Vorträge

Sometimes it is convenient to have a bi-infinite enumeration of the basis elements in the domain and image spaces of an operator A - leading to a representation of A by a bi-infinite matrix.
Shifting one of these enumerations shifts the matrix and hence changes the main diagonal. So which diagonal is ''the'' main diagonal? Isreal Gohberg once diplomatically said that in a bi-infinite matrix, it is every diagonal's right to claim to be the main diagonal. However, there are concrete problems in numerics and in matrix algebra that require a concrete choice - and, as it turns out, the choices coincide: From a certain point of view, there is one distinguished diagonal that deserves being called the main diagonal (a bit more than the others). We show how to find it and we discuss examples.

This is joint work with Gilbert Strang.

We adapt the grammar introduced by Chapuy, Fusy, Kang and Shoilekova to study bipartite graph families which are defined by their 3-connected components. More precisely, in this talk I will explain how to get the counting formulas for bipartite series-parallel graphs (and more generally of the Ising model over this family of graphs), as well as asymptotic estimates for the number of such graphs with a fixed size. This talk is based in a work in progress joint with Kerstin Weller.

... ist vielleicht nur für die Analytiker interessant.

We describe a general approach to the strong hypergraph regularity lemma, which we call 'regular slices', which avoids many of the usual technical complications and retains the features one would like to use in extremal hypergraph theory. This talk will avoid painful technical details in so far as that is possible and focus on an application, proving a hypergraph extension of the Erdos-Gallai theorem.

This is joint work with Julia Böttcher, Oliver Cooley and Richard Mycroft.