[Topology.beta] Friday fish

[Rot, T.O. (Thomas)]

Dear all,

Our friends in Utrecht are organizing a reading club on differentiable stacks (think equivariant mathematics, lie groupoids / orbifolds / geometric structures like foliations). 

Best,
Thomas

On 03/02/2025, 19:27, "friday-fish on behalf of Douwe Hoekstra via friday-fish" <friday-fish-bounces at lists.science.uu.nl <mailto:friday-fish-bounces at lists.science.uu.nl> on behalf of friday-fish at lists.science.uu.nl <mailto:friday-fish at lists.science.uu.nl>> wrote:


Dear all,


The plan is to have a 6-lecture learning seminar on 
differentiable stacks on Fridays starting the 14th of February. The aim 
of this seminar is to first understand stacks in an abstract categorical 
sense and then relate this back to the concrete setting of Lie groupoids 
up to Morita equivalence. The key point of the seminar will be to 
understand the three definitions of stack as given on wikipedia 
<https://en.wikipedia.org/wiki/Differentiable_stack> <https://en.wikipedia.org/wiki/Differentiable_stack>>. 
In addition, we will focus on why these three definitions are equivalent 
and we will make the theory more concrete using plenty of examples. A 
preliminary overview of the intended topics and a tentative schedule are 
given below. The seminar will probably be livestreamed and recorded. If 
you would be interested in giving a talk at the seminar, feel free to 
contact Douwe (d.hoekstra at uu.nl <mailto:d.hoekstra at uu.nl>) or Jaime (j.pedregalpastor at uu.nl <mailto:j.pedregalpastor at uu.nl>). 
Also, feel free to forward this information to anyone that you think 
would be interested.


The information below will also be posted on the webpage of the seminar 
<https://utrechtgeometrycentre.nl/friday-fish-seminar-online-edition/> <https://utrechtgeometrycentre.nl/friday-fish-seminar-online-edition/>>. 
For the latest information see this webpage of the per-talk announcement 
on the Friday Fish mailing list 
<https://mailman.science.uu.nl/mailman/listinfo/friday-fish> <https://mailman.science.uu.nl/mailman/listinfo/friday-fish>>.


*Intended topics *(the references are just suggestions :))
1. Stacks of triangles [AS, Chapter 1]
2. Categorical prerequisites [W, TMS, DSG, PASLG]
3. Abstract definitions of stacks [W, TMS, DSG, PASLG]
4. Lie groupoids up to Morita equivalence as stacks [TMS, DSG, LGDS, PASLG]
5. Morphisms of stacks in the Lie groupoid picture [TMS, DSG, LGDS, PASLG]
6. Optional: C^*-algebras or some other topic.


*References
*[AS] Behrend, Introduction to Algebraic Stacks.
[DSG] Behrend and Xu, Differentiable Stacks and Gerbes.
[TMS] Metzler, Topological and Smooth Stacks.
[LGDS] del Hoyo, Lie Groupoids and Differentiable Stacks.
[PASLG] Bursztyn, Noseda and Zhu, Principal actions of stacky Lie groupoids.
[W] Wikipedia, Differentiable stacks.


*Tentative schedule*


* Friday February 14th, 15:00-17:00: Bas Wensink
* Thursday February 20th, 12:00-14:00: Jaime Pedregal Pastor
* Friday February 28th, 15:00-17:00: Douwe Hoekstra
* Friday March 14th, 15:00-17:00: Sven Holtrop
* Friday March 21st, 15:00-17:00: ?
* Friday March 28th, 11:00-15:00: ? *


*: May be moved because of the DDT&G.


Kind regards,
Bas, Douwe, Jaime and Sven
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