* Beck-Chevalley for presheaves on groupoids?
@ 2007-05-02 17:55 John Baez
0 siblings, 0 replies; 2+ messages in thread
From: John Baez @ 2007-05-02 17:55 UTC (permalink / raw)
To: categories
Hi -
One of you must know the answer to this!
Suppose we have a weak pullback (= pseudo-pullback) square of
groupoids:
G -> H
| |
v v
K -> L
Suppose we take presheaves on all four. We can get a square
hom(G^{op},Set) -> hom(H^{op},Set)
^ ^
| |
hom(K^{op},Set) -> hom(L^{op},Set)
where the arrows pointing forward - in the same direction as
the original arrows - are defined using pushforward, and the arrows
pointing backward are defined using pullback.
Does this square commute up to natural isomorphism? Do you
know a reference somewhere?
Some side remarks:
1) This seems related to the "Beck-Chevalley condition".
2) It may work for categories as well as groupoids, but I happen to
need it only for groupoids.
3) I really need it with the category Vect replacing Set, so
if you know a general result for any sufficiently nice category
playing the role of Set here, that would be wonderful.
Best,
jb
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Beck-Chevalley for presheaves on groupoids?
@ 2007-05-04 20:07 RJ Wood
0 siblings, 0 replies; 2+ messages in thread
From: RJ Wood @ 2007-05-04 20:07 UTC (permalink / raw)
To: categories
If
G -> H
| <= |
v v
K -> L
is a square (with a 2-cell as shown) in cat then
applying ^ =((-)^{op},Set) gives
G^<- H^
^ => ^
| |
K^<- L^
and taking the mate with respect to the horizontal adjunctions
given by left Kan extension gives
G^-> H^
^ => ^
| |
K^-> L^
If the original square is a comma square then the 2-cell in
the third square is invertible. There are squares other than
comma squares, cocomma squares for example, for which the
2-cell in the third square is invertible. See Rene Guitart's
early work on exact squares.
Best, Rj Wood
> One of you must know the answer to this!
>
> Suppose we have a weak pullback (= pseudo-pullback) square of
> groupoids:
>
> G -> H
> | |
> v v
> K -> L
>
> Suppose we take presheaves on all four. We can get a square
>
> hom(G^{op},Set) -> hom(H^{op},Set)
> ^ ^
> | |
> hom(K^{op},Set) -> hom(L^{op},Set)
>
> where the arrows pointing forward - in the same direction as
> the original arrows - are defined using pushforward, and the arrows
> pointing backward are defined using pullback.
>
> Does this square commute up to natural isomorphism? Do you
> know a reference somewhere?
>
> Some side remarks:
>
> 1) This seems related to the "Beck-Chevalley condition".
>
> 2) It may work for categories as well as groupoids, but I happen to
> need it only for groupoids.
>
> 3) I really need it with the category Vect replacing Set, so
> if you know a general result for any sufficiently nice category
> playing the role of Set here, that would be wonderful.
>
> Best,
> jb
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