Ruhr-Universität Bochum

Fakultät für Mathematik

IB 2/133

Universitätsstrasse 150

D-44780 Bochum

Tel.: +49 (0)234/32-28304

Fax: +49 (0)234/32-14025

eMail: gerhard.roehrle@rub.de

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Bildarchiv des Mathematischen Forschungsinstituts Oberwolfach

## Arbeitsgebiete

- Algebraische Lie Theorie
- Algebraische Gruppen
- Darstellungstheorie
- Endliche Gruppen vom Lie-Typ
- Hyperebenenarrangements
- Spiegelungsarrangements

- DFG project: On Ziegler Extensions of Multiarrangements (within the DFG Priority Programme Combinatorial Synergies).

- DFG project: On connected subgraph arrangements (within the DFG Priority Programme Combinatorial Synergies).

- DFG project: Serre's notion of Complete Reducibility and Geometric Invariant Theory (within the
DFG Priority Programme in Representation Theory).

- DFG project: Computational aspects of the Cohomology of Coxeter arrangements: On Conjectures of Lehrer-Solomon and Felder-Veselov (within the
DFG Priority Programme "Algorithmic and experimental methods in Algebra,
Geometry and Number Theory").

- DFG project: Arrangements of complex reflection groups: Geometry and combinatorics (joint with M. Cuntz (Kaiserslautern) within the
DFG Priority Programme "Algorithmic and experimental methods in Algebra, Geometry and Number Theory").

- DFG project: Inductive freeness and rank-generating functions for arrangements of ideal type: Two conjectures of Sommers and Tymoczko revisited (within the DFG GEPRIS).

- DFG project: On the Cohomology of complements of complex reflection arrangements (within the DFG GEPRIS).

- DFG project: Inductive freeness of Ziegler's canonical multiplicity (within the DFG GEPRIS).

- DFG project: Overgroups of distinguished unipotent elements in reductive groups (within the DFG GEPRIS).

- DFG project: On hyperfactored and recursively factored arrangements (within the DFG GEPRIS).

- Publications on
MathSciNet.

- Recent preprints on the
ArXiv.

- Latest preprints:

**Free multiderivations of connected subgraph arrangements**

with Paul Mücksch and Sven Wiesner

Abstract:

Cuntz and Kühne introduced the class of connected subgraph arrangements A_G, depending on a graph G, and classified all graphs G such that the corresponding arrangement A_G is free. We extend their result to the multiarrangement case and classify all graphs G for which the corresponding arrangement A_G supports some multiplicity m such that the multiarrangement (A_G,m) is free.

math.CO/2406.19866

**The subgroup structure of pseudo-reductive groups**

with Michael Bate, Ben Martin, and Damian Sercombe

Abstract:

Let $k$ be a field. We investigate the relationship between subgroups of a pseudo-reductive $k$-group $G$ and its maximal reductive quotient $G'$, with applications to the subgroup structure of $G$. Let $k'/k$ be the minimal field of definition for the geometric unipotent radical of $G$, and let $\pi':G_{k'} \to G'$ be the quotient map. We first characterise those smooth subgroups $H$ of $G$ for which $\pi'(H_{k'})=G'$. We next consider the following questions: given a subgroup $H'$ of $G'$, does there exist a subgroup $H$ of $G$ such that $\pi'(H_{k'})=H'$, and if $H'$ is smooth can we find such a $H$ that is smooth? We find sufficient conditions for a positive answer to these questions. In general there are various obstructions to the existence of such a subgroup $H$, which we illustrate with several examples. Finally, we apply these results to relate the maximal smooth subgroups of $G$ with those of $G'$.

math.GR/2406.11286

G-complete reducibility and saturation

Michael Bate, Sören Böhm, Alastair Litterick, and Benjamin Martin

Abstract:

Let H < G be connected reductive linear algebraic groups defined over an algebraically closed field of characteristic p> 0. In our first principal theorem we show that if a closed subgroup K of H is H-completely reducible, then it is also G-completely reducible in the sense of Serre, under some restrictions on p, generalising the known case for G = GL(V). Our second main theorem shows that if K is H-completely reducible, then the saturation of K in G is completely reducible in the saturation of H in G (which is again a connected reductive subgroup of G), under suitable restrictions on p, again generalising the known instance for G = GL(V). We also study saturation of finite subgroups of Lie type in G. Here we generalise a result due to Nori from 1987 in case G = GL(V).

math.RT/2401.16927

Inductive Freeness of Ziegler's Canonical Multiderivations for Restrictions of Reflection Arrangements

with Torsten Hoge and Sven Wiesner

Abstract:

Let A be a free hyperplane arrangement. In 1989, Ziegler showed that the restriction A'' of A to any hyperplane endowed with the natural multiplicity k is then a free multiarrangement. Recently, in [Hoge-Röhrle2022], an analogue of Ziegler's theorem for the stronger notion of inductive freeness was proved: if A is inductively free, then so is the free multiarrangement (A'',k)$. In [Hoge-Röhrle2018], all reflection arrangements which admit inductively free Ziegler restrictions are classified. The aim of this paper is an extension of this classification to all restrictions of reflection arrangements utilizing the aforementioned fundamental result from [Hoge-Röhrle2022].

math.GR/2210.00436

Invariants and semi-invariants in the cohomology of the complement of a reflection arrangement

with J. Matthew Douglass and Götz Pfeiffer

Abstract:

Suppose V is a finite dimensional, complex vector space, A is a finite set of codimension one subspaces of V, and G is a finite subgroup of the general linear group GL(V) that permutes the hyperplanes in A. In this paper we study invariants and semi-invariants in the graded QG-module H^*(M(A)), where M(A) denotes the complement in V of the hyperplanes in A and H^* denotes rational singular cohomology, in the case when G is generated by reflections in V and A is the set of reflecting hyperplanes determined by G, or a closely related arrangement. Our first main result is the construction of an explicit, natural (from the point of view of Coxeter groups) basis of the space of invariants, H^*(M(A))^G. In addition to providing a conceptual proof of a conjecture due to Felder and Veselov for Coxeter groups, this result extends the latter to all finite complex reflection groups. Moreover, we prove that determinant-like characters of complex reflection groups do not occur in H^*(M(A)). This extends to all finite complex reflection groups a result proved for Weyl groups by Lehrer.

math.RT/2009.12847

**
Former and current PhD students:**

Simon Goodwin
(University of Birmingham)

Michael Bate
(University of York)

Russell Fowler (Npower)

Glenn Ubly (NHS)

Sebastian Herpel (Barmenia Allgemeine Versicherungs-AG)

Peter Mosch (Volkswohl Bund)

Nils Amend (Volkswohl Bund)

Anne Schauenburg (Aldi International Services)

Maike Gruchot (Barmenia Krankenversicherung AG)

Tilman Möller

Falk Bannuscher

Sören Böhm

Sven Wiesner

Lorenzo Giordani

Editorial Activity:

Advanced Studies: Euro-Tbilisi Mathematical Journal
.

**Computer Programs**