From: "John Baez" <baez@math.ucr.edu>
To: categories@mta.ca (categories)
Subject: Re: fibrations as ...
Date: Sun, 20 Nov 2005 14:48:13 -0800 (PST) [thread overview]
Message-ID: <200511202248.jAKMmEc05975@math-cl-n03.ucr.edu> (raw)
Jim Stasheff wrote:
> John and anyone else who cares to weigh in,
> here are some comments from the purely topological
> or rather homotopy theory side:
>
> For both bundles and fibrations (e.g. over a paracompact base), your
> last slogan is the oldest:
>
> FIBRATIONS WITH FIBER F OVER THE SPACE B
> ARE "THE SAME" AS
> MAPS FROM B TO THE CLASSIFYING SPACE OF AUT(F)
>
> `the same as' referring to homotopy classes.
It's certainly old, but I mentioned another that may be older:
COVERING SPACES OVER B WITH FIBER F
ARE "THE SAME" AS
HOMOMORPHISMS FROM THE FUNDAMENTAL GROUP OF B TO AUTOMORPHISMS OF F.
although one usually sees this special case (which I didn't bother
to mention):
CONNECTED COVERING SPACES OVER B WITH FIBER F
ARE "THE SAME" AS
TRANSITIVE ACTIONS OF THE FUNDAMENTAL GROUP OF B ON F
which is usually disguised as follows:
CONNECTED COVERING SPACES OVER B
ARE "THE SAME" AS
SUBGROUPS OF THE FUNDAMENTAL GROUP OF B
Anyway, I wasn't trying to present things in historical order.
I was trying present them roughly in order of increasing
"dimension", starting with extensions of groups, then going up to
2-groups, then expanding out to groupoids, then going up to n-groupoids,
and finally omega-groupoids... which are the same as homotopy types!
And here, as usual, the n-category theorists meet up with the
topologists - and find that the topologists have already done everything
there is to do with omega-groupoids ... but usually by thinking of
them of them as *spaces*, rather than omega-groupoids!
It's sort of like climbing a mountain, surmounting steep cliffs with
the help of ropes and other equipment, and then finding a Holiday Inn
on top and realizing there was a 4-lane highway going up the other side.
So, as usual, the main point of calling homotopy types "omega-groupoids"
instead of "spaces" is not to reinvent topology, but to see how ideas
from topology generalize to n-category theory. Think of spaces as
omega-groupoids but use those as a springboard for omega-categories -
or at least n-categories, perhaps just for low values of n if one is
feeling tired.
In the case at hand, the omega-groupoidal slogan:
FIBRATIONS OF OMEGA-GROUPOIDS WITH FIBER F AND BASE B
ARE "THE SAME" AS
WEAK OMEGA-FUNCTORS FROM B TO AUT(F)
is just a reformulation of:
FIBRATIONS WITH FIBER F OVER THE SPACE B
ARE "THE SAME" AS
MAPS FROM B TO THE CLASSIFYING SPACE OF AUT(F)
but it suggests a grandiose generalization:
FIBRATIONS OF OMEGA-CATEGORIES WITH BASE B
ARE "THE SAME" AS
WEAK OMEGA-FUNCTORS FROM B^{op} TO THE OMEGA-CATEGORY OF OMEGA-CATEGORIES!
I guess we can thank Grothendieck for making precise and proving a
version of this with omega replaced by n = 1:
FIBRATIONS OF CATEGORIES WITH BASE B
ARE "THE SAME" AS
WEAK 2-FUNCTORS FROM B^{op} TO THE 2-CATEGORY OF CATEGORIES.
More recently people have been thinking about the n = 2 case, especially
Claudio Hermida:
22) Claudio Hermida, Descent on 2-fibrations and strongly 2-regular
2-categories, Applied Categorical Structures, 12 (2004), 427-459.
Also available at http://maggie.cs.queensu.ca/chermida/papers/2-descent.pdf
He states something that hints at this:
FIBRATIONS OF 2-CATEGORIES WITH BASE B
ARE "THE SAME" AS
WEAK 3-FUNCTORS FROM B^{op} TO THE WEAK 3-CATEGORY OF 2-CATEGORIES.
where I'm using B^{op} to mean B with the directions of both 1-morphisms
and 2-morphisms reversed.
(Hermida follows tradition and calls this B^{coop} - "op" for reversing
1-morphisms and "co" for reversing 2-morphisms. But, it looks like we'll
be needing to reverse all kinds of morphisms in n-category case, so we'll
need a short name for that.)
Best,
jb
next reply other threads:[~2005-11-20 22:48 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-11-20 22:48 John Baez [this message]
-- strict thread matches above, loose matches on Subject: below --
2005-11-20 14:18 jim stasheff
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