30.10.2018 um 16:15 Uhr in 69/125:
Markus Wageringel (Universität Osnabrück)
Moment ideals of local Dirac mixture
We study a special case of finite mixture models, the Dirac local mixtures, from an algebraic and geometric point of view, highlighting connections to algebraic statistics, symmetric tensor decomposition and signal processing. We focus on the moment varieties of first order local mixtures, providing generators for the ideals and showing a connection to moment ideals of Pareto distributions. Further, we consider mixture models of these distributions and investigate the problem of recovering the parameters of such a distribution from its moments.
This is joint work with Alexandros Grosdos Koutsoumpelias.
06.11.2018 um 16:15 Uhr in 69/125:
Dominik Nagel (Universität Osnabrück)
Condition numbers of Vandermonde matrices with nearly-colliding nodes
The condition number of rectangular Vandermonde matrices with nodes on the complex unit circle is important for the stability analysis of algorithms that solve trigonometric moment problems, e.g. Prony’s method. In the univariate case and with well-separated nodes, this condition number is well understood, but if nodes are nearly-colliding, the situation becomes more complicated. After recalling Prony’s method, results for the condition number of Vandermonde matrices with pairs of nearly-colliding nodes are presented.
Joint work with: Stefan Kunis.
12.11.2018 um 14:15 Uhr in 69/E15 :
Emily King (Universität Bremen)
Combinatorics and Discrete Geometry in (Hilbert Space) Frames
Frames are collections of vectors in Hilbert spaces which have reconstruction properties akin to orthonormal bases. In order for such a representation system to be robust in applications, one often asks that the vectors be geometrically spread apart; that is, the pairwise angles between the lines they span should be as large as possible. It ends up that structures in algebraic combinatorics, like difference sets and balanced incomplete block designs (BIBDs), can be used in different ways to construct optimal configurations. Furthermore, the linear dependencies of the vectors are often encoded as BIBDs, like affine geometry. In this talk, these and other connections between frames and algebraic combinatorics, combinatorial design theory, algebraic graph theory, and more will be presented. A couple of open conjectures will also be discussed, with one in particular possibly being amenable to combinatorial methods. Frames, difference sets, BIBDs, and affine geometry will be defined and explained, making at least part of the talk accessible to a more general audience.
27.11.2018 um 16:15 Uhr in 69/125:
Lorenzo Venturello (Universität Osnabrück)
Balanced triangulations on few vertices
A natural and challenging problem in combinatorial and computational topology asks for the minimum number of vertices needed to triangulate a certain manifold. Moreover since in dimension greater than 2 the number of vertices does not uniquely determine the number of higher dimensional faces, one can ask what conditions are posed on those by the topology. In this talk we focus on the family of balanced simplicial complexes, that is d-dimensional complexes whose underlying graph is (d+1)-colorable, and we exhibit balanced triangulations of surfaces and 3-manifolds on few vertices which are the result of an implementation of local flips preserving the coloring condition. This serves as an excuse to give an overview of classical results on face enumeration, both in the general and in the balanced setting.
11.12.2018 um 16:15 Uhr in 69/125 :
Gaku Liu (Max Planck Institut Leipzig)
Cubical Pachner moves and cobordisms of immersions
We study various analogues of theorems from PL topology for cubical complexes. In particular, we characterize when two PL homeomorphic cubulations are equivalent by Pachner moves by showing the question to be equivalent to the existence of cobordisms between immersions of hypersurfaces. This solves a question and conjecture of Habegger and Funar. The main tool is a theorem to show that any cubical PL decomposition of a disk is regular after some cubical stellar subdivision; this extends a result of Morelli. Joint work with Karim Adiprasito.