* Re: groupoids versus homotopy 1-types
@ 2007-01-05 17:06 Ronnie Brown
0 siblings, 0 replies; 2+ messages in thread
From: Ronnie Brown @ 2007-01-05 17:06 UTC (permalink / raw)
To: categories
I don't have a specific reference in 2-category language but the following
should be relevant:
The notion of a homotopy theory for groupoids was set up in my 1968 topology
book now revised and republished as Topology and Groupoids. In particular
\pi_1: spaces \to groupoids preserves homotopies. Fibrations were introduced
in an exercise, and developed in later editions. See also Philip Higgins'
Categories and Groupoids, (1971) now available as a TAC reprint. The nerve
and classifying space of a groupoid are in Graeme Segal's `Classifying
spaces and spectral sequences' (IHES) utilising Grothendieck's nerve of a
category. These preserve homotopy. The fact that for a CW-complex X, [X,BG]
\cong [\pi_1 X, G] is also well known: P.Olum Ann Math 1958?
There is also relevant material in Gabriel-Zisman's book, but I do not have
it with me.
People should also look at 2 papers on groupoids by P A Smith in the Annals,
1951.
Hope that helps.
Ronnie
www.bangor.ac.uk/r.brown
----- Original Message -----
From: "John Baez" <baez@math.ucr.edu>
To: "categories" <categories@mta.ca>
Sent: Wednesday, December 27, 2006 6:53 PM
Subject: categories: groupoids versus homotopy 1-types
> Dear Categorists -
>
> The following claim should be well-known (or false),
> but I don't know a reference:
>
> Let Gpd be the 2-category consisting of
>
> groupoids
> functors
> natural transformations
>
> and let 1Type be the 2-category consisting of
>
> homotopy 1-types
> continuous maps
> homotopy classes of homotopies
>
> where for present purposes "homotopy 1-types" means "CW complexes with
> vanishing higher homotopy groups regardless of the choice of basepoint".
>
> Claim: Gpd and 1Type are equivalent (or "biequivalent",
> in older terminology).
>
> In fact I bet there is an explicit pseudo-adjunction between them,
> with the "fundamental groupoid" 2-functor going one way and the
> "Eilenberg-Mac Lane space" 2-functor going the other way.
>
> Does anyone know for sure? Know a reference?
>
> Best,
> jb
>
>
>
>
>
>
>
> --
> Internal Virus Database is out-of-date.
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^ permalink raw reply [flat|nested] 2+ messages in thread
* groupoids versus homotopy 1-types
@ 2006-12-27 18:53 John Baez
0 siblings, 0 replies; 2+ messages in thread
From: John Baez @ 2006-12-27 18:53 UTC (permalink / raw)
To: categories
Dear Categorists -
The following claim should be well-known (or false),
but I don't know a reference:
Let Gpd be the 2-category consisting of
groupoids
functors
natural transformations
and let 1Type be the 2-category consisting of
homotopy 1-types
continuous maps
homotopy classes of homotopies
where for present purposes "homotopy 1-types" means "CW complexes with
vanishing higher homotopy groups regardless of the choice of basepoint".
Claim: Gpd and 1Type are equivalent (or "biequivalent",
in older terminology).
In fact I bet there is an explicit pseudo-adjunction between them,
with the "fundamental groupoid" 2-functor going one way and the
"Eilenberg-Mac Lane space" 2-functor going the other way.
Does anyone know for sure? Know a reference?
Best,
jb
^ permalink raw reply [flat|nested] 2+ messages in thread
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