Bildarchiv des Mathematischen Forschungsinstituts Oberwolfach


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

  • Recent preprints on the ArXiv.
  • Latest preprints:

    Relative complete reducibility and normalised subgroups

    with M. Gruchot and A. Litterick
    We study a relative variant of Serre's notion of G-complete reducibility for a reductive algebraic group G. We let K be a reductive subgroup of G, and consider subgroups of G which normalise the identity component K^o. We show that such a subgroup is relatively G-completely reducible with respect to K if and only if its image in the automorphism group of K^o is completely reducible. This allows us to generalise a number of fundamental results from the absolute to the relative setting. We also derive analogous results for Lie subalgebras of the Lie algebra of G, as well as `rational' versions over non-algebraically closed fields.

    On the Invariants of the Cohomology of Complements of Coxeter Arrangements

    with J.M. Douglass and G. Pfeiffer
    We refine Brieskorn's study of the cohomology of the complement of the reflection arrangement of a finite Coxeter group W. As a result we complete the verification of a conjecture by Felder and Veselov that gives an explicit basis of the space of W-invariants in this cohomology ring.

    A Hodge filtration of logarithmic vector fields for well-generated complex reflection groups

    with T. Abe, C. Stump and M. Yoshinaga
    Given an irreducible well-generated complex reflection group, we construct an explicit basis for the module of vector fields with logarithmic poles along its reflection arrangement. This construction yields in particular a Hodge filtration of that module. Our approach is based on a detailed analysis of a flat connection applied to the primitive vector field. This generalizes and unifies analogous results for real reflection groups.

    Relative GL(V)-complete reducibility

    with M. Bate and M. Gruchot
    Let K be a reductive subgroup of a reductive group G over an algebraically closed field k. Using the notion of relative complete reducibility, in previous work of Bate-Martin-Roehrle-Tange a purely algebraic characterization of the closed K-orbits in G^n was given, where K acts by simultaneous conjugation on n-tuples of elements from G. This characterization generalizes work of Richardson and is also a natural generalization of Serre's notion of G-complete reducibility. In this paper we revisit this idea, focusing on the particular case when the ambient group G is a general linear group, giving a representation-theoretic characterization of relative complete reducibility. Along the way, we extend and generalize several results from the aforementioned work of Bate-Martin-Roehrle-Tange.

    Arrangements of ideal type are inductively free

    with M. Cuntz and A. Schauenburg
    Extending earlier work by Sommers and Tymoczko, in 2016 Abe, Barakat, Cuntz, Hoge, and Terao established that each arrangement of ideal type A_I stemming from an ideal I in the set of positive roots of a reduced root system is free. Recently, Röhrle showed that a large class of the A_I satisfy the stronger property of inductive freeness and conjectured that this property holds for all A_I. In this article, we confirm this conjecture.

    The K(pi,1)-problem for restrictions of complex reflection arrangements

    with Nils Amend
    In 1962, Fadell and Neuwirth showed that the configuration space of the braid arrangement is aspherical. Having generalized this to many real reflection groups, Brieskorn conjectured this for all Coxeter groups. This follows from Deligne's seminal work from 1972, where he showed that the complexification of every real simplicial arrangement is a K(pi,1)-arrangement. In the 1980s Nakamura and Orlik-Solomon established this property for all but six irreducible unitary reflection groups. These outstanding cases were resolved in 2015 in stunning work utilizing Garside theory by Bessis. In this paper we show that the K(pi,1)-property extends to all restrictions of complex reflection arrangements with the possible exception of only 11 restrictions stemming from some non-real exceptional groups.

    Freeness of multi-reflection arrangements via primitive vector fields

    with T. Hoge, T. Mano, and C. Stump
    In 2002, Terao showed that every reflection multi-arrangement of a real reflection group with constant multiplicity is free by providing a basis of the module of derivations. We first generalize Terao's result to multi-arrangements stemming from well-generated unitary reflection groups, where the multiplicity of a hyperplane depends on the order of its stabilizer. Here the exponents depend on the exponents of the dual reflection representation. We then extend our results further to all imprimitive irreducible unitary reflection groups. In this case the exponents turn out to depend on the exponents of a certain Galois twist of the dual reflection representation that comes from a Beynon-Lusztig type semi-palindromicity of the fake degrees.


    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 (Universität Hannover)
    Anne Schauenburg
    Maike Gruchot
    Tilman Möller
    Falk Bannuscher

    Editorial Activity:

    Tbilisi Mathematical Journal .

    Computer Programs