**Ort und Zeit:**

Mi 14-15.30 Uhr, Seminarraum 1 (Raum 005)

**Sommersemester 2017**

19.04. | Nico Schreiber |

26.04. | Mohammad Bardestani, Jonas Mahnkopp |

03.05. | Stefan Krupp |

14.06. | Anna Gundert |

21.06. | Frank Vallentin |

28.06. | Fabian Hillmann |

05.07. | Daniel Rumi, Michael Lohaus |

12.07. | Marc Christian Zimmermann |

11.09 | Sören Stahlmann |

28.09 | Björn Rahn |

**Wintersemester 2016/17**

26.10. | Jan Arntzen |

2.11. | Halil Ibrahim Ertik |

9.11. | Fabian Hillmann |

16.11. | Alexander Pütz |

23.11. | Kein Oberseminar |

30.11. | Jan Rolfes |

7.12. | Kein Oberseminar |

14.12. | Workshop "Combinatorial Optimization meets Parameterized Complexity" in Bonn |

21.12. | Katharina Kehrle |

11.1. | Kein Oberseminar |

18.1. | Kein Oberseminar |

25.1. | Maryna Viazovska - The sphere packing problem in dimensions 8 and 24 Institutskolloquium |

1.2. | Kein Oberseminar |

8.2. | Kein Oberseminar |

14.2. (10 Uhr) | Niklas Niemczyk |

**Sommersemester 2016**

13.04. | Vorbesprechung Seminar: Konvexe Optimierung |

20.04. | Cristóbal Guzmán (CWI Amsterdam) - An optimal affine-invariant smooth minimization algorithm |

27.04. | Kein Oberseminar |

04.05. | Aron Rahman, Cordian Riener (Aalto) |

11.05. | Jan Arntzen |

18.05. | Pfingstferien |

25.05. | Raman Sanyal (FU Berlin) |

01.06. | Kein Oberseminar |

08.06. | Anna Karasoulou (Athen) - Approximating Multidimensional Subset Sum and Minkowski Decomposition of polytopes |

15.06. | Maria Infusino (Konstanz) |

22.06. | Maria Dostert |

29.06. | Kein Oberseminar |

06.07. | Fernando Mário de Oliveira Filho - Flag algebras, a first glance |

13.07. | Kein Oberseminar |

06.09. | Sondertermin 14:00 Uhr SR2 (MI): Tim-Lasse Bohm |

**Wintersemester 2015/16**

21.10. | Kein Oberseminar |

28.10. | Kein Oberseminar |

04.11. | Anna Gundert |

11.11. | Kein Oberseminar |

18.11. | Hasan Oruc |

25.11. | Robert Scheidweiler (RWTH Aachen) - Reconstructing finite subsets of the plane up to some groups of isometries (abstract) |

02.12. | Katharina Kehrle |

09.12. | Alexandre d'Aspremont - Ranking from pairwise comparisons using Seriation im Institutskolloquium |

16.12. | Carina Schmitz |

13.01. | Kein Oberseminar |

20.01. | Kein Oberseminar |

27.01. | Alexander Pütz |

03.02. | Kein Oberseminar |

10.02. | Kein Oberseminar |

26.02. 15Uhr | Fabrício C. Machado |

**Sommersemester 2015**

8.4. | kein Oberseminar |

15.4. | kein Oberseminar |

22.4. | Wiebke Lindt, Carina Schmitz |

29.4. | Frederik von Heymann |

6.5. | Frank Vallentin |

13.5. | kein Oberseminar |

20.5. | Berk Öcal |

27.5. | Pfingstferien |

3.6. | Christine Bachoc, Aron Rahman |

10.6. | Lydia Maier |

17.6. | Greta Fischer |

24.6. | Jan Rolfes |

1.7. | Matthias Schwarz, Hasan Oruc |

8.7. | kein Oberseminar |

15.7. | kein Oberseminar |

**Wintersemester 2014/15**

8.10. | Moritz Firsching (FU Berlin) - Applications of nonlinear optimization and integer relations algorithms in computational geometryshow abstract
Given two 3-dimensional polyhedra $P$ and $Q$, we can ask for the largest |

15.10. | kein Oberseminar |

22.10 | Robin Daniels, Lea Loepke |

29.10. | kein Oberseminar |

5.11. | Fernando Mário de Oliveira Filho (University of São Paulo) |

12.11. | Sven Labusch |

19.11. | kein Oberseminar |

26.11. | David de Laat (TU Delft) - Sum of squares polynomials: a symmetrized version of Putinar's theorem |

3.12. | Anna Gundert |

10.12. | Kristóf Huszár (IST Austria) - Random Latin Squares and 2-Dimensional Expandersshow abstract
For more than forty years, expander graphs have been playing an important role in many areas of mathematics, especially in combinatorics and theoretical computer science. In recent years, there is an increasing interest in high-dimensional generalizations of expansion, as well. After introducing the notion of $\mathbb{F}_2$-coboundary expansion for simplicial complexes, we will discuss the key ideas of a paper by Lubotzky and Meshulam, in which the authors establish the existence of an infinite family of 2-dimensional expanders with bounded edge degree. |

17.12. | Lydia Maier, Wiebke Lindt |

7.1. | kein Oberseminar |

14.1. | Jan Rolfes |

21.1. | Greta Fischer, Julia Cynarski |

28.1. | Maria Dostert |

4.2. | Vorbesprechungen Sommersemester 2015 |

4.3. 13 Uhr | David de Laat (TU Delft) |

18.3. 13 Uhr | Sven Labusch |

**Sommersemester 2014**

16.4. | Kein Seminar |

23.4. | Walid Schalesi |

30.4. | Mathieu Dutour Sikiric (Rudjer Bosković Institute) - Polytopes, Polyhedral tessellations, cohomology, Hecke operators and modular forms.show abstract
The computation of modular forms is a major problem in number theory. Here, we propose to consider the space of modular as a cohomology group and to compute it with polyhedral methods. |

7.5. | May Szedlák (ETH Zürich) - Higher Dimensional Discrete Cheeger Inequalitiesshow abstract
For graphs there exists a strong connection between spectral expansion and edge expansion. This is expressed, e.g., by the Cheeger inequality, which states that $\lambda(G) \leq h(G)$, where $\lambda(G)$ is the second smallest eigenvalue of the Laplacian of $G$ and $h(G)$ the Cheeger constant measuring the edge expansion of $G$. We are interested in generalizations of expansion properties to finite simplicial complexes of higher dimension. Whereas for simplicial complexes, higher dimensional Laplacians were introduced already in 1945 by Eckmann, the generalization of edge expansion is not straightforward. Recently, a topologically motivated notion analogous to edge expansion was introduced by Gromov and independently by Linial, Meshulam and Wallach and by Newman and Rabinovich. It is known that for this generalization there is no higher dimensional analogue of the lower bound of the Cheeger inequality. A different, combinatorially motivated generalization of the Cheeger constant, denoted by $h(X),$ was studied by Parzanchevski, Rosenthal and Tessler. They showed that indeed $\lambda(X) \leq h(X)$, where $\lambda(X)$ is the smallest non-trivial eigenvalue of the Laplacian, for the case of $k$-dimensional simplicial complexes $X$ with complete $(k-1)$-skeleton. Whether this inequality also holds for k-dimensional complexes with non-complete $(k-1)$-skeleton has been an open question. We give two different strengthenings of the inequality for arbitrary complexes. |

14.5. | Jan Rolfes, Anna-Lena Tychsen |

21.5. | Raman Sanyal (FU Berlin), Marius Recher Raman Sanyal - Relative Upper Bound Theorems show abstract
The Upper Bound Theorem, conjectured by Motzkin and proved by McMullen, is one of the cornerstones of discrete geometry: Neighborly simplicial polytopes maximize the number of k-dimensional faces among all d-dimensional convex polytopes with a fixed number of vertices. Stanley vastly generalized the Upper Bound Theorem by showing that it even holds for general triangulations of spheres (and beyond). In this talk I will present a generalization of Stanley’s approach that yields an algebraic framework for treating relative upper bound problems. As showcases I will present solutions to combinatorial isometric problems and an Upper Bound Theorem for Minkowski sums. The talk will be a scenic tour from geometry to algebra and back. This is joint work with Karim Adiprasito (IHES). |

28.5. | Nina Berghoff, Stefan Hoppe |

4.6. | Kein Seminar |

18.6. | Elena Kremser, Robin Daniels |

25.6. | Fabian Hillmann, Jan Arntzen |

2.7. | Fernando Mário de Oliveira Filho (University of São Paulo) |

9.7. | Kein Seminar |

16.7. | Stefan Krupp, Matthias Schwarz (Zeit: 16-17:30 Uhr, Ort: MI Übungsraum 1) |

29.9. | Sondertermin: Kolloquien der Bachelorarbeiten. Seminarraum 2 im MI10-10:45 Uhr Tychsen 11-11:45 Uhr Berghoff 13-13:45 Uhr Hoppe 14-14:45 Uhr Arntzen 15-15:45 Uhr Hillmann 16-16:45 Uhr Krupp |

**Wintersemester 2013/14**

23.10.2013 | Marius Recher |

30.10.2013 | Frederik von Heymann |

6.11.2013 | David de Laat (TU Delft) - Flat extensions in noncommutative optimization |

13.12.2013 | Kein Seminar |

20.11.2013 | Lea Loepke, Elena Kremser |

27.11.2013 | Kein Seminar |

4.12.2013 | Evan DeCorte (TU Delft) - Low-dimensional spherical sets avoiding orthogonal pairs of pointsshow abstract
Let $M_n$ be the maximum surface measure of a subset of the $n$-dimensional unit sphere not containing a pair of orthogonal vectors. A 1974 conjecture of H.S. Witsenhausen states that the extremal configuration is given by two diametrically opposite spherical caps of equal size. In the same article, Witsenhausen proves $M_n \le 1/n$. A result of A.M. Raigoroskii from 1999 gives exponentially decreasing upper bounds for $M_n$ via a Frank-Wilson type approach. However for $n=3$, the original $1/3$ bound of Witsenhausen is the best known. In this talk we outline a proof that $M_3$ is strictly less than $1/3$. This is joint work with Oleg Pikhurko. |

11.12.2013 | Walid Schalesi |

18.12.2013 | Katharina Kehrle |

15.1.2014 | Timo de Wolff (Saarbrücken) - Amoebas, Nonnegative Polynomials and Sums of Squares Supported on Circuitsshow abstract
We completely characterize sections of the cones of nonnegative polynomials, convex polynomials and sums of squares with polynomials supported on circuits – a genuine class of sparse polynomials. In particular, nonnegativity is characterized by an invariant, which can be immediately derived from the initial polynomial. Based on these results, we obtain a completely new class of nonnegativity certificates independent from sums of squares certificates. Furthermore, nonnegativity of such polynomials f coincides with solidness of the amoeba of f, i.e., the Log-absolute-value image of the variety of f. To the best of our knowledge this is the first connection established between amoeba theory and nonnegativity of polynomials. These results generalize earlier works both in amoeba theory and real algebraic geometry by Fidalgo, Kovacec, Reznick, Theobald and myself. This talk is based on work in progress joint with Sadik Iliman. |

22.1.2014 | Oliver Schaudt (Köln) - Frankl's conjectureshow abstract
Frankl’s conjecture (1979) says that in a finite family of sets, closed under union, there is an element contained in at least half of the sets. Although simple-sounding, the conjecture is considered as one of the tough problems in combinatorics. In this talk I will sketch the history of the conjecture and the most successful approaches so far. Joint work with Henning Bruhn, Pierre Charbit, and Jan-Arne Telle. |

29.1.2014 | Sjoerd Dirksen (Hausdorff Center Bonn) - A Johnson-Lindenstrauss embedding for general data setsshow abstract
The Johnson-Lindenstrauss lemma says that one can embed a set of n points in a (high dimensional) Euclidean space into an m-dimensional space using a random map, while approximately preserving the distances between the points in the set. Surprisingly, the embedding dimension m does not depend on the original dimension of the data: it scales logarithmically in terms of the number of points. This result has found many applications, e.g. in nearest-neighbor search. In my talk I will present a generalization of the Johnson-Lindenstrauss lemma, which allows one to embed arbitrary data sets (i.e. also of infinite cardinality) in a low-dimensional space using a random map. The result unifies (and improves) several dimensionality reduction results for specific data sets in the literature, such as sets of sparse vectors, low rank matrices and smooth manifolds. |

5.2.2014 | Vorbesprechungen für Sommersemester 2014 |

31.3.2014 | Katharina Kehrle - Kolloquium der Bachelorarbeit |