From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.io/gmane.science.mathematics.categories/6739 Path: news.gmane.org!not-for-mail From: Eduardo Dubuc Newsgroups: gmane.science.mathematics.categories Subject: size_question_encore Date: Tue, 05 Jul 2011 20:29:19 -0300 Message-ID: Reply-To: Eduardo Dubuc NNTP-Posting-Host: lo.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Trace: dough.gmane.org 1309969582 8719 80.91.229.12 (6 Jul 2011 16:26:22 GMT) X-Complaints-To: usenet@dough.gmane.org NNTP-Posting-Date: Wed, 6 Jul 2011 16:26:22 +0000 (UTC) To: Categories Original-X-From: majordomo@mlist.mta.ca Wed Jul 06 18:26:17 2011 Return-path: Envelope-to: gsmc-categories@m.gmane.org Original-Received: from smtpx.mta.ca ([138.73.1.30]) by lo.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1QeUvY-0008AM-T4 for gsmc-categories@m.gmane.org; Wed, 06 Jul 2011 18:26:17 +0200 Original-Received: from mlist.mta.ca ([138.73.1.63]:52441) by smtpx.mta.ca with esmtp (Exim 4.71) (envelope-from ) id 1QeUtY-0005J7-Ep; Wed, 06 Jul 2011 13:24:12 -0300 Original-Received: from majordomo by mlist.mta.ca with local (Exim 4.71) (envelope-from ) id 1QeUtX-0005Up-Op for categories-list@mlist.mta.ca; Wed, 06 Jul 2011 13:24:11 -0300 Precedence: bulk Xref: news.gmane.org gmane.science.mathematics.categories:6739 Archived-At: 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. [For admin and other information see: http://www.mta.ca/~cat-dist/ ]