## May 18, 2009

### Higher Structures in Göttingen III

#### Posted by John Baez

Göttingen was famous as a center of mathematics during the days of Gauss, Riemann, Dirichlet, Klein, Minkowksi, Hilbert, Weyl and Courant. One of the founders of category theory, Saunders Mac Lane, studied there! He wrote:

In 1931, after graduating from Yale and spending a vaguely disappointing year of graduate study at Chicago, I was searching for a really first-class mathematics department which would also include mathematical logic. I found both in Göttingen.

It’s worth reading Mac Lane’s story of how the Nazis eviscerated this noble institution.

But now, thanks to the Courant Research Centre on Higher-Order Structures, Göttingen is gaining fame as a center of research on higher structures (like $n$-categories and $n$-stacks) and their applications to geometry, topology and physics! They’re having another workshop soon:

Here are the talks:

• Dmitry Roytenberg, Differential graded manifolds, infinity-stacks and generalized geometries. (3 hours.)

Abstract: Differential graded manifolds are supermanifolds equipped with an additional grading and differential in the structure sheaf. They can be thought of as a simultaneous generalization of Lie algeboids and L-infinity algebras. DG manifolds arise naturally as first-order approximations to a very wide class of geometric constructions. Conversely, every DG manifold gives rise naturally to an infinity-stack (in the formalism of simplicial presheaves). Except for a few special cases, the question of representability of any finite truncation of this stack by a finite-dimensional simplicial manifold is still open. DG manifolds can be thought of as generalized tangent bundles, thus leading to generalized differential geometries, the ordinary geometry corresponding to the de Rham complex. The goal of these lectures is to give a brief introduction to this circle of ideas.

Plan: In the first lecture l will give an introduction to supermanifolds and dg manifolds as supermanifolds with an action of diffeomorphisms of the odd line. I will define the tangent complex of a dg manifold, generalizing the tangent bundle of a manifold and the anchor of a Lie algebroid. I will give a number of examples. Lastly, I will discuss additional structures on dg manifolds, such as differential forms and vector fields, and the formalism of derived brackets; this will be the framework for the generalized geometries mentioned above.

In the second lecture I will give a brief introduction to the language of infinity stacks in the formalism of simplicial presheaves. I will sketch Severa’s theory of dg manifolds as first approximations. Conversely, I will describe a construction of an infinity-stack from a dg manifold and discuss possible approaches to the representability problem. time permitting, I will describe the computation, due to Andre Henriques, of the homotopy sheaves of a reduced dg manifold, and a generalization of the van Est homomorphism.

The third lecture will be devoted to the special case of Courant algebroids. I will show that these are equivalent to dg manifolds with a certain kind of additional sympelctic structure, and how Hitchin’s generalized geometry in the presence of a gerbe can be naturally interpreted in this framework. I will also describe Bressler’s obstruction-theoretic interpretation of the first Pontryagin class in terms of Courant algebroids. Lastly, I will describe an explicit algebraic construction of a differential graded algebra associated to a Courant-Dorfman algebra and give several examples.

• Eckhard Meinrenken, Twisted $K$-theory and group valued moment maps. (3 hours.)
• Behrang Noohi, Stacks and 2-groups. (2 hours.)
• Patrick Iglesias-Zemmour, Fiber bundles in diffeology. (2 hours.)

It is nice to see that perhaps diffeological spaces are catching on as a convenient framework for geometry — something we sorely need as we investigate the interaction between geometry and higher categories! I recommend Iglesias-Zemmour’s online book for anyone wanting to get started on diffeological spaces.

It is also nice to see that Dmitry Roytenberg is giving a series of talks on the relation between differential graded manifolds and $\infty$-stacks modelled as simplicial presheaves — two popular frameworks designed to blend the theory of manifolds and the theory of $\infty$-categories.

Eckhard Meinrenken’s work on twisted $K$-theory and the basic gerbe of a compact simple group is also wonderful and important, as is Behrang’s Noohi’s work on 2-groups.

So, it should be a great workshop!

Posted at May 18, 2009 5:41 PM UTC

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## 1 Comment & 1 Trackback

### Re: Higher Structures in Göttingen III

With regard to the last entry in John’s post, Behrang informs me of a mistake in the published version of his Notes on 2-groupoids, 2-groups and crossed-modules, which appeared in HHA. He has provided an updated and corrected version on the archive at the link that John provided.

Posted by: Tim Porter on May 19, 2009 11:19 AM | Permalink | Reply to this
Read the post Higher Structures in Göttingen IV
Weblog: The n-Category Café
Excerpt: Announcement of the fourth workshop "Higher Structures in Göttingen".
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