From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.io/gmane.science.mathematics.categories/6178 Path: news.gmane.org!not-for-mail From: Thomas Streicher Newsgroups: gmane.science.mathematics.categories Subject: why it matters that fibrations are "evil" Date: Thu, 16 Sep 2010 10:50:15 +0200 Message-ID: References: Reply-To: Thomas Streicher NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: dough.gmane.org 1284689065 17094 80.91.229.12 (17 Sep 2010 02:04:25 GMT) X-Complaints-To: usenet@dough.gmane.org NNTP-Posting-Date: Fri, 17 Sep 2010 02:04:25 +0000 (UTC) To: categories@mta.ca Original-X-From: majordomo@mlist.mta.ca Fri Sep 17 04:04:24 2010 Return-path: Envelope-to: gsmc-categories@m.gmane.org Original-Received: from smtpy.mta.ca ([138.73.1.139]) by lo.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1OwQJL-0003Vi-NF for gsmc-categories@m.gmane.org; Fri, 17 Sep 2010 04:04:24 +0200 Original-Received: from mlist.mta.ca ([138.73.1.63]:60541) by smtpy.mta.ca with esmtp (Exim 4.71) (envelope-from ) id 1OwQGr-0002cL-BN; Thu, 16 Sep 2010 23:01:49 -0300 Original-Received: from majordomo by mlist.mta.ca with local (Exim 4.71) (envelope-from ) id 1OwQGk-0002ey-Sw for categories-list@mlist.mta.ca; Thu, 16 Sep 2010 23:01:43 -0300 Content-Disposition: inline In-Reply-To: Precedence: bulk Xref: news.gmane.org gmane.science.mathematics.categories:6178 Archived-At: Let BB be a (base) topos. Then split fibrations over BB correspond to categories internal to Psh(BB). (Well, in Psh(BB) = Set^{BB^op} one has to choose Set big enough!). Thus in Psh(B) one can speak about split fibrations over BB as categories internal to Psh(BB). To which extent can this be extended to arbitrary fibrations P : XX -> BB typically not split. In a recent paper by Mike Shulman this has been exercised for the fundamental fibration P_BB : BB^2 -> BB. (Well, he was even exploiting that P_BB is a stack w.r.t. the regular topology on BB). But it works for every fibration P. One can speak about P in a first order dependently typed language L(P) without equality of objects. Generalized elements of sort "object" at stage I are objects of the fibre P(I) and similarly for morphisms. Now this language can be given a Kripke-Joyal interpretation in BB. If P is a JJ-stack then one thinks of BB as endowed with the topology JJ and incorporates this into the Kripke-Joyal interpretation. The key trick is that implicitly one quantifies over all possible reindexings. First I was quite enthusiastic about this because I thought that it augments the usual fibrational foundations of category theory over an arbitrary base topos with a linguistic framework entending that of the internal language of the base (or rather (pre)sheaves over it). But when trying to test the viability of this approach I came across the following problem. It is not enough to quantify over objects but one also has to quantify over families of objects indexed objects of BB. This leads one to consider also the family fibration Fam(P) over BB together with the cartesian functor FFam(P) from Fam(P) to P_BB. One can now easily cook up a language speaking about P, Fam(P) and the cartesian functor FFam(P). The first thing one wants to show is that the proposition "Fam(P) is a fibration" holds under the Kripke-Joyal interpretation. But when writing this down one sees that one cannot escape equality of objects which has been dropped to make the whole thing possible. Thus it might be easier to replace P by an equivalent split fibration Sp(P) and reason about this in Psh(BB). Nothing seems to be lost when one considers everything up to isomorphism. The question rather is whether this excessive prevention of "evil" is appropriate. I have some doubts since it prevents us from speaking about fibrations in a natural way. Thomas [For admin and other information see: http://www.mta.ca/~cat-dist/ ]