (OvGU Magdeburg) | 29.11.17 | 16:30 | MA 406 | Latest news on Hensley's conjecture |

Abstract: The talk is about the maximum volume of d-dimensional lattice polytopes with a fixed positive number of interior lattice points. Several researchers conjectured that the maximum is attained on a nice family of polytopes arising from the so-called Sylvester sequence. I will report on the recent progress on Hensley's conjecture. This is joint work with Benjamin Nill and Jan Krümpelmann. |
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Didier Henrion (LAAS-CNRS Univ Toulouse, FR and Czech Tech Univ Prague, CZ) | 22.11.17 | 16:30 | MA 621 | Convergence rates of moment-sum-of-squares hierarchies for volume approximation of semialgebraic sets |

Abstract: Moment-sum-of-squares hierarchies of semidefinite programs can be used to approximate the volume of a given compact basic semialgebraic set K. The idea consists of approximating from above the indicator function of K with a sequence of polynomials of increasing degree d, so that the integrals of these polynomials generate a convergence sequence of upper bounds on the volume of K. In this talk we show how we could derive an asymptotic rate of convergence for this approximation scheme. Joint work with Milan Korda, see arXiv:1609.02762. |
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Henning Seidler | 15.11.17 | 16:30 | MA621 | Circuit Polynomials for Simplex Newton Polytopes |

Abstract: Finding the minimum of a multivariate real polynomial is a well-known hard problem with various applications. We present an implementation to approximate such lower bounds via sums of non-negative circuit polynomials (SONCs). We provide a test-suite, where we compare our approach, using different solvers, with several solvers for sums os squares (SOS), including sostools and gloptipoly. It turns out that the circuit polynomials yield bounds competitive to SOS in several cases, but using much less time and memory. |
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Samuli Leppänen (Dalle Molle Institute for Artificial Intelligence Research) | 08.11.17 | 16:30 | MA621 | Some integrality gap results for the 0/1 SoS Hierarchy |

Abstract: In this talk we present the SoS hierarchy in the context of 0/1 optimization along with some of our recent integrality gap/lower bound results. We introduce the relevant concepts and give a brief derivation of the hierarchy. As for our results, we characterize the possible gap instances on the round n-1 of the hierarchy. Then we show an unbounded integrality gap after nonconstant number of rounds for a problem that admits a polynomial time algorithm. Finally we present a simplification of the positive semidefiniteness constraints when a high degree of symmetry can be assumed, and some applications to integrality gaps and lower Bounds. |
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Daryl Cooper | 13.09.17 | 16:30 | MA621 | Limits of Algebraic varieties: towards a continuous Nullstellansatz |

Abstract: The map that sends a polynomial ideal to an algebraic variety in complex affine space is not continuous with respect to the obvious topology on the domain. However it is continuous on the closure of the set of ideals generated by polynomials of degree at most d. Moreover the closure of this subspace is compact. This is work in progress, and is joint with Ricky Demer. |
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Dr. Iskander Aliev (Cardiff, Wales) | 05.09.17 | 16:30 | MA 406 | Sparse solutions to the systems of linear Diophantine equations |

Abstract: We present structural results on sparse nonnegative integer solutions to underdetermined systems of linear equations. |
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Marta Panizzut | 30.08.17 | 16:30 | MA 621 | The hunt for K3 polytopes |

Abstract: The connected components of the complement of a tropical hypersurface are called regions, and they are convex polyhedra. A smooth tropical quartic surface of genus one has exaclty one bounded region. We say that a 3- dimensional polytope is a K3 polytope if it arises as the bounded region of a smooth tropical quartic surface. In a joint project with Gabriele Balletti, we are investigating properties of K3 polytopes. In this talk I will report on the progress of this work. |
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Timo de Wolff | 23.08.17 | 16:30 | MA 621 | Intersections of Amoebas |

Abstract: |
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Georg Loho | 16.08.17 | 16:30 | MA 621 | Monomial Tropical Cones for Multicriteria Optimization |

Abstract: We introduce a special class of tropical cones which we call 'monomial tropical cones'. They arise as a crucial tool in the description of discrete multicriteria optimization problems. We employ them to derive an algorithm for computing all nondominated points. Furthermore, monomial tropical cones are intimately related with monomial ideals. We sketch directions for further work. This is based on joint work with Michael Joswig. |
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Johannes Hofscheier | 19.07.17 | 16:30 | MA 621 | A Combinatorial Smoothness Criterion for Spherical Varieties |

Abstract: In this talk we will present algorithms to check smoothness for the class of spherical varieties which contains those of toric varieties, flag varieties and symmetric varieties, and which forms a remarkable class of algebraic varieties with an action by an algebraic group having an open dense orbit. In the toric case there is a well-known simple combinatorial smoothness criterion whereas in the spherical case Brion, Camus and Gagliardi have shown smoothness criteria for spherical varieties which either are rather involved or rely on certain classification results. We suggest a purely combinatorial smoothness criterion by introducing a rational invariant which only depends on the combinatorics of the spherical variety. We have a conjectural inequality this invariant should satisfy where the equality case would imply that the spherical variety has a toric torus action. Our conjecture would also imply the generalized Mukai conjecture for spherical varieties. We complete our talk by summarizing in which cases the above mentioned approach is known to be true. This is joint work with Giuliano Gagliardi. |
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Sinai Robins (University of São Paulo) | 06.07.17 | 16:30 | MA 406 | The Ehrhart quasipolynomials of polytopes that come from trivalent graphs, and a discrete version of Hilbert's 3rd problem for the unimodular group |

Abstract: A graph all of whose nodes have degree \(1\) or \(3\) is called a \(\{1,3\}\)-graph. Liu and Osserman associated to each \(\{1, 3\}\)-graph a polytope. They studied these polytopes and their Ehrhart quasi-polynomials. We prove a conjecture of Liu and Osserman stating that the associated polytopes of all connected \(\{1,3\}\)-graphs with the same number of nodes and edges have the same Ehrhart quasi-polynomial. We also present some structural properties of these polytopes and we show a relation between them and the well-known metric polytopes. This study is related to a question of Hasse and McAllister on a discrete version of Hilbert's 3rd problem for the unimodular group. |
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Matthias Lenz | 28.06.17 | 16:30 | MA 621 | On powers of Plücker coordinates and representability of arithmetic matroids |

Abstract: Given \(k\in \mathbb{R}_{\ge 0}\) and a vector \(v\) of Plücker
coordinates of a point in the real Grassmannian, is the vector obtained by
taking the \(k\)th power of each entry of \(v\) again a vector of
Plücker coordinates? We will show that for \(k\neq 1\), this is true if
and only if the corresponding matroid is regular. Similar results hold over
other fields. We will also describe the subvariety of the Grassmannian that
consists of all the points that define a regular matroid. |
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Jean-Philippe Labbé / Michael Joswig | 21.06.17 | 16:30 | MA 621 | polymake/sage |

Abstract: Jean-Philippe will give an introduction to the new polymake interface in sage. Michael will report basic design principles underlying polymake. This is followed by a common coding session and beer. |
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Héctor Andrade Loarca | 14.06.17 | 16:30 | MA 621 | Fast Multidimensional Signal Processing with Shearlab.jl. |

Abstract: We live in the age of data, huge amount of Data its been generated and acquired everyday in different forms and flavors. One would like to have a technique to store and process optimally this data; this is possible since the relevant information in almost all data found in typical applications is sparse due the high correlation of its elements. The challenge will lie in the search of appropriate dictionary that can represent optimally this information. Along the history very talented scientist have proposed different signal transforms based on certain representation dictionaries, starting with the Fourier Transform and Short Time Fourier Transform; in 1980s the wavelet transform was proposed representing a breakthrough in one dimensional signal representation, with the ability to not just optimally represent the data but also capture certain singularities, regularities and other features; the flaw of the wavelets transforms is its lack of directional sensitivity which does not aloud it to represent optimally multidimensional singularities like curves that are predominant in general multidimensional signals (for instance images and videos). The Shearlet transform is a product of almost 10 years of research on sparsifying transform with directional sensitivity that attains the optimal best N-term approximation error, it was proposed by G. Kutyniok, D. Labate and K Guo in 2015 and since then it's been known to be the best of its type. |
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Hiroshi Hirai | 31.05.17 | 16:30 | MA 621 | Maximum vanishing subspace problem, CAT(0)-space relaxation, and block-triangularization of partitioned matrix |

Abstract: In this paper, we address the following algebraic generalization of the bipartite stable set problem. We are given a block-structured matrix (partitioned matrix) \(A=(A_{\alpha\beta})\), where \(A_{\alpha\beta}\) is an \(m_{\alpha}\) by \(n_{\beta}\) matrix over field F for \(\alpha=1,2,\ldots,\mu\) and \(\beta=1,2,\ldots,\nu\). The maximum vanishing subspace problem (MVSP) is to maximize \(\sum_{\alpha}\mbox{dim}X_{\alpha}+\sum_{\beta}\mbox{dim}X_{\beta}\) over vector subspaces \(X_{\alpha} \subseteq F^{m_{\alpha}}\) for \(\alpha=1,2,\ldots,\mu\) and \(Y_{\beta}\subseteq F^{n_{\beta}}\) for \(\beta=1,2,\ldots,\nu\) such that each \(A_{\alpha\beta}\) vanishes on \(X_{\alpha}\times Y_{\beta}\) when \(A_{\alpha\beta}\) is viewed as a bilinear form \(F^{m_{\alpha}}\times F^{n_{\beta}}\rightarrow F\). This problem arises from a study of a canonical block-triangular form of \(A\) by Ito, Iwata, and Murota (1994). We prove that MVSP can be solved in polynomial time. Our proof is a novel combination of submodular optimization on modular lattice and convex optimization on CAT(0)-space. We present implications of this result on block-triangularization of partitioned matrix. |
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Ted Bisztriczky (University of Calgary) | 31.05.17 | 16:30 | MA 406 | Erdos-Szekeres type theorems for planar convex sets |

Abstract: A family \(\mathcal{F}\) of sets is in convex position if none of its members is contained in the convex hull of the union of the others. The members of \(\mathcal{F}\) are ovals (compact convex sets) in the plane that have a certain property. An Erdos-Szekeres type theorem concerns the existence, for any integer \(n\geq 3\), of a smallest positive integer \(N(n)\) such that if \(|\mathcal{F}|≥N(n)\) then there are \(n\) ovals of \(\mathcal{F}\) in convex position. |
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Dimitrios Dais (University of Crete) | 29.05.17 | 17:00 | MA 406 | Toric log del Pezzo surfaces with a unique singularity |

Abstract: The toric log del Pezzo surfaces are constructed by means of the so-called LDP-lattice polygons. In the talk it will be explained how one classifies the subclass of surfaces of this kind which have a unique singularity. |
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Richard Gardner (Western Washington University) | 17.05.17 | 16:30 | MA 406 | Open problems in Geometric Tomography |

Abstract: This talk will focus on open problems in Geometric Tomography, which aims to retrieve information about a geometric object (such as a convex body, star body, finite set, etc.) from data concerning its intersections with planes or lines and/or projections (i.e., shadows) on planes or lines. The problems, which span nearly a hundred years of mathematics, are diverse. Many have a common thread, however, since they are linked to various integral transforms: the X-ray transform, divergent beam transform, circular Radon transform, cosine transform, or spherical Radon transform. |
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Michael Joswig | 03.05.17 | 16:30 | MA 621 | Computing Invariants of Simplicial Manifolds |

Abstract: This is a brief survey on algorithms for computing basic algebraic invariants such as homology, cup products, and intersection forms. |
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Romanos Malikiosis | 26.04.17 | 16:30 | MA 406 | Formal duality in finite cyclic groups |

Abstract: Numerical computations by Cohn, Kumar, and Schürmann in energy minimizing periodic configurations of density \(\rho\) and \(1/\rho\) revealed an impressive kind of symmetry, the so–called formal duality. Formal dual subsets of \(\mathbb{R}^n\) satisfy a generalized version of Poisson summation formula, and it is conjectured that the only periodic subsets of \(\mathbb{R}\) of density \(1\) possessing a formal dual subset, are \(\mathbb{Z}\) and \(2\mathbb{Z}\cup(2\mathbb{Z}+\frac12)\). |
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Manuel Radons | 12.04.17 | 16:30 | MA 621 | Piecewise linear methods in nonsmooth optimization |

Abstract: Several techniques in numerical analysis, e.g. Newton's methods and ODE solvers, are based on local linear approximations of a smooth function \(f: \mathbb R^n\rightarrow \mathbb R^n\). If \(f\) is piecewise smooth such approximations may be arbitrarily bad near nondifferentiabilites. Piecewise linear approximations -- or, briefer, piecewise linearizations -- restore the structural correspondence between approximation and underlying function. We give an overview of the generalized numerical methods based on piecewise linearizations and their properties, e.g. convergence results, as well as the techniques used in their investigation, which include degree theory for piecewise affine functions and nonsmooth analysis. |
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Philipp Jell | 05.04.17 | 16:30 | MA 621 | Differential forms on Berkovich spaces and their relation to tropical geometry |

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Takayuki Hibi (Osaka University) | 29.03.17 | 16:30 | MA 621 | Order Polytopes and Chain Polytopes |

Abstract: Given a finite partially ordered set P, we associate two poset polytopes, viz, the order polytope O(P) and the chain polytope C(P). In my talk, after reviewing fundamental materials on O(P) and C(P), the question when O(P) and C(P) are unimodularly equivalent will be discussed. Then, in the frame of Gröbner bases, various reflexive polytopes arising from O(P) and C(P) will be presented. Finally, some questions and conjectures will be summarized. No special knowledge will be required to understand my talk. |
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Lucía López de Medrano (UNAM Cuernavaca) | 22.03.17 | 17:00 | MA 621 | On the genus of tropical curves |

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Felipe Rincón (University of Oslo) | 22.03.17 | 16:15 | MA 621 | Tropical Ideals |

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Skip Jordan | 22.03.17 | 15:30 | MA 621 | Parallel Vertex and Facet Enumeration with mplrs |

Abstract: We introduce mplrs, a new parallel vertex/facet enumeration program based on the reverse-search code lrs. The implementation uses MPI and can be used on single machines, clusters and supercomputers. We describe the budgeted parallel tree search implemented in mplrs and compare performance with other sequential and parallel programs for vertex/facet enumeration. In some instances, mplrs achieves almost linear scaling with more than 1000 cores. The approach used in mplrs can be easily applied to various other problems. This is joint work with David Avis. |
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Alex Fink | 22.02.17 | 16:30 | MA 621 | The characteristic polynomial two ways |

Abstract: The theory of hyperplane arrangements and matroids derives great utility from the application of algebraic and algebro-geometric methods. The characteristic polynomial often appears in familiar guise in these methods, as an enumerator of the no broken circuit sets. The characteristic polynomial also appears, on the face of it unrelatedly, in the recent magisterial resolution of Rota's conjecture by Huh, Katz, and Adiprasito. In joint work with David Speyer and Alex Woo we explain how these two manifestations are related after all. |
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Akiyoshi Tsuchiya | 08.02.17 | 16:30 | MA 621 | Gorenstein simplices and the associated finite abelian groups |

Abstract: A lattice polytope is a convex polytope each of whose vertices has integer coordinates. It is known that a lattice simplex of dimension \(d\) corresponds to a finite abelian subgroup of \((\mathcal{R}/\mathcal{Z})^{d+1}\). Conversely, given a finite abelian subgroup of \((\mathcal{R}/\mathcal{Z})^{d+1}\) such that the sum of all entries of each element is an integer, we can obtain a lattice simplex of dimension \(d\). In this talk, we discuss a characterization of Gorenstein simplices in terms of the associated finite abelian groups. Gorenstein polytopes are of interest in combinatorial commutative algebra, mirror symmetry and tropical geometry. In particular, we present complete characterizations of Gorenstein simplices whose normalized volume equal \(p\), \(p^2\) and \(pq\), where \(p,q\) are prime integers. |
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Robert Löwe | 25.01.17 | 16:30 | MA 621 | Secondary fans of a punctured Riemann surface |

Abstract: The secondary fan of a finite point configuration in \(R^n\) stratifies the space of height functions by the combinatorial types of regular subdivisions. Now given a punctured Riemann surface, similar techniques yield a polyhedral fan whose cones correspond to horocyclic Delaunay tessellation in the sense of Penner's convex hull construction. The purpose of the talk is to give an introduction to this topic and the corresponding problems that arise naturally. |
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Eva Maria Feichtner (Universität Bremen) | 18.01.17 | 16:30 | MA 406 | A Leray model for Orlik-Solomon algebras |

Abstract: Although hyperplane arrangement complements are rationally formal,
we note that they have non-minimal rational (CDGA) models which are topologically and combinatorially significant. We construct a family of CDGAs which interpolates between the Orlik-Solomon algebra and the cohomology algebras of arrangement compactifications. Our construction is combinatorial and extends to all matroids, regardless of their (complex) realizability. |
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Benjamin Schröter | 04.01.17 | 16:30 | MA 621 | Fundamental Polytopes of Metric Trees via Hyperplane Arrangements |

Abstract: In this talk I will present a summary of the recent paper 'Fundamental Polytopes of Metric Trees via Hyperplane Arrangements' (arXiv:1612.05534) of Emanuele Delucchi and Linard Hossly. In this paper they develop a formula for the f-vector of the fundamental polytope of a finite metric space and its dual the Lipschitz polytope. |
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Georg Loho | 04.01.17 | 17:15 | MA 621 | Slicing and Dicing Polytopes |

Abstract: I present the recent work 'Slicing and dicing polytopes' (arXiv:1608.05372) by Patrik Norén on cellular resolutions. I introduce diced and sharp polytopes which are necessary for the construction and give a brief introduction to cellular resolutions. The main result, introducing a new class of polyhedral subdivisions which support cellular resolutions, is illustrated along some examples. |
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Marta Panizzut | 26.10.16 | 13:30 | MA 621 | Linear systems on metric graphs, gonality sequence and lifting problems |

Abstract: A combinatorial theory of linear systems on graphs and metric graphs has been introduced in analogy with the one on algebraic curves. The interplay is given by the Specialization Lemma. Let \(X\) be a smooth curve over the field of fractions of a complete discrete valuation ring and let \(\mathfrak{X}\) be a strongly semistable regular model of \(X\). It is possible to specialize a divisor on the curve to a divisor on the dual graph of the special fiber of \(\mathfrak{X}\); through this process the rank of the divisor can only increase. The complete graph \(K_d\) pops up if we take a model of a smooth plane curve of degree d degenerating to a union of d lines. Moreover omitting edges from \(K_d\) can be interpreted as resolving singularities of a plane curve. In this talk we present some results on linear systems on complete graphs and complete graphs with a small number of omitted edges, and we will compare them with the corresponding results on plane curves. In particular, we compute the gonality sequence of complete graphs and the gonality of graphs obtained by omitting edges. We explain how to lift these graphs to curves with the same gonality using models of plane curves with nodes and harmonic morphisms. This is partially a joint work with Filip Cools. |
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Fei Xue (BMS) | 19.10.16 | 16:30 | MA 406 | On Lattice Coverings by Simplices |

Abstract: By studying the volume of a generalized difference body, this talk presents the first nontrivial lower bound for the lattice covering density by \(n\)-dimensional simplices. |
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Ngoc Tran | 05.10.16 | 14:45 | MA 621 | Tropical geometry and mechanism design |

Abstract: How to ensure an outcome based on collective decisions is 'better for all' when a) everyone acts in their personal interest only, and b) we do not know the exact 'motives' of each person? Crudely, this is the central problem in mechanism design, a branch of economics. In this talk, I show how insights from tropical geometry can solve such problems. |
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Thomas Kahle | 05.10.16 | 13:30 | MA 621 | The geometry of rank-one tensor completion |

Abstract: tba |
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André Wagner | 28.09.16 | 16:30 | MA 621 | Veronesean almost binomial almost complete intersections |

Abstract: The second Veronese ideal I_n contains a natural complete intersection J_n generated by the principal 2-minors of a symmetric (n × n)-matrix. In this talk I show, how to determine the subintersections of the primary decomposition of J_n where one intersectand is omitted. These subintersections can be described via combinatorial method and yield interesting insights into binomial ideals. This talk is based on joint work with Thomas Kahle. |
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Laura Silverstein (TU Wien) | 27.07.16 | 16:30 | MA 406 | Tensor Valuations on Lattice Polytopes |

Abstract: A classification of symmetric tensor valuations on lattice polytopes in \(\mathbb{R}^n\) that intertwine the special linear group over the integers is established for the cases in which the rank of the tensor is at most \(n\). The scalar-valued case was classified by Betke and Kneser where it was shown that the only such valuations are the coefficients of the Ehrhart polynomial. Extending this result, the coefficients of the discrete moment tensor form a basis for the symmetric tensor valuations. |
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Apostolos Giannopoulos (University of Athens) | 20.07.16 | 16:30 | MA 406 | Inequalities about sections and projections of convex bodies |

Abstract: We discuss lower dimensional versions of the slicing problem and of the Busemann-Petty problem, both in the classical setting and in the generalized setting of arbitrary measures in place of volume. We introduce an alternative approach which is based on the generalized Blaschke-Petkantschin formula, on asymptotic estimates for the dual affine quermassintegrals and on some new Loomis-Whitney type inequalities in the spirit of the uniform cover inequality of Bollobas and Thomason. |
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Gerassimos Barbatis (University of Athens) | 13.07.16 | 16:30 | MA 406 | On the Hardy constant of some non-convex planar domains |

Abstract: The Hardy constant of a simply connected domain \(\Omega\subset\mathbb{R}^2\) is the best constant for the inequality \[ \int_\Omega |\nabla u|^2 dx \geq c\int_\Omega \frac{u^2}{\operatorname{dist}(x,\partial\Omega)^2} dx, \quad u\in C_c^\infty(\Omega). \] After the work of Ancona where the universal lower bound 1/16 was obtained, there has been a substantial interest on computing or estimating the Hardy constant of planar domains. In this talk we determine the Hardy constant of an arbitrary quadrilateral as well as of some other planar domains (joint work with Achilles Tertikas). |
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Matthew Trager | 15.06.16 | 16:30 | MA 621 | Geometric models for computer vision |

Abstract: The goal of computer vision is to recover real-world information, either semantic (e.g., object recognition) or geometric (3D reconstruction), from sets of photographs or videos. In a broad sense, my research aims at investigating the fundamental geometric models, both synthetic and analytic, that can be used to describe shapes, cameras, and image contours, in processes of visual inference. In this talk, I will present some properties of 'silhouettes' that are obtained as projections of objects in space. I will also discuss some ongoing work that recasts traditional multi-view vision in terms of the Grassmannian of lines in P^3. This is joint work with Jean-Charles Faugere, Xavier Goaoc, Martial Hebert, Jean Ponce, Mohab Safey El Din and Bernd Sturmfels. |
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Axel Flinth | 18.05.16 | 16:30 | MA 406 | High Dimensional Geometry in Compressed Sensing |

Abstract: The main aim of compressed sensing is to recover a low-dimensional object (e.g. a sparse vector, a low-rank matrix, ...) from few linear measurements. Many algorithms for this task (e.g. Basis Pursuit, nuclear norm-minimization) consists of minimizing a structure-promoting function \(f\) over the set of possible solutions. |
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Amanda Cameron | 04.05.16 | 16:30 | MA 621 | A lattice point counting generalisation of the Tutte polynomial |

Abstract: The Tutte polynomial for matroids is not directly applicable to polymatroids. For instance, deletion-contraction properties do not hold. We construct a polynomial for polymatroids which behaves similarly to the Tutte polynomial of a matroid, and in fact contains the same information as the Tutte polynomial when we restrict to matroids. This polynomial is constructed using lattice point counts in the Minkowski sum of the base polytope of a polymatroid and scaled copies of the standard simplex. We also show that, in the matroid case, our polynomial has coefficients of alternating sign, with a combinatorial interpretation closely tied to the Dawson partition. |
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Paul Breiding | 27.04.16 | 16:30 | MA 621 | The spectral Atheory of tensors |

Abstract: The spectral theory of tensors aims at generalizing the concept of eigenvalues and eigenvectors of matrices to higher dimensional arrays. In this talk I will focus on the definition of E-eigenvalues and Z-eigenvalues given by Qi. After giving the definition itself I will explain various applications, give the higher dimensional equivalent of the characteristic polynomial and talk about the number of eigenvalues of both real and complex tensors. The last part of the talk will be about an algorithm to compute eigenvalues of tensors using homotopy methods and the complexity of this algorithm. |
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Shiri Artstein-Avidan (Tel Aviv University) | 27.04.16 | 11:00 | MA 406 | On Godbersen’s conjecture and related inequalities |

Abstract: - |
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Stefan Weltge (Otto-von-Guericke Universität Magdeburg) | 20.04.16 | 16:30 | MA 406 | Tight bounds on discrete quantitative Helly numbers |

Abstract: Given a subset S of R^n, let c(S,k) be the smallest number t such that whenever finitely many convex sets have exactly k common points in S, there exist at most t of these sets that already have exactly k common points in S. For S = Z^n, this number was introduced by Aliev et al. [2014] who gave an explicit bound showing that c(Z^n,k) = O(k) holds for every fixed n. Recently, Chestnut et al. [2015] improved this to c(Z^n,k) = O(k (log log k)(log k)^{-1/3} ) and provided the lower bound c(Z^n,k) = Omega(k^{(n-1)/(n+1)}). We provide a combinatorial description of c(S,k) in terms of polytopes with vertices in S and use it to improve the previously known bounds as follows: We strengthen the bound of Aliev et al. [2014] by a constant factor and extend it to general discrete sets S. We close the gap for Z^n by showing that c(Z^n,k) = Theta(k^{(n-1)/(n+1)}) holds for every fixed n. Finally, we determine the exact values of c(Z^n,k) for all k <= 4. |
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Anna Lena Birkmeyer | 16.03.16 | 16:30 | MA 621 | Realizability of Tropical Hypersurfaces in Matroid Fans |

Abstract: In tropical geometry, an important aim is to understand which tropical varieties arise as tropicalizations of algebraic varieties. We investigate this question in a relative setting: Given a matroid fan F coming from a linear space W, we ask if a tropical hypersurface in F is the tropicalization of an algebraic subvariety of W. We present an algorithm able to decide for any tropical hypersurface in F if it is realizable in W. Moreover, we use this algorithmic approach to describe the structure of the realization space of a tropical hypersurface H in F, i.e. the space of algebraic subvarieties of W tropicalizing to H, and show that the space of all realizable tropical hypersurfaces in F is an abstract polyhedral set. |
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Martin Genzel | 02.03.16 | 16:30 | MA 406 | Convex Recovery of Structured Signals from Non-Linear Observations |

Abstract: In this talk, we study the problem of estimating a structured signal \(x_0\in\mathbb{R}\) linear Gaussian observations. Supposing that \(x_0\) belongs to a certain convex subset \(K\subset\mathbb{R}\) we will see that an accurate recovery is possible as long as the number of observations exceeds the effective dimension of \(K\), which is a common measure for the complexity of signal classes. Interestingly, it will turn out that the (possibly unknown) non-linearity of our model affects the error rate only by a multiplicative constant. This achievement is based on recent works by Yaniv Plan and Roman Vershynin, who have suggested to treat the non-linearity rather as noise which perturbs a linear measurement process. Using the concept of restricted strong convexity, we show that their results for the generalized Lasso can be extended to a fairly large class of convex loss functions. This is especially appealing for practical applications, since in many real-world scenarios, adapted loss functions empirically perform better than the classical square loss. To this end, the presented results provide a unified and general framework for signal reconstruction in high dimensions, covering various challenges from the fields of compressed sensing, signal processing, and statistical learning. |
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Mihalis Kolountzakis (University of Crete) | 03.02.16 | 16:30 | MA 406 | Periodicity for tilings and spectra |

Abstract: We will talk about periodicity (and structure, more generally) in the study of tilings by translation, where the tile is a set or a function in an Abelian group, and also in the study of spectra of sets (sets of characters which form an orthogonal basis for L^2 of the set). There are connections to harmonic analysis, number theory, combinatorics and computation, and these make this subject so fascinating. Starting from the Fuglede conjecture, now disproved in dimension at least 3, which would connect tilings with spectra, we will go over cases where periodicity always holds, cases where it is optional and cases where it's never true, in one dimension (most positive results) and higher dimension (most interesting questions). |
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Simon Hampe | 20.01.16 | 16:30 | MA 621 | Matroids over hyperfields |

Abstract: This talk is a summary of the very recent paper by Matthew Baker of the same title (http://arxiv.org/pdf/1601.01204.pdf). Various flavours of ''matroids with additional structure'', such as valuated and oriented matroids have been around for quite a while, with often very little connection between the different matroid species. Matroids over hyperfields are a generalization of all those concepts - thus allowing a unified treatment. In this talk I will first give a short introduction to hyperfields. I will then define what matroids over hyperfields are, give examples and discuss generalizations of familiar matroid operations such as duality and minors. |
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Silouanos Brazitikos (University of Athens) | 13.01.16 | 16:30 | MA 406 | Quantitative versions of Helly's theorem |

Abstract: We provide a new quantitative version of Helly's theorem: there exists an absolute constant\(\alpha >1\)with the following property: if\(\{P_i: i\in I\}\)is a finite family of convex bodies in\({\mathbb R}^n\)with\({\rm int}\left (\bigcap_{i\in I}P_i\right)\neq\emptyset\), then there exist\(z\in {\mathbb R}^n\),\(s\ls \alpha n\)and\(i_1,\ldots i_s\in I\)such that \begin{equation*} z+P_{i_1}\cap\cdots\cap P_{i_s}\subseteq cn^{3/2}\left(z+\bigcap_{i\in I}P_i\right), \end{equation*} where\(c>0\)is an absolute constant. This directly gives a version of the 'quantitative' volume and diameter theorem of > B\'{a}r\'{a}ny, Katchalski and Pach, with a polynomial dependence on the dimension. |
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Kristin Shaw | 06.01.16 | 16:30 | MA 621 | What is Étale cohomology? |

Abstract: Wished for by Weil and defined by Grothendieck, Étale cohomology is to varieties over fields of finite characteristic what usual cohomology is to complex or real algebraic varieties when they are equipped with the analytic, respectively Euclidean topology. This very short introduction will motivate and try to explain the basic ingredients of Étale cohomology. Throughout I will make analogies to the complex and real situations and stick to simple examples in hopes to provide some intuition into this deep theory. |
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Benjamin Schröter | 02.12.15 | 16:30 | MA 621 | The degree of a tropical basis |

Abstract: In this talk I will present my work with Michael Joswig. I will give an explicit bound on the degree of a tropical basis. Our result is derived from the algorithm of Bogart, Jensen, Speyer, Sturmfels and Thomas that computes a tropical variety via Groebner bases and saturation. Furthermore I will give examples that illustrate the difference between tropical and Groebner bases. |
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Bernardo González Merino (TU München) | 18.11.15 | 16:30 | MA 406 | On the Minkowski measure of symmetry |

Abstract: The Minkowski measure of symmetry s(K) of a convex body K, is the smallest positive dilatation of K containing a translate of -K. In this talk we will explain some of its basic properties in detail. |
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André Wagner | 04.11.15 | 16:30 | MA 621 | Introduction to singular value decomposition |

Abstract: In this short talk I will give a brief introduction to singular value decomposition, some of it's applications and efficient ways to compute it. |
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Jonathan Spreer | 28.10.15 | 16:30 | MA 621 | Algorithms and complexity for Turaev-Viro invariants |

Abstract: The Turaev-Viro invariants are a powerful family of topological invariants for distinguishing between different 3-manifolds. They are invaluable for mathematical software, but current algorithms to compute them require exponential time. I will discuss this family of invariants, and present an explicit fixed-parameter tractable algorithm for arbitrary r which is practical -- and indeed preferable -- to the prior state of the art for real computation. |
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Alexander Gamkrelidze | 11.08.15 | 15:00 | MA 621 | Algorithms for Convex Topology: Computation of Hom-Complexes |

Abstract: After a brief introduction to the field of polytopal complexes and affine maps between them, we define the set Hom(P,Q) of all possible affine mappings between two polytopal complexes P and Q that builds a polytopal complex in a higher dimensional space. There are efficient software packages like Polymake that deal (among other things) with the computation of Hom-complexes between two polytopes, but no complexes. In this talk, we introduce algorithms that compute the above mentioned structures and their homotopies. At the end of the talk we introduce a conjectured geometric analog to a well known formula Hom(P,ker(f))=kerHom(P,f): Hom(P,\(\Sigma\)(f))=\(\Sigma\) Hom(P,f) and discuss some ideas to compute the Sigma operator and the possible ways to prove the equation. In case the above conjecture holds, the wide area of the algebraic theory of polytopes can be established. |

# Vorträge

Name | Datum | Uhrzeit | Ort | Titel |
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Michael Joswig | 19.10.15 | 14:15 | MA 041 | Smooth Fano Polytopes With Many Vertices |

# Konferenzen/Workshops

Titel | Datum | Ort | Homepage |
---|---|---|---|

8th polymake workshop | February 2-4, 2017 | TU Berlin | 8th polymake workshop |

7th polymake workshop | January 28-30, 2016 | TU Berlin | 7th polymake workshop |

Meeting on Algebraic Vision 2015 | October 8-9, 2015 | TU Berlin | MAV |

Tropical geometry in Europe | March 30-31, 2015 | Berlin | TGE |

6th polymake workshop | December 5,2014 | TU Berlin | 6th polymake workshop |

5th polymake workshop | March 28, 2014 | TU Berlin | 5th polymake workshop |

4th polymake conference and developer meeting at TU Berlin | November 1-2, 2013 | TU Berlin | 4th polymake conference and developer meeting |

Delaunay Geometrie: Polytopes, Triangulations and Spheres | October 7-9,2013 | FU Berlin | Second ERC "SDModels" Workshop |