Baer Colloquium
June 24, 2017 — Ghent, Belgium

Organizers: T. De Medts and H. Van Maldeghem

Location: Krijgslaan 281, campus "De Sterre", building S25, lectures in room "Emmy Noether"

Schedule of Talks

Saturday, June 24, 2017
10:00h - 10:30h Coffee and tea
10:30h - 11:30h Bertrand Rémy  (École Polytechnique, Palaiseau)
Wonderful compactifications of Bruhat–Tits buildings

We will introduce the general theme of compactifying buildings attached to semisimple groups over local fields. In the case of a split group, we can identify equivariantly the maximal Satake–Berkovich compactification of the corresponding Euclidean building with the compactification obtained by embedding the building in the Berkovich analytic space associated to the wonderful compactification of the group. The relationship between the structures at infinity, one coming from strata of the wonderful compactification and the other from Bruhat–Tits buildings, can be described.

➡  Related paper on arXiv

11:30h - 12:00h Coffee break
12:00h - 13:00h Michael Joswig  (T.U. Berlin)
Matroids from hypersimplex splits

Studying regular subdivisions of a class of convex polytopes which are called hypersimplices naturally leads to a new class of matroids, which we call split matroids. Very many matroids are of this type; e.g., the paving matroids arise as special cases. It turns out that the structural properties of the split matroids can be exploited to obtain new results in tropical geometry. Interestingly, prominent examples of finite geometries, such as the Fano plane, play an important role. Joint work with Benjamin Schröter.

➡  Download Presentation in PDF

13:00h - 14:30h Lunch break
14:30h - 15:30h Karsten Naert  (Universiteit Gent)
Suzuki–Ree groups as algebraic groups over \( \mathbb{F}_{\sqrt{p}} \)

Amongst the finite simple groups, the Suzuki–Ree groups are peculiar, because they "are not algebraic groups"; more precisely they are not in a natural way (derived subgroups of groups) of the form \( G(K) \), where \( G \) is an algebraic \( k \)-group and \( K \) a \( k \)-algebra. Intuitively though, they should be algebraic groups over the field with \( \sqrt p \) elements.

We will make this intuition precise by constructing the 'field' \( \mathbb F_{\sqrt p} \) and algebro-geometric objects (rings, varieties, schemes, algebraic groups, ...) over it, and we will explain how in this setting Suzuki–Ree groups are algebraic groups. The construction is categorical in nature, and expands the category of schemes over \( \mathbb F_p \) to a category of twisted schemes, which can be interpreted as a category of 'schemes over \( \mathbb F_{\sqrt p} \)'.
This approach also suggests a hidden world of objects over \( \mathbb F_p \) which we relate to Tits's mixed groups and buildings and to exotic pseudo-reductive groups.

➡  Download Presentation in PDF
➡  Related paper on arXiv

15:30h - 16:00h Coffee break
16:00h - 17:00h Jost-Hinrich Eschenburg  (Universität Augsburg)
Rosenfeld planes

The following is mainly work of my PhD student Erich Dorner.

The most important compact symmetric spaces of exceptional type have dimensions 16, 32, 64, 128, and their isometry groups are the exceptional groups of type \( F_4 \), \( E_6 \), \( E_7 \), \( E_8 \). The first space is the octonionic projective plane. Hans Freudenthal has developed an own incidence geometry for each of these spaces. But Boris Rosenfeld back in 1956 tried to describe them as projective planes over the algebra \( \mathbb{K} \otimes \mathbb{O} \) where \( \mathbb{O} \) denotes the octonions and \( \mathbb{K} \) one of the 4 normed division algebras \( \mathbb{R},\mathbb{C},\mathbb{H},\mathbb{O} \).

Rosenfeld's approach failed, but these spaces do have a strong relationship to \( (\mathbb{K}\otimes\mathbb{O})^2 \). We show that \( (\mathbb{K}\otimes\mathbb{L})^2 \) for any two normed division algebras \( \mathbb{K}\subset\mathbb{L} \) (with dimension \( k,l \)) carries the spin representation for \( \operatorname{Spin}_{k+l} \) which allows a simple construction of the exceptional Lie algebras and the Rosenfeld planes. We strongly use \( (\mathbb{K}\otimes\mathbb{L})^2 \) also for associative \( \mathbb{L} \) which correspond to certain Grassmannians. They greatly help understanding the structure of the Rosenfeld planes in which they are contained.

➡  [preprint available upon request]


Registration is closed.

Getting around in Ghent

In case you plan to arrive on Friday, this list of hotels might help you. If you are interested to visit Ghent (which is a beautiful historical city, voted "most pleasant city of Flanders" in 2005), the website is a good starting point to find more information.

This map might help you to find the precise location of the department of mathemematics, where the event will take place. It can easily be reached by train (15 minute walk from the main station) or by highway (E17 or E40).

More information

If you need more information, you can contact one of the organizers, by sending an email message to  or

(last update: June 26, 2017)