A pdf-copy of the program is available here.
Thursday May 11
  • 14:00 - 15:00: Pflaum
  • 15:15 - 16:15: Arici
  • 16:15 - 16:45: Coffee break
  • 16:45 - 17:45: Lang
  • 18:30: Conference Dinner (in the Faculty Club)
Friday May 12
  • 09:45 - 10:00: Coffee
  • 10:00 - 11:00: Bailey
  • 11:15 - 12:15: Werndli
  • 12:15 - 14:00: Lunch break
  • 14:00 - 15:00: Klaasse
  • 15:00 - 15:30: Coffee break
  • 15:30 - 16:30: Crisp


All talks take place in room HG00.071 in the Huygensgebouw, Heyendaalseweg 135, Nijmegen, The Netherlands (Travel Directions)


This is the ninth in a series of short workshops jointly organized by the Geometry/Topology groups in Bonn, Nijmegen, and Utrecht, all situated along the Lower Rhine. The focus lies on the development and application of new structures in geometry and topology such as Lie groupoids, differentiable stacks, Lie algebroids, generalized complex geometry, topological quantum field theories, higher categories, homotopy algebraic structures, higher operads, derived categories, and related topics.

Previous Meetings

Earlier editions of these workshops were: I (MPIM Bonn), II (Nijmegen), III (Utrecht), IV (MPIM Bonn)V (Nijmegen), VI (Utrecht), VII (Leuven), VIII (MPIM Bonn)


This workshop is organized by Christian Blohmann, Marius Crainic, Ioan Mărcuț, Ieke Moerdijk and Steffen Sagave.


If you like to attend the workshop, please register so that we can estimate the number of participants. There is no registration fee.

Speakers, titles and abstracts

  • Francesca Arici (Radboud University Nijmegen)
    Quantum lattice gauge theories and groupoid C*-algebras
    Abstract: We will describe the quantization of gauge theories on a graph in terms of their algebras of observables and of the Hilbert space on which the algebra is represented. The algebra of observables for the quantum system admits a natural geometric realization as a groupoid C*-algebra. I will describe the behaviour of such algebras under lattice refinements and the resulting continuum limit of the theory. Based on joint work with R. Stienstra and W. van Suijlekom.
  • Michael Bailey (Utrecht University)
    Some remarks about complex structures on stacks
    Abstract: I will explain how a generalized complex structure (a la Gualtieri) may be understood as a complex structure on a shifted symplectic stack. From this point of view, the local classification result for these structures may be understood as a special case of a (so-far-conjectural) "derived Newlander-Nirenberg theorem", which I will also discuss if I have time. Involves joint work with Marco Gualtieri and Brent Pym.
  • Tyrone Crisp (Radboud University Nijmegen)
    Descent of operator modules
    Abstract: I will describe how some basic notions of categorical algebra, namely monadicity and descent, play out in the setting of Haagerup's tensor product for algebras and modules of Hilbert space operators. For an important class of operator algebras – the nuclear C*-algebras – the descent problem turns out to have a very satisfactory solution: every monomorphism satisfies effective descent for modules. I will also explain the motivation for this work, coming from the representation theory of reductive Lie groups.
  • Ralph Klaasse (Utrecht University)
    Constructing A-Symplectic Structures
    Abstract: In this talk we discuss how to construct A-symplectic structures for a Lie algebroid A by adapting Gompf-Thurston techniques to Lie algebroid morphisms. As an application we obtain both log-symplectic and stable generalized complex structures out of log-symplectic structures. In particular we define a class of maps called boundary Lefschetz fibrations and show they equip their total space with a stable generalized complex structure. This is based on joint work with Gil Cavalcanti.
  • Honglei Lang (MPIM Bonn)
    Double Principal Bundles
    Abstract: We define double principal bundles (DPBs), for which the frame bundle of a double vector bundle, double Lie groups and double homogeneous spaces are basic examples. It is shown that a double vector bundle can be realized as the associated bundle of its frame bundle. Also dual structures, gauge transformations and connections in DPBs are investigated. This is a joint work with Yanpeng Li and Zhangju Liu.
  • Markus Pflaum (University of Colorado Boulder)
    Inertia Groupoids and the concept of stratified groupoids
    Abstract: The inertia space of a compact Lie group action or more generally of a proper Lie groupoid has an interesting singularity structure. Unlike the quotient space of the group action, respectively the groupoid, the inertia space can not be stratified by orbit types, in general. In the talk we explain this phenomenon and provide a stratification and local description of the inertia space. Moreover, we show that that leads naturally to the concept of a stratified groupoid which lies in between the one of a Lie groupoid and the one of a topological groupoid. Finally we show that a de Rham theorem holds for inertia spaces and explain the connection of the inertia space with the non- commutative geometry of the underlying groupoid.
  • Kay Werndli (Utrecht University)
    Higher Homotopy Excision via Bousfield Classes
    Abstract:In the 90s, Chachòlski initiated a programme to reformulate and strengthen the classical homotopy excision theorem due to Blakers and Massey in terms of the then newly developed language of Bousfield classes. Some twenty years later, we reached this goal in a highly satisfactory manner, albeit only for homotopy pushout squares. Since then, some results have been obtained about cubical diagrams, though this question is mostly still open. In this talk, we will quickly discuss the necessary formalism of Bousfield classes, state the result for squares and give a first glimpse on how the cubical case can be approached.