From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.io/gmane.science.mathematics.categories/6462 Path: news.gmane.org!not-for-mail From: JeanBenabou Newsgroups: gmane.science.mathematics.categories Subject: Re: Fibrations in a 2-category Date: Wed, 12 Jan 2011 07:50:08 +0100 Message-ID: References: Reply-To: JeanBenabou NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 (Apple Message framework v753.1) Content-Type: text/plain; charset=ISO-8859-1; delsp=yes; format=flowed Content-Transfer-Encoding: quoted-printable X-Trace: dough.gmane.org 1294881323 20513 80.91.229.12 (13 Jan 2011 01:15:23 GMT) X-Complaints-To: usenet@dough.gmane.org NNTP-Posting-Date: Thu, 13 Jan 2011 01:15:23 +0000 (UTC) To: Ross Street , Categories Original-X-From: majordomo@mlist.mta.ca Thu Jan 13 02:15:13 2011 Return-path: Envelope-to: gsmc-categories@m.gmane.org Original-Received: from smtpx.mta.ca ([138.73.1.114]) by lo.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1PdBmS-0000UV-FS for gsmc-categories@m.gmane.org; Thu, 13 Jan 2011 02:15:12 +0100 Original-Received: from mlist.mta.ca ([138.73.1.63]:33352) by smtpx.mta.ca with esmtp (Exim 4.71) (envelope-from ) id 1PdBm1-0006yN-Uy; Wed, 12 Jan 2011 21:14:45 -0400 Original-Received: from majordomo by mlist.mta.ca with local (Exim 4.71) (envelope-from ) id 1PdBlu-0005ZW-Lu for categories-list@mlist.mta.ca; Wed, 12 Jan 2011 21:14:39 -0400 In-Reply-To: Precedence: bulk Xref: news.gmane.org gmane.science.mathematics.categories:6462 Archived-At: Dear Ross, Thank you for answering me and agreeing with me about the difference =20 between internal fibrations and you call "representable" ones.I had =20 started to write a few comments about what you said on the =20 description of the Eilenberg- Moore category as sheaves on the =20 Kleisli category for a generalized topology. I shall give a single =20 answer to the two mails because they have some common features. I apologize if this answer is very sketchy. Each of these questions =20 would deserve a long development, which I can't give for two reasons. 1- Long mails are not accepted on the official list. (I don't want =20 our moderator to think that any criticism is hinted at by this =20 remark, but some questions, especially those dealing with =20 "foundations", do need long developments if they are to be discussed =20 seriously. Thus the official list does not permit such discussions. =20 Can anybody tell me where they can take place publicly?) 2- I'm supposed to give a 2 hours lecture on Feb. 5 on "Transcendent =20 " methods in Category Theory. The audience is quite "mixed": =20 mathematicians, philosophers, linguists, and even... musicians! Quite =20= a challenge since some of them have only the faintest notions about =20 Category Theory. Thus most of my time and energy are devoted to its =20 preparation. =A71- EILENBERG-MOORE VESUS KLEISLI 1.1. I have no objection to the terminology; "sheaves for a =20 generalized topoloogy". You could even drop "generalized" provided =20 you indicate precisely what you mean by "topology" and "sheaf". After =20= all Grothendieck did precisely that when he used these two words for =20 his "topologies' on categories which were indeed "generalized" from =20 "usual" topology. 1.2. There is an ambiguity in your mail when you write. "After all, I believe Linton's work aimed at generalizing to all monads the correspondence between monads of finite rank on Set and Lawvere theories, under which Eilenberg-Moore algebras become product-preserving presheaves on part of the Kleisli category)" As far as I remember Linton did not deal with "all" monads but with =20 monads ON SETS. The same ambiguity can be found e.g. in Lack's mail where he writes: "Similarly, the Elienberg-Moore algebras can be seen as the =20 presheaves on the Kleisli category which send certain diagrams to limits." He never mentions the fact that the monad is ON SETS. I suppose your =20 "sheaf-interpretation" holds only for monads on Set, am I wrong? What if we replace Set by another category, say S? I don't want to =20 under estimate Lawvere's, or Linton's or your work, but; I apologize =20 to theses authors this is a bit of "glorious past-time story", Let me look at simple example, namely the "notion" of group. In the case of sets there are many closely related notions, let me =20 describe a few ones. (i) The category Grp of groups which is monadic over Set (ii)The Kleisli category of this monad (iii) The Lawvere theory of groups, say Th(Grp) If we replace Set by a category S with finite products, and denote by =20= Grp(S) the category of internal groups of S; what is (part of ) the =20 general picture? (a) Grp(S) is the category of product preserving functors Th(Grp) --> =20= S, which you can view as "S-valued sheaves on Th(Grp) for the obvious =20= (generalized) topology . It is equipped with the forgetful functor U; =20= Grp(S) --> S "evaluation at 1 (I apologize for such trivialities) (b) Suppose U has a left adjoint F and let T be the associated monad. (b.1) Is it obvious that Grp(S) is monadic for the monad T? (b.2) What is the precise relation between Th(Grp) and the Kleisli =20 category Kl(T) of T ? (b.3) Can Grp(S) be interpreted as S valued sheaves on K(T) for a =20 suitable topology? A partial answer to these questions can be given when S is a topos =20 with NNO, but,for me at least, even in that case, there remain many =20 important questions which I can't answer. Has anybody been interested =20= by the kind of questions raised in the previous subsection? =A72 FIBRATIONS AND "REPRESENTABLE" FIBRATIONS. Thank you Ross for agreeing with me about the difference between =20 (internal) fibrations and what you call "representable" ones. Sorry if I disagree with you, but I tend to prefer the first ones.It =20 is very easy to generalize important notions of Category Theory to 2-=20 categories by making them "representable" but to me the real problem =20 is to "internalize" these notions,( that is easy by using the =20 internal language which I introduced for precisely that purpose) and =20= to STUDY THE PROPERTIES of these internalized notions; I am long past =20= believing that ,apart from size conditions, ZF, with or without =20 Universes or AC is enough to express all mathematical possibilities. =20 As an,example is the important notion of "definability" which took me =20= a long time to understand, by going "outside" of ZF In your mail you say: "An internal fibration between groups in a topos E is a group =20 morphism whose underlying morphism in E is an epimorphism; for a =20 representable fibration, it is a split epimorphism in E. Jack Duskin =20 alerted me to this many years ago." I do not know when Duskin "alerted" you. What I now is that I found =20 the remark in the original paper of Grothendieck on fibrations (1961) =20= and that I "internalized" it in 1970 when I introduced internal =20 languages. I talked many times of this example in my seminar as an =20 illustration of what internal languages could do. And Duskin attended =20= my seminar for a whole year, and many other times for shorter periods =20= But let's forget about this "detail" and concentrate about more =20 important things. For more than 20 years I have tried to convince people that =20 fibrations and indexed categories are not "the same thing", even if =20 we use AC and universes. For a long time I didn't convince you. I =20 remember having offered 6 bottles of champagne to anyone who could =20 prove, using only indexed categories, that the composite of two =20 fibrations is a fibration. And I got an answer from you where you had =20= to go through the Grothendieck construction for one of the indexed =20 categories. Thus you didn't get the champagne. Of course, if you =20 visit me in Paris, I'll be very glad to share with you a bottle, for =20 old time's sake. I contend that indexed categories are not the same even as cloven =20 fibrations. They are the same IN SETS. But if you you go back to =A71, =20= you'll immediately see the difference. Fibrations and cloven =20 fibrations can be internalized, e.g. in a topos (although this =20 assumption is much too strong); Indexed categories cannot. And even =20 if by some very complicated construction one could, in some special =20 case, internalize them, unless you add some very strong artificial =20 assumptions they would not coincide with internal fibrations, not =20 even cloven ones. Let me give a final trivial example to try again to convince you and =20 a few other ones. Let me call for short "surjective" morphism of =20 groups a morphism of groups such the the underlying morphism is an =20 epi (better be a regular epi). Obviously they are stable under =20 composition. How would you formulate this in terms of "internal =20 indexed categories", assuming you have defined such notion? There is a lot more I could say but the mail is already very long, =20 and I hope it will be forwarded. Thanks for reading me. Best wishes, Jean=20 =20= [For admin and other information see: http://www.mta.ca/~cat-dist/ ]