RE: size_question_encore

RE: size_question_encore

[André Joyal]

dear Marta,

I apologise, I had forgoten our conversation!
My memory was never good, and it is getting worst.

You wrote:

  >No, I am not thinking of the analogue of Steve Lack's model  
structure since, strictly speaking,
  >it has nothing to do with stacks. Comments to that effect (with  
which Steve agrees) are included
  >in the Bunge-Hermida paper. It was actually a surprise to discover  
that after trying to do what
  >you suggest and failing.

I disagree with your conclusion. I looked at your paper with Hermida.
We are not talking about the same model structure. The fibrations in  
2Cat(S) defined
by Steve Lack (your definition 7.1) are too weak when the topos S does  
not satify the axiom of choice.
Equivalently, his generating set of trivial cofibrations is too small.

Nobody has read my paper with Myles
<A.Joyal, M.Tierney: Classifying spaces for sheaves of simplicial  
groupoids, JPAA, Vol 89, 1993>.

Best,
André



-------- Message d'origine--------
De: Marta Bunge [mailto:martabunge@hotmail.com]
Date: sam. 09/07/2011 13:41
À: Joyal, André; edubuc@dm.uba.ar; categories@mta.ca
Objet : RE: RE : RE : categories: size_question_encore


Dear Andre,
You were indeed aware of my work and that with Pare on stacks since  
you are one of the few we thank for useful conversations! There were  
two ways to define stacks and one of them was your suggestion. One  
could say that one is expressed directly in terms of descent and the  
other in terms of weak equivalences. As it turns out, both are needed  
in my construction of the stack completion and similarly in the 2- 
dimensional case.
As for which method is preferable, I do not know. Whether one  
constructs stack completions for categories in a Grothendieck topos  
using the carving out from presheaf toposes (my method), or by means  
of a model structure (yours), one has to resort to the existence of a  
generating family to keep them small.
No, I am not thinking of the analogue of Steve Lack's model structure  
since, strictly speaking, it has nothing to do with stacks. Comments  
to that effect (with which Steve agrees) are included in the Bunge- 
Hermida paper. It was actually a surprise to discover that after  
trying to do what you suggest and failing. I attach my paper with  
Hermida in this connection. Section 3 makes clear what happens with  
Lack's model structure in dimension 1, and Section 7 considers the 2- 
dimensional analogue, also not suitable to get the 2-stack completion.
I really meant an extension of the Joyal-Tierney model structure.  
Thanks for pointing out Moerdijk's work, and your old one with  
Tierney. I will eventually look into those.
No need to respond to this.
Best regards, and many thanks,Marta



  > Subject: RE : RE : categories: size_question_encore
  > Date: Sat, 9 Jul 2011 12:18:45 -0400
  > From: joyal.andre@uqam.ca
  > To: martabunge@hotmail.com; edubuc@dm.uba.ar; categories@mta.ca
  >
  > Dear Marta,
  >
  > I thank you for your message and for drawing my attention to your  
work.
  > I apologise for not having refered to it.
  >
  > >More recently (Bunge-Hermida, MakkaiFest, 2011), we have carried  
out the 2-analogue of the 1-dimensional
  > >case along the same lines of the 1979 papers, by constructing the  
2-stack completion of a 2-gerbe in "exactly the same way". >Concerning  
this, I have a question for you. Is there a model structure on 2- 
Cat(S) (or 2-Gerbes(S)), for S a Grothedieck topos, >whose weak  
equivalences are the weak 2-equivalence 2-functors, and whose fibrant  
objects are precisely the (strong) 2-stacks? >Although not needed for  
our work, the question came up naturally after your paper with Myles  
Tierney. We could find no such >construction in the literature.
  >
  > I guess you are thinking of having the analog of Steve Lack's model  
structure
  > but for the category of 2-categories internal to a Grothendieck  
topos S.
  > That is a good question. I am not aware that this has been done  
(but my knowledge of the litterature is lacunary).
  > You may also want to establish the analog of Moerdijk's model  
structure for the category of internal 2-groupoids.
  > I am confident that these model structure exists.
  > They should be closely related to a model structure on internal  
simplicial groupoids
  > <A.Joyal, M.Tierney: Classifying spaces for sheaves of simplicial  
groupoids, JPAA, Vol 89, 1993>.
  > And also related to the model structure on simplicial sheaves,  
described in my letter
  > to Grothendieck in 1984, but unfortunately not formally published.
  >
  > Best regards,
  > Andre
  >

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stacks (was: size_question_encore)

[David Roberts]

Dear Marta, André, and others,

this is perhaps a bit cheeky, because I am writing this in reply to Marta's
email to André, quoted below. To me it almost feels like reading anothers' mail;
please forgive the stretch of etiquette.

---

Marta raised an interesting point that stacks can be described in (at least) two
ways: via a model structure and via descent. The former implicitly (in the case
of topoi: take all epis) or explicitly needs a pretopology on the base category
in question. This is to express the notion of essential surjectivity.

However, I would advertise a third way, and that is to localise the (or a!)
2-category of categories internal to the base directly, rather than using  a
model category, which is a tool (among other things) to localise the 1-category
of internal categories. Dorette Pronk proved a few special cases of this in her
1996 paper discussing bicategorical localisations, namely algebraic,
differentiable and topological stacks, all of a fixed sort.

By this I mean she took a static definition of said stacks, rather than working
with a variable notion of cover, as one finds, for example in algebraic
geometry: Artin stacks, Deligne-Mumford stacks, orbifolds etc, or as in Behrang
Noohi's 'Foundations of topological stacks', where one can have a variable class
of 'local fibrations', which control the behaviour of the fibres of
source/target maps of a presenting groupoid.

With enough structure on the base site (say, existence and stability under
pullback of reflexive coequalisers), then one can define (in roughly historical
order, as far as I know):

representable internal distributors/profunctors
= meriedric morphisms (generalising Pradines)
= Hilsum-Skandalis morphisms
= (internal) saturated anafunctors
= (incorrectly) Morita morphisms
= right principal bibundles/bitorsors

and then (it is at morally true that) the 2-category of stacks of groupoids is
equivalent to the bicategory with objects internal groupoids and 1-arrows  the
above maps (which have gathered an interesting collection of names), both  of
which are a localisation of the same 2-category at the 'weak equivalences'.

Without existence of reflexive coequalisers (say for example when working  in
type-theoretic foundations), then one can consider ordinary (as opposed to
saturated) anafunctors. Whether these also present the 2-category of stacks is a
(currently stalled!) project of mine. The question is a vast generalisation of
this: without the 'clutching' construction associating to a Cech cocyle a  actual
principal bundle, is a stack really a stack of bundles, or a stack of
cocycles/descent data.

The link to the other two approaches mentioned by Marta is not too obscure: the
class of weak equivalences in the 2-categorical and 1-categorical approaches are
the same, and if one has enough projectives (of the appropriate variety),  then
an internal groupoid A (say) with object of objects projective satisfies

Gpd(S)(A,B) ~~> Gpd_W(S)(A,B)

for all other objects B, and where Gpd_W(S) denotes the 2-categorical
localisation of Gpd(S) at W.

One more point: Marta mentioned the need to have a generating family. While in
the above approach one keeps the same objects (the internal
categories/groupoids), there is a need to have a handle on the size of the
hom-categories, to keep local smallness. One achieves this by demanding that for
every object of the base site there is a *set* of covers for that object cofinal
in all covers for that object. Then the hom-categories for the localised
2-category are essentially small.

All the best,

David

Quoting André Joyal <joyal.andre@uqam.ca>:

> dear Marta,
>
> I apologise, I had forgoten our conversation!
> My memory was never good, and it is getting worst.
>
> You wrote:
>
>  >No, I am not thinking of the analogue of Steve Lack's model
> structure since, strictly speaking,
>  >it has nothing to do with stacks. Comments to that effect (with
> which Steve agrees) are included
>  >in the Bunge-Hermida paper. It was actually a surprise to discover
> that after trying to do what
>  >you suggest and failing.
>
> I disagree with your conclusion. I looked at your paper with Hermida.
> We are not talking about the same model structure. The fibrations in
> 2Cat(S) defined
> by Steve Lack (your definition 7.1) are too weak when the topos S does
> not satify the axiom of choice.
> Equivalently, his generating set of trivial cofibrations is too small.
>
> Nobody has read my paper with Myles
> <A.Joyal, M.Tierney: Classifying spaces for sheaves of simplicial
> groupoids, JPAA, Vol 89, 1993>.
>
> Best,
> André
>

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RE: stacks (was: size_question_encore)

[Marta Bunge]

Dear David,

 

 

 

Whatever is published in categories is
of public domain so anyone can intervene. You have nothing to apologize for.

 

 

 

I am not acquainted with recent work of
Dorette Pronk, but I read her 1995 Utrecht thesis in detail since I was asked
to do so by her advisor. In it, she refers to my paper (Marta Bunge, "An
application of descent to a classification theorem for toposes" , Math.
Proc. Camb. Phil. Soc. 107 (1990) 59-79), where I prove, in Corollary 5.4 to
the main Theorem 5.1, the following, which is, oin the case of groupoids, what
you call the third way. It says that, if W is the class of isomorphisms classes
of weak equivalences of etale complete groupoids (ECG), then W admits a
calculus of right fractions, and the functor B from Gpd to Top induces an
equivalence ECG[W-1] \iso [Top], where [Top} denotes the category of
Grotehndieck toposes (over a base S not necessarily Sets) and isomorphism
classes of geometric morphisms. So, the purpose of the third way, in my  view,
is to prove classification theorems. However, I am not au courant of more
recent developments.

 

 

 

As for the other two approaches I
mentioned in my correspondence with Andre Joyal, their equivalence is not  that
obvious. In the 1-dimensional case, this is done in Bunge-Pare (1979)
Proposition 2.7, and in the 2-dimensional case it is done in Bunge-Hermida
(2010) Theorem 4-9.

 

 

 

Concerning size matters, let me observe
that my construction of the stack completion (Bunge, Cahiers 1979) is
meaningful regardless of size questions, that is, for any base topos S.  The
outcome, however, of applying it to an internal category need no longer  be
internal. For this reason I introduce an "axiom of stack completions"
which guarantees that stack completions of internal categories be again
internal,and which is satisfied by any S a Grothehdieck topos. The question of
stating such an axiom as an additional axiom to the ones for elementary toposes
was proposed as a problem by Lawvere in his Montreal lectures in 1974. 

 

 

 

Good luck with your projects.

Marta



> Date: Mon, 11 Jul 2011 15:06:38 +0930
> From: david.roberts@adelaide.edu.au
> To: joyal.andre@uqam.ca; martabunge@hotmail.com
> CC: categories@mta.ca
> Subject: stacks (was: size_question_encore)
> 
> Dear Marta, André, and others,
> 
> this is perhaps a bit cheeky, because I am writing this in reply to Marta's
> email to André, quoted below. To me it almost feels like reading anothers' mail;
> please forgive the stretch of etiquette.
> 
> ---
> 
> Marta raised an interesting point that stacks can be described in (at least) two
> ways: via a model structure and via descent. The former implicitly (in the case
> of topoi: take all epis) or explicitly needs a pretopology on the base category
> in question. This is to express the notion of essential surjectivity.
> 
> However, I would advertise a third way, and that is to localise the (or a!)
> 2-category of categories internal to the base directly, rather than using a
> model category, which is a tool (among other things) to localise the 1-category
> of internal categories. Dorette Pronk proved a few special cases of this in her
> 1996 paper discussing bicategorical localisations, namely algebraic,
> differentiable and topological stacks, all of a fixed sort.
> 
> By this I mean she took a static definition of said stacks, rather than  working
> with a variable notion of cover, as one finds, for example in algebraic
> geometry: Artin stacks, Deligne-Mumford stacks, orbifolds etc, or as in Behrang
> Noohi's 'Foundations of topological stacks', where one can have a variable class
> of 'local fibrations', which control the behaviour of the fibres of
> source/target maps of a presenting groupoid.
> 
> With enough structure on the base site (say, existence and stability under
> pullback of reflexive coequalisers), then one can define (in roughly historical
> order, as far as I know):
> 
> representable internal distributors/profunctors
> = meriedric morphisms (generalising Pradines)
> = Hilsum-Skandalis morphisms
> = (internal) saturated anafunctors
> = (incorrectly) Morita morphisms
> = right principal bibundles/bitorsors
> 
> and then (it is at morally true that) the 2-category of stacks of groupoids is
> equivalent to the bicategory with objects internal groupoids and 1-arrows  the
> above maps (which have gathered an interesting collection of names), both of
> which are a localisation of the same 2-category at the 'weak equivalences'.
> 
> Without existence of reflexive coequalisers (say for example when working  in
> type-theoretic foundations), then one can consider ordinary (as opposed  to
> saturated) anafunctors. Whether these also present the 2-category of stacks is a
> (currently stalled!) project of mine. The question is a vast generalisation of
> this: without the 'clutching' construction associating to a Cech cocyle a  actual
> principal bundle, is a stack really a stack of bundles, or a stack of
> cocycles/descent data.
> 
> The link to the other two approaches mentioned by Marta is not too obscure: the
> class of weak equivalences in the 2-categorical and 1-categorical approaches are
> the same, and if one has enough projectives (of the appropriate variety), then
> an internal groupoid A (say) with object of objects projective satisfies
> 
> Gpd(S)(A,B) ~~> Gpd_W(S)(A,B)
> 
> for all other objects B, and where Gpd_W(S) denotes the 2-categorical
> localisation of Gpd(S) at W.
> 
> One more point: Marta mentioned the need to have a generating family. While in
> the above approach one keeps the same objects (the internal
> categories/groupoids), there is a need to have a handle on the size of the
> hom-categories, to keep local smallness. One achieves this by demanding  that for
> every object of the base site there is a *set* of covers for that object cofinal
> in all covers for that object. Then the hom-categories for the localised
> 2-category are essentially small.
> 
> All the best,
> 
> David
> 

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RE: stacks (was: size_question_encore)

[Michael Shulman]

Is the "axiom of stack completions" related to the "axiom of small
cardinality selection" used by Makkai to prove that the bicategory of
anafunctors is cartesian closed?  I think I recall a remark in
Makkai's paper to the effect that the stack completion of a category C
is at least morally the same as the category Ana(1,C) of "ana-objects"
of C.

Are there known examples of elementary toposes which violate the axiom
of stack completions?

On Mon, Jul 11, 2011 at 5:32 AM, Marta Bunge <martabunge@hotmail.com> wrote:
> Concerning size matters, let me observe
> that my construction of the stack completion (Bunge, Cahiers 1979) is
> meaningful regardless of size questions, that is, for any base topos S.  The
> outcome, however, of applying it to an internal category need no longer  be
> internal. For this reason I introduce an "axiom of stack completions"
> which guarantees that stack completions of internal categories be again
> internal,and which is satisfied by any S a Grothehdieck topos. The question of
> stating such an axiom as an additional axiom to the ones for elementary toposes
> was proposed as a problem by Lawvere in his Montreal lectures in 1974.


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Re: size_question_encore

[Toby Bartels]

Michael Shulman wrote in part:
>We can make finitely many choices without any axiom of
>choice.  Thus, for any natural number n, by applying collection n
>times, we can find *some* n^th iterate of the "construction".
>(Formally, we prove this by induction on n.)  Applying the axiom of
>collection again over the natural numbers, we obtain a set which
>contains at least one n^th iterate of the "construction" for every
>natural number n.  Taking the union of this set, we should obtain a
>set of objects whose corresponding full subcategory contains at least
>one limit of every finite diagram therein.

OK, I buy that.  The part where we take the union
is the step that doesn't generalise to arbitrary applications of DC.


--Toby


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Re: size_question_encore

[Michael Shulman]

On Wed, Jul 13, 2011 at 9:10 PM, Toby Bartels
<categories@tobybartels.name> wrote:
>>the axiom of collection, which implies that we can find some *set* of
>>objects containing *at least one* limit for every finite diagram in
>>the original small subcategory; and then we can iterate countably many
>>times to obtain a small category which contains at least one limit for
>>any finite diagram therein.
>
> The axiom of collection guarantees only *some* appropriate set of objects,
> so you need to choose one.  To iterate this countably many times,
> you might need dependent choice.

That's a good point.  However, I think we can get around it as
follows.  We can make finitely many choices without any axiom of
choice.  Thus, for any natural number n, by applying collection n
times, we can find *some* n^th iterate of the "construction".
(Formally, we prove this by induction on n.)  Applying the axiom of
collection again over the natural numbers, we obtain a set which
contains at least one n^th iterate of the "construction" for every
natural number n.  Taking the union of this set, we should obtain a
set of objects whose corresponding full subcategory contains at least
one limit of every finite diagram therein.

Mike


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Re: size_question_encore

[Toby Bartels]

Mike Shulman wrote in part:

>Even in a category of sets, I don't see why choice is necessary in
>order to complete a small subcategory under finite limits and obtain a
>small subcategory.  It seems to me that what is needed is rather the
>axiom of collection, which implies that we can find some *set* of
>objects containing *at least one* limit for every finite diagram in
>the original small subcategory; and then we can iterate countably many
>times to obtain a small category which contains at least one limit for
>any finite diagram therein.

The axiom of collection guarantees only *some* appropriate set of objects,
so you need to choose one.  To iterate this countably many times,
you might need dependent choice.

Unless I'm not understanding what you're doing.


--Toby


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Re: size_question_encore

[Michael Shulman]

Even in a category of sets, I don't see why choice is necessary in
order to complete a small subcategory under finite limits and obtain a
small subcategory.  It seems to me that what is needed is rather the
axiom of collection, which implies that we can find some *set* of
objects containing *at least one* limit for every finite diagram in
the original small subcategory; and then we can iterate countably many
times to obtain a small category which contains at least one limit for
any finite diagram therein.  There is of course no canonical result,
and the various results obtained will not necessarily be strongly
equivalent, but it seems to me that they should all be weakly
equivalent.

And it also seems to me that the same approach should work internal to
any topos.  Collection is true internally to any topos (essentially by
the internal definition of "indexed family"), so it should still be
possible to enlarge a small internal site of definition to one that
has finite limits.  Unless there is some other subtlety that I'm not
seeing.

Mike

On Tue, Jul 5, 2011 at 4:29 PM, Eduardo Dubuc <edubuc@dm.uba.ar> wrote:
> I have now clarified (to myself at least) that there is no canonical
> small category of finite sets, but a plethora of them. The canonical one
> is large. With choice, they are all equivalent, without choice not.
>
> When you work with an arbitrary base topos (assume grothendieck) "as if
> it were Sets" this may arise problems as they are beautifully
> illustrated in Steven Vickers mail.
>
> In Joyal-Tierney galois theory (memoirs AMS 309) page 60, they say S_f
> to be the topos of (cardinal) finite sets, which is an "internal
> category" since then they take the exponential S^S_f. Now, in between
> parenthesis you see the word "cardinal", which seems to indicate to
> which category of finite sets (among all the NON equivalent ones) they
> are referring to.
>
> Now, it is well known the meaning of "cardinal" of a topos ?.
> I imagine there are precise definitions, but I need a reference.
>
> Now, it is often assumed that any small set of generators determine a
> small set of generators with finite limits. As before, there is no
> canonical small finite limit closure, thus without choice (you have to
> choose one limit cone for each finite limit diagram), there is no such a
> thing as "the" small finite limit closure.
>
> Working with an arbitrary base topos, small means internal, thus without
> choice it is not clear that a set of generators can be enlarged to have
> a set of generators with finite limits (not even with a terminal
> object). Unless you add to the topos structure (say in the hypothesis of
> Giraud's Theorem) the data of canonical finite limits.
>
> For example, in Johnstone book (the first, not the elephant) in page 18
> Corollary 0.46 when he proves that there exists a site of definition
> with finite limits, in the proof, it appears (between parenthesis) the
> word "canonical" with no reference to its meaning. Without that word,
> the corollary is false, unless you use choice. With that word, the
> corollary is ambiguous, since there is no explanation for the technical
> meaning of "canonical". For example, in theorem 0.45 (of which 0.46 is a
> corollary), the word does not appear. A topos, is not supposed to have
> canonical (whatever this means) finite limits.
>
> e.d.
>


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RE: size_question_encore

[Marta Bunge]

Dear Andre,

I believe that you misunderstood the comment made in Bunge-Hermida (2011) concerning model structures. In particular, I do not know what you meant by your saying "I disagree with your conclusion". We say precisely in Section 3 (Remark 3.6) that the two model structures mentioned - namely the Joyal-Tierney and the Lack model structures on Cat(S), S a Grothendieck topos, are different, and why. I think then that we agree. 
Best,Marta
 

> From: joyal.andre@uqam.ca
> To: categories@mta.ca
> Subject: categories: RE: size_question_encore
> Date: Sun, 10 Jul 2011 09:30:07 -0400
> 
> dear Marta,
> 
> I apologise, I had forgoten our conversation!
> My memory was never good, and it is getting worst.
> 
> You wrote:
> 
>  >No, I am not thinking of the analogue of Steve Lack's model  
> structure since, strictly speaking,
>  >it has nothing to do with stacks. Comments to that effect (with  
> which Steve agrees) are included
>  >in the Bunge-Hermida paper. It was actually a surprise to discover  
> that after trying to do what
>  >you suggest and failing.
> 
> I disagree with your conclusion. I looked at your paper with Hermida.
> We are not talking about the same model structure. The fibrations in  
> 2Cat(S) defined
> by Steve Lack (your definition 7.1) are too weak when the topos S does  
> not satify the axiom of choice.
> Equivalently, his generating set of trivial cofibrations is too small.
> 
> Nobody has read my paper with Myles
> <A.Joyal, M.Tierney: Classifying spaces for sheaves of simplicial  
> groupoids, JPAA, Vol 89, 1993>.
> 
> Best,
> André
> 
> 
> 

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RE: size_question_encore

[André Joyal]

Dear Marta,

I thank you for your message and for drawing my attention to your work.
I apologise for not having refered to it.

  >More recently (Bunge-Hermida, MakkaiFest, 2011), we have carried out  
the 2-analogue of the 1-dimensional
  >case along the same lines of the 1979 papers, by constructing the 2- 
stack completion of a 2-gerbe in "exactly the same way". >Concerning  
this, I have a question for you. Is there a model structure on 2- 
Cat(S) (or 2-Gerbes(S)), for S a Grothedieck topos, >whose weak  
equivalences are the weak 2-equivalence 2-functors, and whose fibrant  
objects are precisely the (strong) 2-stacks? >Although not needed for  
our work, the question came up naturally after your paper with Myles  
Tierney. We could find no such >construction in the literature.

I guess you are thinking of having the analog of Steve Lack's model  
structure
but for the category of 2-categories internal to a Grothendieck topos S.
That is a good question. I am not aware that this has been done (but  
my knowledge of the litterature is lacunary).
You may also want to establish the analog of Moerdijk's model  
structure for the category of internal 2-groupoids.
I am confident that these model structure exists.
They should be closely related to a model structure on internal  
simplicial groupoids
<A.Joyal, M.Tierney: Classifying spaces for sheaves of simplicial  
groupoids, JPAA, Vol 89, 1993>.
And also related to the model structure on simplicial sheaves,  
described in my letter
to Grothendieck in 1984, but unfortunately not formally published.

Best regards,
Andre




-------- Message d'origine--------
De: Marta Bunge [mailto:martabunge@hotmail.com]
Date: ven. 08/07/2011 09:00
À: Joyal, André; edubuc@dm.uba.ar; categories@mta.ca
Objet : RE : categories: size_question_encore


Dear Andre,I welcome your suggestion of involving stacks in order to  
test  universality when the base topos S does not have Choice. I have  
been exploiting this implicitly but systematically several times since  
my own construction of the stack completion of a category object C in  
any Grothendieck topos S (Cahiers, 1979). For instance, I have used it  
crucially in my paper on Galois groupoids and covering morphisms  
(Fields, 2004), not only in distinguishing between Galois groupoids  
from fundamental groupoids, but also for a neat way of (well) defining  
the fundamental groupoid topos of a Grothendieck topos as the limit of  
a filtered 1-system of discrete groupoids, obtained from the naturally  
arising bifiltered 2-system of such by taking stack completions. This  
relates to the last remark you make in your posting. Concerning S_fin,  
it does not matter if, in constructing the object classifier, one uses  
its stack completion instead, since S is a stack (Bunge-Pare, Cahiers,  
1979). In my opinion, stacks should be the staple food of category  
theory without Choice. For instance, an anafunctor (Makkai's  
terminology) from C to D is precisely a functor from C to the stack  
completion of D. More recently (Bunge-Hermida, MakkaiFest, 2011), we  
have carried out the 2-analogue of the 1-dimensional case along the  
same lines of the 1979 papers, by constructing the 2-stack completion  
of a 2-gerbe in "exactly the same way". Concerning this, I have a  
question for you. Is there a model structure on 2-Cat(S) (or 2- 
Gerbes(S)), for S a Grothedieck topos, whose weak equivalences are the  
weak 2-equivalence 2-functors, and whose fibrant objects are precisely  
the (strong) 2-stacks? Although not needed for our work, the question  
came up naturally after your paper with Myles Tierney. We could find  
no such construction in the literature.  With best regards, Marta

  > Subject: categories: RE : categories: size_question_encore
  > Date: Wed, 6 Jul 2011 21:23:36 -0400
  > From: joyal.andre@uqam.ca
  > To: edubuc@dm.uba.ar; categories@mta.ca
  >
  > Dear Eduardo,
  >
  > I would like to join the discussion on the category of finite sets.
  >

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size_question_encore

[Eduardo Dubuc]

I have now clarified (to myself at least) that there is no canonical
small category of finite sets, but a plethora of them. The canonical one
is large. With choice, they are all equivalent, without choice not.

When you work with an arbitrary base topos (assume grothendieck) "as if
it were Sets" this may arise problems as they are beautifully
illustrated in Steven Vickers mail.

In Joyal-Tierney galois theory (memoirs AMS 309) page 60, they say S_f
to be the topos of (cardinal) finite sets, which is an "internal
category" since then they take the exponential S^S_f. Now, in between
parenthesis you see the word "cardinal", which seems to indicate to
which category of finite sets (among all the NON equivalent ones) they
are referring to.

Now, it is well known the meaning of "cardinal" of a topos ?.
I imagine there are precise definitions, but I need a reference.

Now, it is often assumed that any small set of generators determine a
small set of generators with finite limits. As before, there is no
canonical small finite limit closure, thus without choice (you have to
choose one limit cone for each finite limit diagram), there is no such a
thing as "the" small finite limit closure.

Working with an arbitrary base topos, small means internal, thus without
choice it is not clear that a set of generators can be enlarged to have
a set of generators with finite limits (not even with a terminal
object). Unless you add to the topos structure (say in the hypothesis of
Giraud's Theorem) the data of canonical finite limits.

For example, in Johnstone book (the first, not the elephant) in page 18
Corollary 0.46 when he proves that there exists a site of definition
with finite limits, in the proof, it appears (between parenthesis) the
word "canonical" with no reference to its meaning. Without that word,
the corollary is false, unless you use choice. With that word, the
corollary is ambiguous, since there is no explanation for the technical
meaning of "canonical". For example, in theorem 0.45 (of which 0.46 is a
corollary), the word does not appear. A topos, is not supposed to have
canonical (whatever this means) finite limits.

e.d.


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