Name | Datum | Uhrzeit | Ort | Titel |
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TBD | 26.07.23 | 16:30 | MA 750 | TBD |

Abstract: TBD |
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Sayan Mukherjee (MPI-MiS) | 19.07.23 | 16:30 | MA 750 | Modeling shapes and surfaces: Geometry meets machine learning |

Abstract: We will consider modeling shapes and fields via topological and lifted-topological transforms. Specifically, we show how the Euler Characteristic Transform and the Lifted Euler Characteristic Transform can be used in practice for statistical analysis of shape and field data. We also state a moduli space of shapes for which we can provide a complexity metric for the shapes. We also provide a sheaf theoretic construction of shape space that does not require diffeomorphisms or correspondence. A direct result of this sheaf theoretic construction is that in three dimensions for meshes, 0-dimensional homology is enough to characterize the shape. We will also discuss Gaussian processes on fiber bundles and applications to evolutionary questions about shapes. Applications in biomedical imaging and evolutionary anthropology will be stated throughout the talk. |
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Stephanie Mui (NYU) | 05.07.23 | 16:30 | MA 750 | TBD |

Abstract: TBD |
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Theo Grundhöfer (U Würzburg) | 21.06.23 | 16:30 | MA 750 | Groups and nearfields |

Abstract: Nearfields are easy to define: just omit one of the two distributive laws in the definition of skew fields. We explain the connection of nearfields to sharply 2-transitive permutation groups. Every nearfield has an intrinsic vector space structure, hence one can study nearfields in terms of groups of linear transformations. All finite nearfields have been classified by Zassenhaus in 1936. We report on classification results for infinite nearfields, and we construct some `wild´ nearfields which are far away from skew fields. |
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Jacob Matherne (Universität Bonn, MPI) | 07.06.23 | 16:30 | MA 750 | Polynomials in combinatorics and representation theory |

Abstract: Many polynomials in combinatorics (and in other areas of mathematics) have nice properties such as having all of their roots being real numbers, or having all of their coefficients being nonnegative. By surveying recent advances in the Hodge theory of matroids (namely, the nonnegativity of Kazhdan-Lusztig polynomials of matroids and Dowling and Wilson's top-heavy conjecture for the lattice of flats of a matroid), I will give several examples of well-behaved polynomials, and I will indicate some connections of these properties to geometry and representation theory. The talk should be understandable to everyone, and should appeal to those with interests in at least one of the following topics: hyperplane arrangements, matroids, log-concavity, real-rooted polynomials, lattice theory, Coxeter groups, and the representation theory of Lie algebras. It will contain results that are joint work with Tom Braden, Luis Ferroni, June Huh, Nicholas Proudfoot, Matthew Stevens, Lorenzo Vecchi, and Botong Wang. |
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Martin Winter (Warwick) | 31.05.23 | 16:30 | MA 750 | Rigidity, Tensegrity and Reconstruction of Polytopes under Metric Constraints |

Abstract: In how far is it possible to reconstruct a convex polytope (up to isometry, affine equivalence or combinatorial equivalence) from its edge-graph and some 'graph-compatible metric data', such as its edge-lengths? In this talk we explore this question and pose the following conjecture: a convex polytope P is uniquely determined by its edge-graph, its edge length and the distance of each vertex from some common interior point of P, across all dimensions and combinatorial types. We conjecture even stronger, that two polytopes P and Q are already isometric if they have the same edge-graph, edges in P are at most as long as in Q, and vertex-point distances in P are at least as long as in Q. In other word, a convex polytope cannot become 'bigger' while its edges become shorter. We verify the conjectures in three special cases: if P and Q are combinatorically equivalent, if P and Q are centrally symmetric, and if Q is a slight perturbation of P. These results were obtained using techniques from the intersection of convex geometry and spectral graph theory. |
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Kris Shaw (University of Oslo) | 24.05.23 | 16:30 | MA 750 | Real phase structures on matroid fans and matroid orientations |

Abstract: In this talk, I will propose a first definition of real structures on polyhedral complexes. I’ll explain that in the case of matroid fans, specifying such a structure is cryptomorphic to providing an orientation of the underlying matroid. Real phase structures are used to patchwork the real part of a tropical variety in any codimension and this gives a closed chain in the real part of a toric variety. This provides a link between oriented matroids and real toric varieties and an obstruction to the orientability of matroids via the homology groups of toric varieties. This is based on joint work with Johannes Rau and Arthur Renaudineau. |
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Aryaman Jal (KTH) | 17.05.23 | 16:30 | MA 750 | Polyhedral geometry of bisectors and bisection fans |

Abstract: Every symmetric convex body induces a norm on its affine hull. The object of our study is the bisector of two points with respect to this norm. A topological description of bisectors is known in the 2 and 3-dimensional cases and recent work of Criado, Joswig and Santos (2022) expanded this to a fuller characterisation of the geometric, combinatorial and topological properties of the bisector. A key object introduced was the bisection fan of a polytope which they were able to explicitly describe in the case of the tropical norm. We discuss the bisector as a polyhedral complex, introduce the notion of bisection cones and describe the bisection fan corresponding to other polyhedral norms. This is joint work with Katharina Jochemko. |
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Andreas Löhne (Universität Jena) | 10.05.23 | 16:30 | MA 750 | Stable algorithms for the 3-dimensional vertex enumeration problem |

Abstract: Vertex enumeration is the fundamental computational problem to determine the vertices of a convex polytope given by affine inequalities. The converse problem, called convex hull problem, is equivalent to polarity. |
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Leonid Monin (EPFL) | 19.04.23 | 16:30 | MA 750 | Combinatorics of toric bundles |

Abstract: Toric bundles are fibre bundles which have a toric variety as a fiber. One particularly studied class of toric bundles are horospherical varieties which are toric bundles over generalized flag varieties. Similar to toric varieties, toric bundles admit a combinatorial description via polyhedral geometry. In my talk, I will explain such a combinatorial description, and describe a couple of results which rely on it. In particular, I will present a generalization of the BKK theorem and the Fano criterion for toric bundles. |
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Frank Sottile (Texas A&M University) | 12.04.23 | 16:30 | TBD | Galois groups in Enumerative Geometry and Applications |

Abstract: In 1870 Jordan explained how Galois theory can be applied to problems from enumerative geometry, with the group encoding intrinsic structure of the problem. Earlier Hermite showed the equivalence of Galois groups with geometric monodromy groups, and in 1979 Harris initiated the modern study of Galois groups of enumerative problems. He posited that a Galois group should be `as large as possible' in that it will be the largest group preserving internal symmetry in the geometric problem. |
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Jeffrey Giansiracusa (Durham University) | 05.04.23 | 16:30 | TBD | An E-infinity structure on the matroid Grassmannian |

Abstract: The matroid Grassmannian is the space of oriented matroids. 30 years ago MacPherson showed us that understanding the homotopy type of this space can have significant implications in manifold topology. In some easy cases, the matroid Grassmannian is homotopy equivalent to the ordinary real Grassmannian, but in most cases we have no idea whether or not they are equivalent. This is known as MacPherson's conjecture. I'll show that one of the important homotopical structures of the ordinary Grassmannians has an analogue on the matroid Grassmannian, namely the monoidal product tof direct sum that is commutative up to all higher homotopies. |
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ArXiv seminar | 22.03.23 | 16:30 | MA 621 | ArXiv seminar |

Abstract: ArXiv seminar |
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SFB retreat | 01.03.23 | -- | Bad Münster am Stein | SFB retreat |

Abstract: |
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Vadim Semenov (NYU) | 22.02.23 | 16:30 | Online | The Gauss Image Problem |

Abstract: The Gauss Image problem is a generalization to the question originally posed by Aleksandrov who studied the existence of the convex body with the prescribed Aleksandrov's integral curvature. A simple discrete case of the Gauss Image Problem can be formulated as follows: given a finite set of directions in Euclidian space and the same number of unit vectors, does there exist a convex polytope in this space containing the origin in its interior with vertices at given directions such that each normal cone at the vertex contains exactly one of the given vectors. In this talk, we are going to discuss the discrete Gauss Image Problem, and its relation to other Minkowski-type problems. Two different proofs of the problem are going to be addressed: A smooth proof based on transportation polytopes and a discrete proof based on Helly’s theorem. Time permitting, we will also discuss the uniqueness statement for the problem. |
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Dustin Ross (San Francisco State University) | 15.02.23 | 16:30 | Online | Putting the “volume” back in “volume polynomials” |

Abstract: Recent developments in tropical geometry and matroid theory have led to the study of “volume polynomials” associated to tropical varieties, the coefficients of which record all possible degrees of top powers of tropical divisors. In this talk, I’ll discuss a volume-theoretic interpretation of volume polynomials of tropical fans; namely, they measure volumes of polyhedral complexes obtained by truncating the tropical fan with normal hyperplanes. I’ll also discuss how this volume-theoretic interpretation inspires a general framework for studying an analogue of the Alexandrov-Fenchel inequalities for degrees of divisors on tropical fans. Parts of this work are joint with Anastasia Nathanson, Lauren Nowak, and Patrick O’Melveny. |
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Iskander Aliev (Cardiff University) | 08.02.23 | 16:30 | MA 850 | Sparse integer points in rational polyhedra: bounds for the integer Carathéodory rank |

Abstract: We will give an overview of the recent results on sparse integer points (that is the integer points with relatively large number of zero coordinates) in the rational polyhedra of the form {x: Ax=b, x>=0}, where A is an integer matrix and b is an integer vector. In particular, we will discuss the bounds on the Integer Caratheodory rank in various settings and proximity/sparsity transference results. |
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Igor Makhlin (Skoltech) | 01.02.23 | 16:30 | Online | Poset polytopes and toric degenerations |

Abstract: In the 1980s Stanley introduced the order polytope and chain polytope associated with a poset while Hibi considered the closely related notions of the Hibi ideal and Hibi ring (as they are now known) associated with a distributive lattice. During the subsequent decades it was realized that these concepts can be applied to the construction and study of toric degenerations of Grassmannians and flag varieties. Moreover, virtually all sufficiently explicit and sufficiently general constructions of such degenerations known today can be interpreted in terms of poset polytopes and Hibi ideals. I will give an overview of these notions and explain how they lead to toric degenerations, reviewing some foundational and some very recent results in this direction. |
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Adam Afandi (University of Münster) | 25.01.23 | 16:30 | MA 850 | An Ehrhart Theory For Tautological Intersection Numbers |

Abstract: The tautological intersection theory of the moduli space of stable pointed curves is an active field of research in modern algebraic geometry. In this talk, I'll explain how Ehrhart theory can give a novel perspective on tautological intersection numbers. In particular, one can arrange tautological intersection numbers into families of evaluations of Ehrhart polynomials of partial polytopal complexes. The proof of this result relies on a theorem of Breuer, which classifies the Ehrhart polynomials of partial polytopal complexes. Time permitting, I will also discuss my current attempts to generalize this Ehrhart phenomenon. |
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Emre Sertöz (Leibniz University of Hannover) | 18.01.23 | 16:30 | MA 850 | Limit periods, arithmetic, and combinatorics |

Abstract: With Spencer Bloch and Robin de Jong, we recently proved that in a nodal degeneration of smooth curves, the periods of the resulting limit mixed Hodge structure (LMHS) contain arithmetic information. More precisely, if the nodal fiber is identified with a smooth curve C glued at two points p and q then the LMHS relates to the Neron--Tate height of p-q in the Jacobian of C. In making this relation precise, we observed that a "tropical correction term" is required that is based on the degenerate fiber. In this talk, I will explain this circle of ideas with the goal of arriving at the tropical correction term. |
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Catherine Babecki (University of Washington) | 11.01.23 | 16:30 | Online | Structure and Complexity of Graphical Designs for Weighted Graphs through Eigenpolytopes |

Abstract: We extend the theory of graphical designs, which are quadrature rules for graphs, to positively weighted graphs. Through Gale duality for polytopes, we show that there is a bijection between graphical designs and the faces of eigenpolytopes associated to the graph. This bijection proves the existence of graphical designs with positive quadrature weights, and upper bounds the size of a graphical design. We further show that any combinatorial polytope appears as the eigenpolytope of a positively weighted graph. Through this universality, we establish two complexity results for graphical designs: it is strongly NP-complete to determine if there is a graphical design smaller than the mentioned upper bound, and it is #P-complete to count the number of minimal graphical designs. Joint work with David Shiroma. https://arxiv.org/abs/2209.06349. |
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Zafeirakis Zafeirakopoulos (Gebze Technical University) | 30.11.22 | 16:30 | MA 850 | Polyhedral Omega (+ applications) |

Abstract: Polyhedral Omega is an algorithm for solving linear Diophantine systems (LDS), i.e., for computing a multivariate rational function representation of the set of all non-negative integer solutions to a system of linear equations and inequalities. Polyhedral Omega combines methods from partition analysis with methods from polyhedral geometry. In particular, we combine MacMahon’s iterative approach based on the Omega operator and explicit formulas for its evaluation with geometric tools such as Brion decomposition and Barvinok’s short rational function representations. This synthesis of ideas makes Polyhedral Omega by far the simplest algorithm for solving linear Diophantine systems available to date. After presenting the algorithm, we will see some applications and generalizations. |
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Katherina von Dichter (TU München / Cottbus) | 23.11.22 | 16:30 | MA 850 | Mean inequalities for symmetrizations of convex sets |

Abstract: The arithmetic-harmonic mean inequality can be generalized for convex sets, considering the intersection, the harmonic and the arithmetic mean, as well as the convex hull of two convex sets. We study those relations of symmetrization of convex sets, i.e., dealing with the means of some convex set C and -C. We determine the dilatation factors, depending on the asymmetry of C, to reverse the containments between any of those symmetrizations, and tighten the relations proven by Firey and show a stability result concerning those factors near the simplex. |
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Omid Amini (École Polytechnique) | 16.11.22 | 16:30 | MA 850 | Polyhedral geometry in higher rank and non-linear settings |

Abstract: New types of polyhedral geometry and combinatorial structures emerge in the study of asymptotic geometry of complex manifolds. This includes a multi-scale version of polyhedral geometry, with links to a theory of higher rank combinatorial structures, which combine parts living in different scales at infinity, and a theory of convexity in a piecewise linear setting. |
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MaRDI Annual Meeting | 09.11.22 | --- | --- | --- |

Abstract: |
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Raman Sanyal (Goethe-Universität Frankfurt) | 02.11.22 | 16:30 | MA 850 | Monotone paths on matroids, pivot rules, and flag polymatroids |

Abstract: The well-known greedy algorithm on matroids was adapted by Edmonds to polymatroid polytopes or, essentially equivalent, generalized permutahedra. For a given objective function, the greedy algorithm traces a monotone path in the graph of the polymatroid polytope. The starting point of this work is the observation that this path is precisely the path taken by the shadow-vertex simplex algorithm. It turns out that all monotone paths are greedy paths and all are coherent in the sense of Billera-Sturmfels. Thus, monotone paths are parametrized by the associated monotone path polytope. We show that these monotone path polytope are again a generalized permutahedron and in the case of matroids are the underlying flag matroids. In this talk I will explain the geometry of these “flag polymatroids” and the connection to certain nestohedra associated to lattices of flats via so-called pivot rule polytopes. If time permits, I will also discuss relations to sorting networks and the enumeration of Young tableaux. This is joint work with Alex Black. |
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Manuel Radons (TU Berlin) | 26.10.22 | 14:00 | MA 313/314 | Ph.D. Defense |

Abstract: |
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Stephane Gaubert (INRIA / CMAP École Polytechnique) | 26.10.22 | 11:00 | MA 313/314 | Ambitropical convexity, injective hulls of metric spaces, and mean-payoff games |

Abstract: Hyperconvexity was introduced by Aronszajn and Panichpadki in the 50', to study nonexpansive mappings between metric spaces. We study a new kind of convexity, defined in terms of lattice properties. We call it ``ambitropical'' as it includes both tropical convexity and its dual, and we relate it with hyperconvexity and mean-payoff games. |
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Leonid Monin (MPI MiS Leipzig) | 19.10.22 | 16:30 | MA 850 | Polyhedral models for K-theory |

Abstract: One can associate a commutative, graded algebra which satisfies Poincare duality to a homogeneous polynomial f on a vector space V. One particularly interesting example of this construction is when f is the volume polynomial on a suitable space of (virtual) polytopes. In this case the algebra A_f recovers cohomology rings of toric or flag varieties. |
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Yulia Alexandr (UC Berlekey) | 12.10.22 | 17:00 | Online | Logarithmic Voronoi cells |

Abstract: Logarithmic Voronoi cells are convex sets, which arise as fibers of the maximum likelihood estimation map. For discrete models, they live inside their log-normal polytopes, and for certain families of statistical models, the two sets coincide. In particular, logarithmic Voronoi cells for such families are polytopes. I will introduce these notions and investigate when this property holds. I will also talk about how this theory generalizes to Gaussian models, with lots of examples. This talk is based on joint works with Alex Heaton and Serkan Hosten. |
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Matthias Schymura (BTU Cottbus-Senftenberg) | 05.10.22 | 16:30 | MA 141 | On the maximal number of columns of a Delta-modular integer matrix |

Abstract: We are interested in a combinatorial question related to the recent and ongoing interest in understanding the complexity of integer linear programming with bounded subdeterminants. Given a number Delta and a full-rank integer matrix A with m rows such that the absolute value of every m-by-m minor of A is bounded by Delta, at most how many pairwise distinct columns can A have? The case Delta = 1 is the classical result of Heller (1957) saying that the maximal number of pairwise distinct columns of a totally unimodular integer matrix with m rows equals m^2 + m + 1. If m >= 3 and Delta >= 3, the precise answer to this combinatorial question is not known, but bounds of order O(Delta^2 m^2) have been proven by Lee et al. (2021). In the talk, I will discuss our approach to the problem which rests on counting columns of A by residue classes of a suitably defined integer lattice. As a result, we obtain the first bound that is linear in Delta and polynomial (of low degree) in the dimension m. Moreover, I explain how one can approach the corresponding classification problem and how this results in the construction of counterexamples to the previously conjectured value of the maximum above. The talk is based on a joint work with Gennadiy Averkov, see https://arxiv.org/abs/2111.06294. |
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Jesús A. De Loera (UC Davis) | 28.09.22 | 16:30 | MA 144 | Open problems arising from trying to understand the Simplex Method |

Abstract: The simplex method is one of the most famous and popular
algorithms in optimization, it was even
named on the top 10 algorithms of the 20th century by SIAM and IEEE. But
despite its great success and fame
we do not fully understand it. There are several natural open questions
arising from the analysis of its performance
This talk highlights several fascinating geometric and combinatorial
problems we wish to answer. Many open questions will be available for the eager
young researcher. I promise! |
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SFB/TRR 195 meeting | 20.09.22 | --- | --- | --- |

Abstract: |
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DMV annual meeting | 13.09.22 | --- | --- | --- |

Abstract: |
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Geometry meets combinatorics in Bielefeld | 06.09.22 | --- | --- | --- |

Abstract: |
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Benjamin Schröter (KTH) | 24.08.22 | 16:30 | H 1029 | Valuative Invariants for Matroids |

Abstract: Valuations on polytopes are maps that combine the geometry of polytopes with relations in a group. It turns out that many important invariants of matroids are valuative on the collection of matroid base polytopes, e.g., the Tutte polynomial and its specializations or the Hilbert–Poincaré series of the Chow ring of a matroid. In this talk I will present a framework that allows us to compute such invariants on large classes of matroids, e.g., sparse paving and elementary split matroids, explicitly. The concept of split matroids introduced by Joswig and myself is relatively new. However, this class appears naturally in this context. Moreover, (sparse) paving matroids are split. I will demonstrate the framework by looking at Ehrhart polynomials and Hilbert–Poincaré series of elementary split matroids. This talk is based on the preprint `Valuative invariants for large classes of matroids' which is joint work with Luis Ferroni. |
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Anton Dochtermann (Texas State University | 20.07.22 | 16:30 | MA 141 | Root polytopes, tropical types, and toric edge ideals |

Abstract: We consider arrangements of tropical hyperplanes, where the apices of the hyperplanes can be `taken to infinity' in certain directions. Such an arrangement defines a decomposition of Euclidean space where a cell is defined by its `type' data, analogous to the covectors of an oriented matroid. By work of Develin-Sturmfels and Fink-Rincon it is known that these `tropical complexes' are dual to (regular) subdivisions of root polytopes, which in turn are in bijection with mixed subdivisions of certain generalized permutohedra. Extending previous work with Joswig-Sanyal, we show how a natural monomial labeling of these complexes describes polynomial relations (syzygies) among `type ideals' which arise naturally from the combinatorial data of the arrangement. In particular, we show that the cotype ideal is Alexander dual to a squarefree initial ideal of the lattice ideal of a root polytope corresponding to the Stanley-Reisner ideal of a regular triangulation of the root polytope. This in turn leads to novel ways of studying algebraic properties of various monomial and lattice ideals. For instance, our methods of studying the dimension of a tropical complex provide new bounds on homological invariants of toric edge ideals of bipartite graphs, which have been extensively studied in the commutative algebra community. This is joint work with Ayah Almousa and Ben Smith. |
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Büsra Sert | 13.07.22 | 16:30 | MA 141 | The Half-Plane Property of Matroids, Related Concepts, and an Algorithm |

Abstract: Studies on homogeneous polynomials with the half-plane property were initially
motivated by the physical theory of electrical networks. They later became of interest
in convex optimization. In particular, a homogeneous polynomial with the half-plane
property is hyperbolic with respect to every point in the positive orthant, having an
associated hyperbolicity cone. Feasible sets of semi-definite programming, i.e.,
spectrahedral cones, are hyperbolicity cones for some polynomials. For the converse, the
generalized Lax conjecture states that every hyperbolicity cone is spectrahedral. |
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Matthias Beck (SFSU) | 22.06.22 | 16:30 | MA 141 | \(f^*\)- and \(h^*\)-vectors |

Abstract: If \(P\) is a lattice polytope (i.e., \(P\) is the convex hull of finitely many integer points in \({\bf R}^d\)), Ehrhart's theorem asserts that the integer-point counting function \(L_P(m) = \#(mP \cap {\bf Z}^d)\) is a polynomial in the integer variable $m$. Our goal is to study structural properties of Ehrhart polynomials---essentiallly asking variants of the (way too hard) question |
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Julian Pfeifle (UPC) | 15.06.22 | 16:30 | Online | Coming soon to polymake: (sometimes) disproving convex realizability of spheres |

Abstract: Every so often, we find a family of simplicial spheres with interesting
combinatorial properties, and would like to know if they are realizable as
boundaries of convex polytopes. For instance, this happened recently to
Zheng; to Novik and Zheng; and to Criado and Santos. |
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CG Week | 08.06.22 | FU Berlin | CG Week | |

Abstract: |
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Enis Kaya (MPI MiS Leipzig) | 01.06.22 | 16:30 | MA 141 | Determining reduction types of Picard curves via tropical invariants |

Abstract: For a separable binary form of degree n over a complete non-archimedean field, there is a canonical metric tree on n leaves associated to it. Furthermore, for every degree n, there are only finitely many tree types, which gives rise to a partition of the space of all non-archimedean binary forms of degree n. |
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Zoe Wellner (Carnegie Mellon University) | 25.05.22 | 16:30 | MA 141 | Colorful Borsuk-Ulam Results, Their Generalizations and Applications |

Abstract: In combinatorics and discrete geometry there are many colorful and rainbow results. Additionally topological tools have proven to be useful in generating interesting combinatorial results. In this line of approach we prove a colorful generalization of the Borsuk–Ulam theorem. We give a short proof of Ky Fan’s Lemma and generalize it from involutions to larger symmetry groups Z/p for prime p. We also present colorful generalizations of these nonexistence results for equivariant maps. As consequences we derive a colorful generalization of the Ham–Sandwich theorem and the necklace splitting theorem among other applications of Borsuk-Ulam. This is joint work with Florian Frick. |
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Niels Lindner (ZIB) | 18.05.22 | 16:30 | MA 141 | On the tropical and zonotopal geometry of periodic timetabling |

Abstract: The timetable is the core of every public transportation system. The standard mathematical tool for optimizing periodic timetabling problems is the Periodic Event Scheduling Problem (PESP). A solution to PESP consists of three parts: a periodic timetable, a periodic tension, and integer periodic offset values. While the space of periodic tension has received much attention in the past, we explore geometric properties of the other two components, establishing novel connections between periodic timetabling and discrete geometry. Firstly, we study the space of feasible periodic timetables, and decompose it into polytropes, i.e., polytopes that are convex both classically and in the sense of tropical geometry. We then study this decomposition and use it to outline a new heuristic for PESP, based on the tropical neighbourhood of the polytropes. Secondly, we recognize that the space of fractional cycle offsets is in fact a zonotope. We relate its zonotopal tilings back to the hyperrectangle of fractional periodic tensions and to the tropical neighbourhood of the periodic timetable space. Finally, we also use this new understanding to give tight lower bounds on the minimum width of an integral cycle basis in terms of the number of spanning trees. |
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Alex Elzenaar (MPI MiS Leipzig) | 04.05.22 | 16:30 | MA 141 | Aspects of computation in hyperbolic geometry |

Abstract: Low-dimensional topology has the advantage that it is relatively easy to visualise-the key word being relatively. Some people have amazing powers of visualisation (famously, William Thurston); but most of us need to rely on visual aids to try to understand what is going on. There is a long history of computer experimentation in the area of hyperbolic geometry and Kleinian groups (groups which arise dually to hyperbolic 3-manifolds), from the Thurston days (Robert Riley's work on knot groups in the 1970s and Jeff Weeks' study of triangulations of 3-manifolds in the 1980s) to the modern era (including but not limited to work by David Mumford, Caroline Series, and David Wright; Masaaki Wada; and Sabetta Matsumoto and Henry Segerman). This talk will discuss some of the highlights of computation in hyperbolic geometry with an emphasis on visualisation, focusing on groups and manifolds related to two-bridge knots (parameterised by the so-called Riley slice). |
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Thomas Blomme (University of Geneva) | 27.04.22 | 16:30 | Online | Enumeration of tropical curves in abelian surfaces |

Abstract: Tropical geometry is a powerful tool that allows one to compute enumerative algebraic invariants through the use of some correspondence theorem, transforming an algebraic problem into a combinatorial problem. Moreover, the tropical approach also allows one to twist definitions to introduce mysterious refined invariants, obtained by counting tropical curves with polynomial multiplicities. So far, this correspondence has mainly been implemented in toric varieties. In this talk we will study enumeration of curves in abelian surfaces and use the tropical geometry approach to prove a multiple cover formula that enables an simple and elegant computation of enumerative invariants of abelian surfaces. |
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Michael Joswig (TU Berlin) | 20.04.22 | 16:30 | Online | Lattice polygons and real roots |

Abstract: It is known from theorems of Bernstein, Kushnirenko and Khovanskii from the 1970s that the number of complex solutions of a system of multivariate polynomial equations can be expressed in terms of subdivisions of the Newton polytopes of the polynomials. For very special systems of polynomials Soprunova and Sottile (2006) found an analogue for the number of real solutions. In joint work with Ziegler we could give a simple combinatorial formula and an elementary proof for the signature of foldable triangulation of a lattice polygon. Via the Soprunova-Sottile result this translates into lower bounds for the number of real roots of certain bivariate polynomial systems. |
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PhD Students of DM/G (TU Berlin) | 13.04.22 | 16:30 | Online | Scientific Writing |

Abstract: |
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Marie Brandenburg (MPI MiS Leipzig) | 16.03.22 | 16:30 | Online | Competitive Equilibrium and Lattice Polytopes |

Abstract: The question of existence of a competitive equilibrium is a game theoretic question in economics. It can be posed as follows: In a given auction, can we make an offer to all bidders, such that no bidder has an incentive to decline our offer? We consider a mathematical model of this question, in which an auction is modelled as weights on a simple graph. The existence of an equilibrium can then be translated to a condition on certain lattice points in a lattice polytope. In this talk, we discuss this translation to the polyhedral language. Using polyhedral methods, we show that in the case of the complete graph a competitive equilibrium is indeed guaranteed to exist. This is based on joint work with Christian Haase and Ngoc Mai Tran. |
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Lorenzo Venturello (KTH) | 09.03.22 | 16:30 | Online | On the gamma-vector of symmetric edge polytopes |

Abstract: Symmetric edge polytopes (SEPs) are special lattice polytopes constructed from a simple graph. They play a role in physics, in the study of metric spaces and optimal transport. We study a numerical invariant of symmetric edge polytopes, called the gamma-vector, whose entries are linear combinations of the \(h^*\) numbers. In particular, we target a conjecture of Ohsugi and Tsuchiya which predicts nonnegativity of the gamma-vector of every SEP. In a joint work with Alessio D'Alì, Martina Juhnke-Kubitzke and Daniel Köhne, we prove nonnegativity for one of the entries of the gamma-vector, and descriube the graphs whose SEP attains equality. Moreover, we consider random graphs in the Erdős–Rényi model, and we obtain asymptotic nonnegativity of the whole gamma-vector in certain regimes. |
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Lei Xue (University of Washington) | 16.02.22 | 16:30 | Online | A Proof of Grünbaum’s Lower Bound Conjecture for polytopes, lattices, and strongly regular pseudomanifolds |

Abstract: In 1967, Grünbaum conjectured that any \(d\)-dimensional polytope with \(d +s \leq 2d\) vertices has at least \( \phi_k (d + s, d) = {d +1 \choose k + 1} + { d \choose k + 1} - { d + 1 - s \choose k + 1} \) \(k\)-faces. In the talk, we will prove this conjecture and discuss equality cases. We will then extend our results to lattices with diamond property (the inequality part) and to strongly regular normal pseudomanifolds (the equality part). We will also talk about recent results on \(d\)-dimensional polytopes with \(2d + 1\) or \(2d + 2\) vertices. |
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Mara Belotti (TU Berlin) | 09.02.22 | 16:30 | Online | An enumerative problem for cubic (hyper)surfaces: point and line conditions |

Abstract: The moduli space of smooth cubic hypersurfaces in \(\mathbb{P}^n\) is an open subset of a projective space. We construct a compactification of the latter which allows us to count the number of smooth cubic hypersurfaces tangent to a prescribed number of lines and passing through a given number of points. We term it a \(1\)-complete variety of cubic hypersurfaces in analogy to the space of complete quadrics. Paolo Aluffi explored the case of plane cubic curves. Starting from his work, we construct such a space in arbitrary dimensions by a sequence of five blow-ups. The counting problem is then reduced to the computation of five total Chern classes. Finally, we arrive at the desired numbers in the case of cubic surfaces. This is joint work with Alessandro Danelon, Claudia Fevola, Andreas Kretschmer. |
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Sara Griffith (Brown University) | 02.02.22 | 16:30 | Online | The Torelli theorem, or how to determine a graph from its algebraic data |

Abstract: The Torelli theorem for graphs, due to Caporaso-Viviani and Su-Wagner, shows that with mild hypotheses a graph's matroid can be obtained from the integral lattice in its vector space of flows. New results of the speaker show that the graph itself can be recovered using additional data related to winnable chip firing games on the graph. |
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Paco Santos (Universidad de Cantabria) | 26.01.22 | 16:30 | Online | Multitriangulations and tropical Pfaffians |

Abstract: Let \(V=\binom{[n]}{2}\) label the possible diagonals among the vertices of a convex \(n\)-gon. A subset of size \(k+1\) is called a \((k+1)\)-crossing if all elements mutually cross, and a general subset is called \((k+1)\)-crossing free if it does not contain a \(k\)-crossing. \((k+1)\)-crossing free subsets form a simplicial complex that we call the \(k\)-associahedron and denote \(Ass_k{n}\) since for \(k=1\) one (essentially) recovers the simplicial associahedron. The \(k\)-associahedron on the \(n\)-gon is known to be (essentially ) a shellable sphere of dimension \(k(n-2k-1)\) and conjectured to be polytopal (Jonsson 2003). It is also a subword complex in the root system of the A. |
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Jonathan Boretsky (Harvard) | 19.01.22 | 16:30 | Online | The Totally Non-Negative Complete Flag Variety and its Tropicalization |

Abstract: A complete flag in \(\mathbb{R}^n\) is a collection of linear subspaces \(\{0\}=V_0\subsetneq V_1\cdots \subsetneq V_n=\mathbb{R}^n\). The set of all complete flags in \(\mathbb{R}^n\) form a variety called the complete flag variety. We give a combinatorial parameterization of the totally non-negative part of this variety. We also give a simple coordinate-wise characterization of which flags lie in the totally non-negative flag variety. We will also define the notion of a tropical variety and the non-negative part of a tropical variety. There are two tropical varieties that can be described using tropical versions of the equations cutting out the complete flag variety. One, the flag Dressian, can be thought of as describing flags of tropical linear spaces and the second, the tropical flag variety, can be thought of as describing realizable flags of tropical linear space. While these tropical varieties are generally different, we will use our parameterization of the non-negative complete flag variety to show that their non-negative parts coincide. |
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Dante Luber (TU Berlin) | 12.01.21 | 16:30 | Online | Generalized permutahedra and positive flag Dressians |

Abstract: A well known result relates matroid subdivisions of hypersimplices to tropical linear spaces. We continue the study of subdivisions of generalized permutahedra into cells that are generalized permutahedra. This talk will focus on subdivisions of the regular permutahedron into cells that correspond to flag matroids and intervals in a partial ordering on the symmetric group. This is joint work with Michael Joswig, Georg Loho, and Jorge Olarte |
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Katharina Jochemko (KTH) | 15.12.21 | 16:30 | Online | The Eulerian transformation |

Abstract: Many polynomials arising in combinatorics are known or conjectured to have only real roots. One approach to these questions is to study transformations that preserve the real-rootedness property. This talk is centered around the Eulerian transformation which is the linear transformation that sends the i-th standard monomial to the i-th Eulerian polynomial. Eulerian polynomials appear in various guises in enumerative and geometric combinatorics and have many favorable properties, in particular, they are real-rooted and symmetric. We discuss how these properties carry over to the Eulerian transformation. In particular, we disprove a conjecture by Brenti (1989) concerning the preservation of real roots, extend recent results on binomial Eulerian polynomials and provide enumerative and geometric interpretations. This is joint work with Petter Brändén. |
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Matthias Köppe (UC Davis) | 08.12.21 | 16:30 | Online | sagemath-polyhedra: A modularized Sage distribution |

Abstract: SageMath is an open-source general-purpose mathematical system based
on Python that integrates computer algebra facilities and general
computational packages. Sage, initially developed by Stein and first
released in 2005, in over 1.5 decades of incubation in its
pseudo-distribution comprising over 300 software packages, has grown
to provide over 500 Cython and over 2000 of its own Python modules,
ranging from sage.algebras.* over sage.geometry.* to sage.tensor.*,
with a total of over 2.2 million lines of code. |
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Benjamin Schröter (KTH) | 01.12.21 | 16:30 | Online | Ehrhart Polynomials of Rank-Two Matroids |

Abstract: There are many questions that are equivalent to the enumeration of lattice
points in convex sets. Ehrhart theory is the systematic study of lattice point counting
in dilations of polytopes. Over a decade ago De Loera, Haws and Köppe conjectured that
the lattice point enumerator of dilations of matroid basis polytopes is a polynomial with
positive coefficients. This intensively studied conjecture has recently been disproved in
all ranks greater than or equal to three. However, the questions of what characterizes
these polynomials remain wide open. |
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Alexander Stoimenow (GIST College) | 24.11.21 | 16:00 | Online | Exchange moves and non-conjugate braid representatives of links |

Abstract: The braid groups $B_n$ were introduced in the 1930s
in the work of Artin. An element in the braid group $B_n$ is called an
$n$-braid. Alexander related braids to knots and
links in 3-dimensional space, by means of a closure
operation. In that realm, it became important to
understand the braid representatives of a given link.
Markov's theorem relates these
representatives by two moves, conjugacy in the braid group,
and (de)stabilization, which passes between braid groups. Markov's
moves and braid group algebra have become fundamental in Jones'
pioneering work and its later continuation towards
quantum invariants. Conjugacy
is, starting with Garside's, and later many others' work, now
relatively well group-theoretically understood. In contrast, the effect
of (de)stabilization on conjugacy classes of braid representatives of
a given link is in general difficult to control. Only in very special
situations can these conjugacy classes be well described. |
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Sinai Robins (Universidade de São Paulo) | 17.11.21 | 16:30 | Online | The null set of a polytope, and the Pompeiu property for polytopes |

Abstract: We study the null set $N(\P)$ of the Fourier transform of a polytope P in $\R^d$, and we find that this null set does not contain (almost all) circles in $\R^d$. As a consequence, the null set does not contain the algebraic varieties $\{ z \in \C^d \mid z_1^2 + \cdots + z_d^2 = \alpha \}$, for each fixed $\alpha \in \C$. In 1929, Pompeiu asked the following question. Suppose we have a convex subset P in R^d, and a function f, defined over R^d, such that the integral of f over P vanishes, and all of the integrals of f, taken over each rigid motion of P, also vanish. Does it necessarily follow that f = 0? If the answer is affirmative, then the convex body P is said to have the Pompeiu property. It is a conjecture that in every dimension, balls are the only convex bodies that do not have the Pompeiu property. Here we get an explicit proof that the Pompeiu property is true for all polytopes, by combining our work with the work of Brown, Schreiber, and Taylor from 1973. Our proof uses the Brion-Barvinok theorem in combinatorial geometry, together with some properties of the Bessel functions. The original proof that polytopes (as well as other bodies) possess the Pompeiu property was given by Brown, Schreiber, and Taylor (1973) for dimension 2. In 1976, Williams observed that the same proof also works for $d>2$ and, using eigenvalues of the Laplacian, gave another proof valid for $d \geq 2$ that polytopes indeed have the Pompeiu property. The null set of the Fourier transform of a polytope has also been used in a different direction, by various researchers, to tackle problems in multi-tiling Euclidean space. Thus, the null set of a polytope is interesting for several applications, including discrete versions of this problem that we will mention, which are generally unsolved. This is joint work with Fabrício Caluza Machado. |
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Austin Alderete (UT Austin) | 10.11.21 | 16:30 | Online | A polymake extension for computing the homology groups of minor-closed classes of matroids |

Abstract: We present a polymake extension for performing computations in the intersection ring of matroids. Addition in the ring is carried out on the indicator vectors of maximal chains of flats while the product is the usual matroid intersection when the intersection is loopfree and zero otherwise. The primary feature of this extension is the ability to compute the homology groups of the intersection ring induced by deletion and contraction as well as those of any subring generated by a minor-closed class of matroids. |
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MaRDI | 03.11.21 | 16:30 | MPI Leipzig | MaRDI Kickoff Workshop |

Abstract: |
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Davide Lofano (TU Berlin) | 27.10.21 | 16:30 | H 0107 | Hadamard Matrix Torsion |

Abstract: After a brief overview about torsion in homology and examples of complexes with small torsion and few vertices we will focus our attention on a particular series of examples. In particular, we will construct a series HMT(n) of 2-dimensional simplicial complexes with huge torsion, |H_1(HMT(n))|=|det(H(n))|=n^(n/2), where the construction is based on the Hadamard matrices H(n) and they have linearly many vertices in n. Our explicit series improves a previous construction by Speyer, narrowing the gap to the highest possible asymptotic torsion growth achieved by Newman via a randomized construction. |
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Sylvain Spitz (TU Berlin) | 20.10.21 | 16:30 | H 0107 | Generalized Permutahedra and Optimal Auctions |

Abstract: We study a family of convex polytopes, called SIM-bodies, which were introduced by Giannakopoulos and Koutsoupias (2018) to analyze so-called Straight-Jacket Auctions. First, we show that the SIM-bodies belong to the class of generalized permutahedra. Second, we prove an optimality result for the Straight-Jacket Auctions among certain deterministic auctions. Third, we employ computer algebra methods and mathematical software to explicitly determine optimal prices and revenues. |
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Petter Brändén (KTH) | 13.10.21 | 16:30 | Online | Lorentzian polynomials on cones and the Heron-Rota-Welsh conjecture |

Abstract: We extend the theory of Lorentzian polynomials to convex cones. This is used to give a short proof of the log-concavity of the coefficients of the reduced characteristic polynomial of a matroid, and reprove the Hodge-Riemann relations of degree one for the Chow ring of a matroid. This is joint work with Jonathan Leake. |
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Georg Loho (University of Twente) | 25.08.21 | 16:30 | Online | Generalized permutahedra and complete classes of valuated matroids |

Abstract: Generalized permutahedra form an important class of polytopes occurring explicitly or implicitly in several branches of mathematics and beyond. I start with an overview of concepts related with generalized permutahedra from optimization and Discrete Convex Analysis. Valuated generalized matroids give rise to a particularly interesting subclass. I show some fundamental constructions for these objects. This leads to an answer to two open questions from auction theory and discrete convex analysis. |
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Chiara Meroni (MPI MiS) | 04.08.21 | 16:30 | Online | Fiber convex bodies |

Abstract: Given a convex polytope P and a projection map it is possible to construct the associated fiber polytope, that intuitively is the average of the fibers of P under this projection and has interesting combinatorial properties. In this joint work with Léo Mathis we study this construction for all convex bodies. The aim is to relate aspects of the original convex body with those of its fiber body. I will discuss theoretical results as well as some concrete examples. |
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Tobias Boege (Universität Magdeburg) | 28.07.21 | 16:30 | Online | The Gaussian conditional independence inference problem |

Abstract: Conditional independence is a ternary relation on subsets of a finite vector of random variables \xi. The CI statement \xi_i \CI \xi_j \mid \xi_K
asserts that ``whenever the outcome of all the variables \xi_k, k \in K,
is known, learning the outcome of \xi_i provides no further information
on \xi_j''. |
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Claudia Yun (Brown University) | 14.07.21 | 16:30 | Online | The S_n-equivariant rational homology of the tropical moduli spaces \Delta_{2,n} |

Abstract: Fix non-negative integers g and n such that 2g-2+n>0, the tropical moduli space \Delta_{g,n} is a topological space that parametrizes isomorphism classes of n-marked stable tropical curves of genus g with total volume 1. These spaces are important because their reduced rational homology is identified equivariantly with the top weight cohomology of the algebraic moduli spaces M_{g,n}, with respect to the S_n-action of permuting marked points. In this talk, we focus on the genus 2 case and compute the characters of \tilde{H}_i(\Delta_{2,n};Q) as S_n-representations for n up to 8. To carry out the computations, we use a cellular chain complex for symmetric \Delta-complexes, a technique developed by Chan, Galatius, and Payne. We will also briefly mention work in progress that extends the computations to n=10, in collaboration with Christin Bibby, Melody Chan, and Nir Gadish. All computations are done in SageMath. |
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Ilia Itenberg (Sorbonne Université) | 07.07.21 | 16:30 | Online | Planes in cubic fourfolds |

Abstract: We discuss possible numbers of 2-planes in a smooth cubic hypersurface in the 5-dimensional projective space. We show that, in the complex case, the maximal number of planes is 405, the maximum being realized by the Fermat cubic. In the real case, the maximal number of planes is 357. The proofs deal with the period spaces of cubic hypersurfaces in the 5-dimensional complex projective space and are based on the global Torelli theorem and the surjectivity of the period map for these hypersurfaces, as well as on Nikulin's theory of discriminant forms. Joint work with Alex Degtyarev and John Christian Ottem. |
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Holger Eble (TU Berlin) | 30.06.21 | 16:30 | Online | Enumerating Chambers of Hyperplane Arrangements with Symmetry |

Abstract: We introduce a new algorithm for enumerating chambers of hyperplane arrangements which exploits their underlying symmetry groups. Our algorithm counts the chambers of an arrangement as a byproduct of computing its characteristic polynomial. We showcase our julia implementation, based on OSCAR, on examples coming from hyperplane arrangements with applications to physics and computer science. This is joint work with Taylor Brysiewicz and Lukas Kuehne. |
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Manuel Radons (TU Berlin) | 23.06.21 | 16:30 | Online | Edge-unfolding nested prismatoids |

Abstract: A 3-Prismatoid is the convex hull of two convex polygons A and B which lie in parallel planes H_A, H_B in R^3. Let A' be the orthogonal projection of A onto H_B. A prismatoid is called nested if A' is properly contained in B, or vice versa. We show that every nested prismatoid has an edge-unfolding to a non-overlapping polygon in the plane. |
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Christoph Hertrich (TU Berlin) | 16.06.21 | 16:30 | Online | Towards Lower Bounds on the Depth of ReLU Neural Networks |

Abstract: We contribute to a better understanding of the class of functions that is represented by a neural network with ReLU activations and a given architecture. Using techniques from mixed-integer optimization, polyhedral theory, and tropical geometry, we provide a mathematical counterbalance to the universal approximation theorems which suggest that a single hidden layer is sufficient for learning tasks. In particular, we investigate whether the class of exactly representable functions strictly increases by adding more layers (with no restrictions on size). We also present upper bounds on the sizes of neural networks required for exact function representation. |
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Matteo Gallet (RICAM) | 09.06.21 | 16:30 | Online | Mobile Icosapods |

Abstract: Pods are mechanical devices constituted of two rigid bodies, the base and the platform, connected via spherical joints by a number of other rigid bodies, called legs. The maximal number, when finite, of legs of a mobile pod is 20. In 1904, Borel designed a technique to construct examples of such 20-pods over the complex numbers. We show that recent results about spectrahedra provide a way to determine real solutions for Borel’s construction. This is a joint work with Georg Nawratil, Jon Selig, and Josef Schicho. |
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Marieke van der Wegen (Utrecht University) | 02.06.21 | 16:30 | Online | Computing stable gonality is hard |

Abstract: Based on analogies between algebraic curves and graphs, there are several graph parameters defined. In this talk we will study the so-called stable gonality. The stable gonality is a measure for the complexity of a graph and is defined using morphisms from a graph to a tree. We show that computing the stable gonality of a graph is NP-hard. This is joint work with Dion Gijswijt and Harry Smit. |
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Guido Montúfar (MPI Leipzig and UCLA) | 26.05.21 | 16:30 | Online | Sharp bounds for the number of regions of maxout networks and vertices of Minkowski sums |

Abstract: We present results on the number of linear regions of the functions that can be represented by artificial feedforward neural networks with maxout units. A rank-k maxout unit is a function computing the maximum of k linear functions. For networks with a single layer of maxout units, the linear regions correspond to the upper vertices of a Minkowski sum of polytopes. We obtain face counting formulas in terms of the intersection posets of tropical hypersurfaces or the number of upper faces of partial Minkowski sums, along with explicit sharp upper bounds for the number of regions for any input dimension, any number of units, and any ranks, in the cases with and without biases. Based on these results we also obtain asymptotically sharp upper bounds for networks with multiple layers. This is joint work with Yue Ren and Leon Zhang. |
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Simon Telen (MPI MiS Leipzig) | 19.05.21 | 16:30 | Online | Likelihood equations and scattering amplitudes |

Abstract: We identify the scattering equations from particle physics as the likelihood equations for a particular statistical model. The scattering potential plays the role of the log-likelihood function. We employ recent methods from numerical nonlinear algebra to solve challenging instances of the scattering equations. We revisit the theory of stringy canonical forms proposed by Arkani-Hamed, He and Lam, introducing positive statistical models and their amplitudes. This is joint work with Bernd Sturmfels. |
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Sophie Rehberg (FU Berlin) | 12.05.21 | 16:30 | Online | Combinatorial reciprocity theorems for generalized permutahedra, hypergraphs, and pruned inside-out polytopes |

Abstract: Generalized permutahedra are a class of polytopes with many interesting combinatorial subclasses. We introduce pruned inside-out polytopes, a generalization of inside-out polytopes introduced by Beck–Zaslavsky (2006), which have many applications such as recovering the famous reciprocity result for graph colorings by Stanley. We study the integer point count of pruned inside-out polytopes by applying classical Ehrhart polynomials and Ehrhart–Macdonald reciprocity. This yields a geometric perspective on and a generalization of a combinatorial reciprocity theorem for generalized permutahedra by Aguiar–Ardila (2017) and Billera–Jia–Reiner (2009). Applying this reciprocity theorem to hypergraphic polytopes allows us to give an arguably simpler proof of a recent combinatorial reciprocity theorem for hypergraph colorings by Aval–Karaboghossian–Tanasa (2020). Our proof relies, aside from the reciprocity for generalized permutahedra, only on elementary geometric and combinatorial properties of hypergraphs and their associated polytopes. |
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Sandra Di Rocco (KTH) | 05.05.21 | 16:30 | Online | Iterated discriminants |

Abstract: The Hyperdeterminant is a celebrated tool in tensor theory, geometrically it represents the discriminant of a Segre embedding. The Segre embedding is a toric map associated to a Cayley sum, probably the basic example of Cayley polytopes. Unlike other discriminants the hyperdeterminant is at times accessible to computation due to a decomposition method developed by Schäfli. We propose a generalisation of the Schäfli method to discriminants associated to general Cayley sums and show how this can be treated as a discriminant of a system of polynomials. This is joint work with A. Dickenstein and R. Morrison. |
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Daniel Corey (University of Wisconsin-Madison) | 28.04.21 | 16:30 | Online | Initial degenerations of Grassmannians and spinor varieties |

Abstract: We construct closed immersions from initial degenerations of Gr_0(d,n)---the open cell in the Grassmannian Gr(d,n) given by the nonvanishing of all Plücker coordinates---to limits of thin Schubert cells associated to diagrams induced by the face poset of the corresponding tropical linear space. These are isomorphisms in many cases, including (d,n) equal to (2,n), (3,6) and (3,7). As an application, Gr_0(3,7) is schön, and the Chow quotient of Gr(3,7) by the maximal torus in PGL(7) is the log canonical compactification of the moduli space of 7 points in P^2 in linear general position, making progress on a conjecture of Hacking, Keel, and Tevelev. Finally, I will discuss recent work on extending these results to the Lie-type D setting. |
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Mario Kummer (TU Dresden) | 21.04.21 | 16:30 | Online | Iso Edge Domains |

Abstract: A Conjecture by Conway and Sloane from the 90s can be phrased as ''Every tropical abelian variety is determined by its tropical theta constants''. It was proved by Conway and Sloane in dimension up to 3 and by Vallentin in dimension 4. We prove it in dimension 5. The prove relies on the classification of iso-edge domains in dimension 5: These are a variant of the iso-Delaunay decomposition introduced by Baranovskii and Ryshkov. We further characterize the so-called matroidal locus in terms of the tropical theta constants in dimension up to 5 and point out connections to the tropical Schottky problem. The talk is based on a joint work with Mathieu Dutour Sikirić. |
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Antony Della Vecchia | 14.04.21 | 16:30 | Online | One Relator Groups and Regular Sectional Curvature |

Abstract: We present the notion of regular sectional curvature for angled 2-complexes. It is known that fundamental groups of angled 2-complexes with negative or nonpositive regular section curvature are coherent and, in certain cases, locally quasi-convex. We state some open problems for one-relator groups, describe a construction of angled 2-complexes for one-relator groups and provide some computer generated results supporting the claims. |
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Nicholas Williams (University of Leicester) | 10.03.21 | 16:30 | Online | New interpretations of the higher Stasheff–Tamari orders |

Abstract: The higher Stasheff–Tamari orders are two partial orders introduced in the 1990s by Kapranov, Voevodsky, Edelman and Reiner on the set of triangulations of a cyclic polytope. Edelman and Reiner conjectured these two orders to coincide, which remains an open problem today. In this talk we give new combinatorial interpretations of the orders. These interpretations differ depending on whether the cyclic polytope is 2d-dimensional or (2d + 1)-dimensional, but are expressed in each case in terms of the d-skeleton of the triangulation. This naturally leads us to also characterise the d-skeletons of triangulations of (2d + 1)-dimensional cyclic polytopes, complementing the description of the d-skeleton of a 2d-dimensional triangulation given by Oppermann and Thomas. Our results make the conjectured equivalence of the orders a more tractable problem, and we use them to run computational experiments checking the conjecture, to which we find no counter-examples. Our results also have interpretations in the representation theory of algebras, but we will not discuss these connections. |
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Kaie Kubjas (Aalto University) | 03.03.21 | 16:30 | Online | Identifying 3D Genome Organization in Diploid Organisms via Euclidean Distance Geometry |

Abstract: The 3D organization of the genome plays an important role for gene regulation. Chromosome conformation capture techniques allow one to measure the number of contacts between genomic loci that are nearby in the 3D space. In this talk, we study the problem of reconstructing the 3D organization of the genome from whole genome contact frequencies in diploid organisms, i.e. organisms that contain two indistinguishable copies of each genomic locus. In particular, we study the identifiability of the 3D organization of the genome and optimization methods for reconstructing it. This talk is based on joint work with Anastasiya Belyaeva, Lawrence Sun and Caroline Uhler. |
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Malte Renken (TU Berlin) | 24.02.21 | 16:30 | Online | What does the Cyclic Polytope have to do with Visibility Graphs? |

Abstract: In the 90's the study of visibility graphs prompted the definition of
"persistent graphs", a class of vertex-ordered graphs defined by three
simple combinatorial properties.
We reveal a surprising connection of these persistent graphs to the
triangulations
of the 3-dimensional cyclic polytope via a natural bijection. |
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Jorge Olarte (TU Berlin) | 10.02.2021 | 17:00 | Online | The tropical symplectic Grassmannian |

Abstract: The symplectic Grassmannian SpGr(k,2n) is the space of a all linear subspaces of dimension k of a vector space of dimension 2n which are isotropic with respect to a symplectic form. We look at several equivalent characterizations of isotropic linear subspaces and formulate a tropical analog for each, such as, for example, being in the tropicalization of the symplectic Grassmannian. It turns out that in the tropical world these characterizations are no longer equivalent and we will see exactly for which k and n does one characterization imply another. This is joint work with George Balla. |
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Oguzhan Yürük (TU Berlin) | 10.02.2021 | 16:30 | Online | Detecting the Regions of Multistaionarity in CRNT via Symbolic Nonnegativity Certificates |

Abstract: Parameterized ordinary differential equation systems are crucial for modeling in biochemical reaction networks under the assumption of mass-action kinetics. Various questions concerning the signs of multivariate polynomials in positive orthant arise from studying the solutions' qualitative behavior with respect to parameter values, in particular the existence of multistationarity. In this work, we revisit a method of detecting multistationarity, in which we utilize the circuit polynomials to find symbolic certificates of nonnegativity for 2-site phosphorylation cycle in a recent work of the speaker joint with Elisenda Feliu, Nidhi Kaihnsa and Timo de Wolff. Moreover, we will give a description of all possible certificates that can arise from 2-site phosphorylation cycle, and provide further insight into the number of positive steady states of the n-site phosphorylation cycle model. |
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Siddarth Kannan (Brown University) | 03.02.2021 | 16:30 | Online | Symmetries of tropical moduli spaces of curves |

Abstract: I will briefly introduce the moduli space of n-marked stable tropical curves of genus g, and then discuss the combinatorial calculation of its automorphism group. I will conclude by talking about natural follow-up questions to this result, motivated by connections to the Deligne-Mumford moduli stack of marked stable algebraic curves and the mapping class group. |
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Abhishek Rathod (TU Munich) | 27.01.2021 | 16:30 | Online | Parameterized inapproximability of Morse matching |

Abstract: In this talk, we look at the problem of minimizing the number of critical simplices (Min-Morse) from the point of view of inapproximability and parameterized complexity. Let n denote the size of a simplicial complex. We first show that Min-Morse can not be approximated within a factor of 2^{\log^{(1-\epsilon)}n}- \delta, for any \epsilon,\delta>0, unless NP \subseteq QP. Our second result shows that Min-Morse is WP-hard with respect to the standard parameter. Next, we show that Min-Morse with standard parameterization has no FPT-approximation algorithm for any approximation factor \rho, unless WP = FPT. Since the gadgets involved in our reduction are 2-complexes, the above hardness results are applicable for all complexes of dimension \geq 2. |
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Christian Steinert (RWTH Aachen University) | 20.01.2021 | 16:30 | Online | A Diagrammatic Approach to String Polytopes |

Abstract: It is a common principle of mathematics to translate problems from one
area of research to another and solve them there. A very particular
example of this approach is the study of string polytopes, originally
introduced and studied by Berenstein and Zelevinsky as well as
Littelmann. Their original motivation stems from representation theory
but they also provide fruitful tools in the algebraic geometry of flag
varieties and more as studied by Caldero as well as Alexeev and Brion.
Although these string polytopes have been around for more than 30
years, results on their combinatorial properties are scarce. |
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Dhruv Ranganathan (Universtiy of Cambridge) | 13.01.2021 | 16:30 | Online | Donaldson-Thomas theory and the secondary polytope |

Abstract: Donaldson-Thomas theory extracts numerical invariants from an algebraic variety X by examining the geometry of the Hilbert schemes of X. While the sister subject of Gromov-Witten theory has had a long and rich interaction with tropical and polyhedral methods, the relationship between the Hilbert scheme and tropical geometry is less developed. I will give an introduction to tropical and logarithmic aspects of Donaldson-Thomas theory, from the point of view of the GKZ secondary polytope construction, and generalizations thereof. This is based on joint work with Davesh Maulik (MIT). |
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Michael Joswig (TU Berlin) | 09.12.2020 | 16:30 | Online | Moduli of tropical curves - data and algorithms |

Abstract: In joint work with Sarah Brodsky, Ralph Morrison and Bernd Sturmfels in 2015 we computed the moduli of tropical plane curves of genus $\leq 4$. After a brief summary of the mathematical results the focus of this presentation is about the computational methods and the 400MB of data produced. We will see how this is being used for further research (ongoing joint work with Dominic Bunnett). On the way we will touch upon general questions concerning data in mathematics. |
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Melody Chan (Brown University) | 02.12.2020 | 16:30 | Online | Tropical abelian varieties |

Abstract: I'll give an introduction to the moduli space of tropical abelian varieties, assuming no background in tropical geometry. Lots of different combinatorics arises, including the beautiful century-old combinatorics of Voronoi reduction theory, perfect quadratic forms, regular matroids, and metric graphs. On the geometric side, it relates to toroidal compactifications of the classical moduli space A_g of abelian varieties. I'll explain how, and, time permitting, I'll report on work-in-progress in which we use tropical techniques to find new rational cohomology classes in A_g in a previously inaccessible range. Joint work with Madeline Brandt, Juliette Bruce, Margarida Melo, Gwyneth Moreland, and Corey Wolfe. |
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Kemal Rose (TU Berlin) | 25.11.2020 | 16:30 | Online | Certifying roots of polynomial systems using interval arithmetic |

Abstract: We report on an implementation of the Krawczyk method which establishes interval arithmetic as a practical tool for certification in numerical algebraic geometry. Our software HomotopyContinuation.jl now has a built-in function certify, which proves the correctness of an isolated solution to a square system of polynomial equations. This implementation dramatically outperforms earlier approaches to certification. |
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Benjamin Schröter (KTH Stockholm) | 18.11.2020 | 16:30 | Online | Reconstructing Matroid Polytopes |

Abstract: This talk deals with two fundamental objects of discrete mathematics
that are closely related - (convex) polytopes and matroids.
Both appear in many areas of mathematics, e.g., algebraic geometry,
topology and optimization. |
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Nick Early (Perimeter Institute for Theoretical Physics) | 11.11.2020 | 16:30 | Online | Combinatorial Geometries: From Polyhedral Subdivisions to Scattering Amplitudes |

Abstract: In 1948, Richard Feynman introduced a formalism in Quantum Field Theory to organize the expansion of a scattering amplitude as a sum of elementary rational functions, labeled by graphs. These graphs, now called Feynman diagrams, specify which singularities of the amplitude are compatible and can appear simultaneously. |
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Dominic Bunnett (TU Berlin) | 04.11.2020 | 16:30 | Online | Embedded stacky fans and tropical moduli spaces |

Abstract: Stacky fans are orbifold objects in discrete geometry
with a rich combinatorial structure. They arise naturally
as the moduli spaces of tropical curves. Explicitly, a
stacky fan is built from polyhedral cones quotiented out
by finite groups. |
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Rainer Sinn (University of Leipzig) | 28.10.2020 | 16:30 | Online | Realization spaces of polytopes |

Abstract: We study a simple model of the realization space of a polytope that lends itself to the application of the implicit function theorem. We explore how far this basic tool can carry us. Combined with quite elementary combinatorial arguments, this recovers most of the known results of “nice” realization spaces. |
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Federico Castillo (MPI Leipzig) | 21.10.2020 | 16:30 | Online | The space of Minkowski Summands |

Abstract: Given a polytope P we study the set of all possible Minkowski summands. Being a Minkowski summand is equivalent to normal (or strongly combinatorial) equivalence. Different points of views provide different looking (but equivalent) parametrizations of the space of Minkowski summands as a polyhedral cone. We will go over several examples like polygons, permutohedra, cubes, and more. |
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Leonid Monin (University of Bristol) | 14.10.2020 | 16:30 | Online | Inversion of matrices, ML-degrees and the space of complete quadrics |

Abstract: In my talk I will discuss the following questions: (1) How many quadrics go through *k* generic points and are tangent to *m * generic hyperplanes? (2) What is the maximum likelihood degree of a general linear concentration model? (3) What is the degree of the variety *L-1* obtained as the closure of the set of inverses of matrices from a generic linear subspace *L* of Sym*n*? In fact, all three questions are equivalent! First I will briefly explain why this is the case, and then I will describe several approaches to the above questions and possible answers they lead to. This is joint work in progress with Laurent Manivel, Mateusz Michalek, Tim Seynnaeve, Martin Vodicka, Andrzej Weber, and Jaroslaw A. Wisniewski. |
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Karin Schaller (FU) | 07.10.2020 | 16:30 | Online | Mirror symmetry constructions for quasi-smooth Calabi-Yau hypersurfaces in weighted projective spaces |

Abstract: We consider a general combinatorial framework for constructing mirrors of quasi-smooth Calabi-Yau hypersurfaces defined by weighted homogeneous polynomials. Our mirror construction shows how to obtain mirrors being Calabi-Yau compactifications of non-degenerate affine hypersurfaces associated to certain Newton polytopes. This talk is based on joint work with Victor Batyrev. |
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Lukas Kühne (Hebrew Universiy of Jerusalem) | 30.09.2020 | 16:30 | Online | The Resonance Arrangement |

Abstract: The resonance arrangement is the arrangement of hyperplanes which has all nonzero 0/1-vectors in R^n as normal vectors. It is also called the all-subsets arrangement. Its chambers appear in algebraic geometry, in mathematical physics and as maximal unbalanced families in economics. |
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Jaeho Shin (Seoul National University) | 26.08.2020 | 16:30 | Online | Polytropes and Tropical Linear Spaces |

Abstract: A polytrope is a convex polytope that is expressed as the tropical convex hull of a finite number of points. It is well known that every bounded cell of a tropical linear space is a polytrope, and its converse statement has been a conjecture. We develop an elementary approach to the relationship between tropical convexity and tropical linearity, and show that the conjecture holds in dimension up to 3 and fails in every higher dimension. Thus, tropical convexity is strictly bigger than tropical linearity. |
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Yue Ren (Swansea University) | 29.07.2020 | 16:30 | Online | TBA |

Abstract: |
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Stephane Gaubert (INRIA and CMAP, Ecole polytechnique, CNRS) | 08.07.2020 | 16:30 | Online | Understanding and monitoring the evolution of the Covid-19 epidemic from medical emergency calls: the example of the Paris area |

Abstract: We portray the evolution of the Covid-19 epidemic during the crisis of March-April 2020 in the Paris area, by analyzing the medical emergency calls received by the EMS of the four central departments of this area (Centre 15 of SAMU 75, 92, 93 and 94). Our study reveals strong dissimilarities between these departments. We show that the logarithm of each epidemic observable can be approximated by a piecewise linear function of time. This allows us to distinguish the different phases of the epidemic, and to identify the delay between sanitary measures and their influence on the load of EMS. This also leads to an algorithm, allowing one to detect epidemic resurgences. We rely on a transport PDE epidemiological model, and we use methods from Perron-Frobenius theory and tropical geometry. |
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Mara Belotti (SISSA) | 24.06.2020 | 16:30 | Online | Topology of rigid isotopy classes of geometric graphs |

Abstract: We study the topology of rigid isotopy classes of geometric graphs on n vertices in a d-dimensional Euclidean space. We consider in particular two different regimes: a large number of vertices for the graphs (n large) and a large dimension of the space in which the graphs live (d large). In both cases we make considerations on the number of such classes and study their Betti numbers. In particular, for the second case we register a "shift to infinity of the topology". This is joint work with A.Lerario and A.Newman. |
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Raman Sanyal (Goethe University Frankfurt) | 17.06.2020 | 16:30 | Online | Inscribable fans, type cones, and reflection groupoids |

Abstract: Steiner posed the question if any 3-dimensional polytope had a realization with vertices on a sphere. Steinitz constructed the first counter examples and Rivin gave a complete resolution. In dimensions 4 and up, Mnev's universality theorem renders the question for inscribable combinatorial types hopeless. In this talk, I will address the following refined question: Given a polytope P, is there a normally equivalent polytope with vertices on a sphere? That is, can P be deformed into an inscribed polytope while preserving its normal fan? It turns out that the answer gives a rich interplay of geometry and combinatorics, involving local reflections and type cones. This is based on joint work with Sebastian Manecke (who will continue the story in the FU discrete geometry seminar next week). |
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Taylor Brysiewicz (Texas A&M University) | 03.06.2020 | 16:30 | Online | The degree of Stiefel manifolds |

Abstract: The Stiefel manifold is the set of orthonormal bases for k-planes in an n-dimensional space. We compute its degree as an algebraic variety in the space of k-by-n matrices using techniques from classical algebraic geometry, representation theory, and combinatorics. We give a combinatorial interpretation of this degree in terms of non-intersecting lattice paths. (This is joint work with Fulvio Gesmundo) |
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Thorsten Theobald (Goethe University Frankfurt) | 27.05.2020 | 16:30 | Online | Conic stabilility of polynomials and spectrahedra |

Abstract: In the geometry of polynomials, the notion of stability is of prominent importance. The purpose of the talk is to discuss its recent generalization to conic stability. Given a convex cone K in real n-space, a multivariate polynomial f in C[z] is called K-stable if it does not have a root whose vector of the imaginary parts is contained in the interior of $K$. If $K$ is the non-negative orthant, then $K$-stability specializes to the usual notion of stability of polynomials. In particular, we focus on $K$-stability with respect to the positive semidefinite cone and develop stability criteria building upon the connection to the containment problem for spectrahedra, to positive maps and to determinantal representations. These results are based on joint work with Papri Dey and Stephan Gardoll. |
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Luis Carlos Garcia Lirola (Kent State University) | 20.05.2020 | 16:30 | Online | Volume product and metric spaces |

Abstract: Given a finite metric space M, the set of Lipschitz functions on M with Lipschitz constant at most 1 can be identified with a convex polytope in R^n. In this talk, we will show that there is a strong connection between the geometric properties of this polytope (as the vertices and the volume product) and the properties of the metric space M. We will also relate this study with a famous open problem in Convex Geometry, the Mahler conjecture, on the product of the volume of a convex body and its polar. This is a joint work with M. Alexander, M. Fradelizi, and A. Zvavitch. |
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Georg Loho (London School of Economics) | 13.05.2020 | 16:30 | Online | Oriented Matroids from Triangulations of Products of Simplices |

Abstract: Classically, there is a rich theory in algebraic combinatorics surrounding the various objects associated with a generic real matrix. Examples include regular triangulations of the product of two simplices, coherent matching fields, and realizable oriented matroids. In this talk, we will extend the theory by skipping the matrix and starting with an arbitrary triangulation of the product of two simplices instead. In particular, we show that every polyhedral matching field induces oriented matroids. The oriented matroid is composed of compatible chirotopes on the cells in a matroid subdivision of the hypersimplex. Furthermore, we give a corresponding topological construction using Viro’s patchworking. This talk will also sketch the relationship between Baker-Bowler’s matroids over hyperfields and our work. This is joint work with Marcel Celaya and Chi-Ho Yuen. |
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Martin Winter (TU Chemnitz) | 06.05.2020 | 16:30 | Online | Edge-Transitive Polytopes |

Abstract: Despite the long history of the study of symmetric polytopes, aside
from two extreme cases, the general transitivity properties of convex
polytopes are still badly understood.
For example, it has long been known that there are five
flag-transitive (aka. regular) polyhedra (called, Platonic solids),
six flag-transitive 4-polytopes and exactly three flag-transitive
d-polytopes for any d>=5. The symmetry requirement of
flag-transitivity is therefore quite restrictive for convex polytopes.
On the other end of the spectrum, the class of vertex-transitive (aka.
isogonal) polytopes is as rich as the category of finite groups.
There seems to be little known about intermediate symmetries, like
transitivity on edges or k-faces for any k>=2, and their interactions. |
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Marek Kaluba | 29.04.2020 | 17:00 | Online | Random polytopes in Machine Learning |

Abstract: In this talk I will describe very recent work on analyzing different
models of auto-encoder neural networks using random polytopes. In general, a
class of neural networks known as auto-encoders can be understood as a low-
dimensional (piecewise linear) embedding ℝᴺ → ℝⁿ (with N >> n), where points
in the domain are e.g. images (in their pixel form) and each dimension the
range extracts a single "feature". |
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Francisco Criado | 29.04.2020 | 16:30 | Online | Borsuk’s problem |

Abstract: In 1932, Borsuk proved that every 2-dimensional convex body can be
divided in three parts with strictly smaller diameter than the
original. He also asked if the same would hold for any dimension d: is
it true that every d-dimensional convex body can be divided in (d+1)
pieces of strictly smaller diameter? |
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Jonathan Spreer (University of Sydney) | 22.04.2020 | 16:30 | MA 621 | Linking topology and combinatorics: Width-type parameters of 3-manifolds |

Abstract: Many fundamental topological problems about 3-manifolds are algorithmically solvable in theory, but continue to withstand practical computations. In recent years some of these problems have been shown to allow efficient solutions, as long as the input 3-manifold comes with a sufficiently "thin" presentation. |
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Ayush Kumar Tewari | 15.04.2020 | 17:00 | Online | Forbidden patterns in tropical planar curves and Panoptagons |

Abstract: |
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Dominic Bunnett | 15.04.2020 | 16:30 | Online | Geometric discriminants and moduli |

Abstract: A discriminant is a subset of a function space consisting of singular functions. We shall consider discriminants of finite dimensional vector spaces of polynomials defining hypersurfaces in a toric variety. In the best case scenario (for plane curves for example), the discriminant is an irreducible hypersurface, however this is not the case for almost any other toric variety. We discuss positive results on the geometry of discriminants of weighted projective spaces and how to compute them via the A-discriminants of Gelfand, Kapranov and Zelevinsky. |
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Robert Loewe | 08.04.2020 | 16:30 | Online | Minkowski decompositions of generalized associahedra |

Abstract: We give an explicit subword complex description of the rays of the type cone of the g-vector fan of an finite type cluster algebra with acyclic initial seed. In particular, this yields a non-recursive description of the Newton polytopes of the F-polynomials as conjectured by Brodsky and Stump. We finally show that for finite type cluster algebras, the cluster complex is isomorphic to the totally positive part of the tropicalization of the cluster variety as conjectured by Speyer and Williams. |
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Lars Kastner | 01.04.2020 | 16:30 | Online | Hyperplane arrangements in polymake |

Abstract: I will report on the implementation of hyperplane arrangements in polymake.
Hyperplane arrangements appear in a wide variety of applications in tropical
and algebraic geometry. An important construction is the induced subdivision.
We will discuss the new HyperplaneArrangement object, its properties and our
simple algorithm for constructing the cell subdivision. |
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Paul Vater | 25.03.2020 | 17:00 | Online | Patchworking real tropical hypersurfaces |

Abstract: We take a look at the tropical version of Viro's combinatorial patchworking method, as well as a recent implementation in polymake. In particular we investigate an efficient way of computing the homology with Z_2 coefficients of such a patchworked hypersurface. |
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Andrew Newman | 25.03.2020 | 16:30 | Online | A two-step random polytope model |

Abstract: We consider a model for generating non-simple, non-simplicial random polytopes. The first step of the random process generates a random polytope P via random hyperplanes tangent to the d-dimensional sphere; the second step is given by the convex hull of a binomial sample of the vertices of P. In this talk we will discuss some ongoing work establishing results about the expected complexity of such polytopes. |
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Oguzhan Yuruk | 11.03.2020 | 16:30 | MA 621 | Understanding the Parameter Regions of Multistationarity in Dual Phosporylation Cycle via SONC |

Abstract: Parameterized ordinary differential equation systems are crucial for modeling in biochemical reaction networks under the assumption of mass-action kinetics. Existence of multiple positive solutions in systems arising from biochemical reaction network are crucial since it is linked to cellular decision making and memory related on/off responses. Recent developments points out that the multistationarity, along with some other qualitative properties of the solutions, is related to various questions concerning the signs of multivariate polynomials in positive orthant. In this work, we provide further insight to the set of kinetic parameters that enable or preclude multistationarity of dual phosphorylation cycle by utilizing circuit polynomials to find symbolic certificates of nonnegativity. This is a joint work with Elisenda Feliu, Nidhi Kaihnsa and Timo de Wolff. |
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Ralph Morrison (Williams College) | 26.02.2020 | 16:30 | MA 621 | Tropically planar graphs: geometry and combinatorics |

Abstract: Tropically planar graphs are graphs that arise as the skeletons of smooth tropical plane curves, which are dual to regular unimodular triangulations of lattice polygons. In this talk we study what geometric properties such graphs have in the moduli space of metric graphs, as well as the combinatorial properties these graphs satisfy. This talk will include older work with Brodsky, Joswig, and Sturmfels; newer work with Coles, Dutta, Jiang, and Scharf; and ongoing work with Tewari. |
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Antonio Macchia (FU Berlin) | 12.02.2020 | 16:30 | MA 621 | Binomial edge ideals of bipartite graphs |

Abstract: Binomial edge ideals are ideals generated by binomials corresponding to the
edges of a graph, naturally generalizing the ideals of 2-minors of a generic
matrix with two rows. They also arise naturally in the context of
conditional independence ideals in Algebraic Statistics. |
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Amy Wiebe (FU Berlin) | 05.02.2020 | 16:30 | MA 621 | Combining Realization Space Models of Polytopes |

Abstract: In this talk I will present a model for the realization space of a polytope which represents a polytope by its slack matrix. This model provides a natural algebraic relaxation for the realization space, and comes with a defining ideal which can be used as a computational engine to answer questions about the realization space. We will see how this model is related to more classical realization space models (representing realizations by Gale diagrams or points of the Grassmannian). In particular, we will see these relationships can be used to improve computational efficiency of the slack model. |
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Matias Villagra (Pontifical Catholic University of Chile) | 29.01.2020 | 16:30 | MA 621 | On a canonical symmetry breaking technique for polytopes |

Abstract: Given a group of symmetries of a polytope, a Fundamental Domain is a set of R^n that aims to select a unique representative of symmetric vectors, i.e. such that each point in the set is a unique representative under its G-orbit, effectively eliminating all isomorphic points of the polytope. The canonical Fundamental Domain found in the literature, which can be constructed for any permutation group, is NP-hard to separate even for structurally simple groups whose elements are disjoint involutions (Babai & Luks 1983). |
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Andres R. Vindas Melendez (University of Kentucky) | 22.01.2020 | 16:30 | MA 621 | The Equivariant Ehrhart Theory of the Permutahedron |

Abstract: In 2010, Stapledon described a generalization of Ehrhart theory with group actions. In 2018, Ardila, Schindler, and I made progress towards answering one of Stapledon's open problems that asked to determine the equivariant Ehrhart theory of the permutahedron. We proved some general results about the fixed polytopes of the permutahedron, which are the polytopes that are fixed by acting on the permutahedron by a permutation. In particular, we computed their dimension, showed that they are combinatorially equivalent to permutahedra, provided hyperplane and vertex descriptions, and proved that they are zonotopes. Lastly, we obtained a formula for the volume of these fixed polytopes, which is a generalization of Richard Stanley's result of the volume for the standard permutahedron. Building off of the work of the aforementioned, we determine the equivariant Ehrhart theory of the permutahedron, thereby resolving the open problem. This project presents combinatorial formulas for the Ehrhart quasipolynomials and Ehrhart Series of the fixed polytopes of the permutahedron, along with other results regarding interpretations of the equivariant analogue of the Ehrhart series. This is joint work with Federico Ardila (San Francisco State University) and Mariel Supina (UC Berkeley). |
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Simon Telen (KU Leuven) | 15.01.2020 | 16:30 | MA 621 | Numerical Root Finding via Cox Rings |

Abstract: In this talk, we consider the problem of solving a system of (sparse) Laurent polynomial equations defining finitely many nonsingular points on a compact toric variety. The Cox ring of this toric variety is a generalization of the homogeneous coordinate ring of projective space. We work with multiplication maps in graded pieces of this ring to generalize the eigenvalue, eigenvector theorem for root finding in affine space. We use numerical linear algebra to compute the corresponding matrices, and from these matrices a set of homogeneous coordinates of the solutions. Several numerical experiments show the effectiveness of the resulting method, especially for solving (nearly) degenerate, high degree systems in small numbers of variables. |