Ruhr-Universität Bochum

Fakultät für Mathematik

Universitätsstrasse 150

D-44780 Bochum

NA 2/33

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

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

eMail: gerhard.roehrle@rub.de

Sprechstunde:

dienstags 12-13 Uhr und nach Vereinbarung

## Arbeitsgebiete

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

- 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").

- Publications on
MathSciNet.

- Recent preprints on the
ArXiv.

- Latest preprints:

Divisionally free Restrictions of Reflection Arrangements

Abstract:

We study some aspects of divisionally free arrangements which were recently introduced by Abe. Crucially, Terao's conjecture on the combinatorial nature of freeness holds within this class. We show that while it is compatible with products, surprisingly, it is not closed under taking localizations. In addition, we determine all divisionally free restrictions of all reflection arrangements.

math.CO/1510.00213

Nice reflection arrangements

with T. Hoge

Abstract:

The aim of this note is a classification of all nice and all inductively factored reflection arrangements. It turns out that apart from the supersolvable instances only the monomial groups G(r,r,3) for r at least 3 give rise to nice reflection arrangements. As a consequence of this and of the classification of all inductively free reflection arrangements from our earlier work, we deduce that the class of all inductively factored reflection arrangements coincides with the supersolvable reflection arrangements. Moreover, we extend these classifications to hereditarily factored and hereditarily inductively factored reflection arrangements.

math.GR/1505.04603

Inductive and Recursive Freeness of Localizations of Multiarrangements

with T. Hoge and A. Schauenburg

Abstract:

The class of free multiarrangements is known to be closed under taking localizations. We extend this result to the stronger notions of inductive and recursive freeness. As an application, we prove that recursively free multiarrangements are compatible with the product construction for multiarrangements. In addition, we show how our results can be used to derive that some canonical classes of free multiarrangements are not inductively free.

math.CO/1501.06312

Cocharacter-closure and the rational Hilbert-Mumford Theorem

with M. Bate, S. Herpel, and B. Martin

Abstract:

For a field k, let G be a reductive k-group and V an affine k-variety on which G acts. Using the notion of cocharacter-closed G(k)-orbits in V, we prove a rational version of the celebrated Hilbert-Mumford Theorem from geometric invariant theory. We initiate a study of applications stemming from this rationality tool. A number of examples are discussed to illustrate the concept of cocharacter-closure and to highlight how it differs from the usual Zariski-closure.

math.GR/1411.7849

Restricting invariants of unitary reflection groups

with N. Amend, A. Berardinelli, and J. M. Douglass

Abstract:

Suppose that G is a finite, unitary reflection group acting on a complex vector space V and X is the fixed point subspace of an element of G. Define N to be the setwise stabilizer of X in G, Z to be the pointwise stabilizer, and C=N/Z. Then restriction defines a homomorphism from the algebra of G-invariant polynomial functions on V to the algebra of C-invariant functions on X. Extending earlier work by Douglass and Roehrle for Coxeter groups, we characterize when the restriction mapping is surjective for arbitrary unitary reflection groups G in terms of the exponents of G and C, and their reflection arrangements. A consequence of our main result is that the variety of G-orbits in the G-saturation of X is smooth if and only if it is normal.

math.RT/1503.00329

**
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 (Gothaer Lebensversicherung AG)

Nils Amend

Anne Schauenburg

Editorial Activity:

Tbilisi Mathematical Journal
.

**Computer Programs**