From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.io/gmane.science.mathematics.categories/4414 Path: news.gmane.org!not-for-mail From: Sam Staton Newsgroups: gmane.science.mathematics.categories Subject: General notions of equivalence and exactness Date: Thu, 29 May 2008 10:01:31 +0100 Message-ID: NNTP-Posting-Host: main.gmane.org Mime-Version: 1.0 (Apple Message framework v753) Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed Content-Transfer-Encoding: 7bit X-Trace: ger.gmane.org 1241019932 13182 80.91.229.2 (29 Apr 2009 15:45:32 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 29 Apr 2009 15:45:32 +0000 (UTC) To: categories@mta.ca Original-X-From: rrosebru@mta.ca Thu May 29 08:30:43 2008 -0300 Return-path: Envelope-to: categories-list@mta.ca Delivery-date: Thu, 29 May 2008 08:30:43 -0300 Original-Received: from Majordom by mailserv.mta.ca with local (Exim 4.61) (envelope-from ) id 1K1gBY-0007hJ-50 for categories-list@mta.ca; Thu, 29 May 2008 08:20:44 -0300 Original-Sender: cat-dist@mta.ca Precedence: bulk X-Keywords: X-UID: 37 Original-Lines: 43 Xref: news.gmane.org gmane.science.mathematics.categories:4414 Archived-At: Hello. In a category with pullbacks, say that a binary relation X <- R -> Y is "z-closed" if it satisfies the following axiom (interpreted as usual): If x R y and x' R y and x' R y' then x R y'. (The "z" in "z-closed" refers to the pattern of variables in the premise.) Z-closedness seems to be a sensible generalization of "equivalence" to relations between two different objects. (e.g. In computer science, it is common to relate the state spaces of two different systems.) Note that an endorelation is an equivalence relation if and only if it is z-closed and reflexive. Also note that, in an abelian category, every relation is z-closed. The [z-closed v. equivalence] connection seems to extend to [pullbacks v. kernel pairs]. Every span that arises from a pullback is a z-closed relation. Say that a category is "z-effective" if every z-closed relation arises as a pullback. - every abelian category is straightforwardly z-effective; - in a topos, every z-closed relation arises as a pullback span. Indeed, an extensive regular category has effective equivalence relations if and only if it is z-effective. These notions and ideas seem quite elementary, even fundamental, and I would be surprised if no-one had thought of them before. I borrowed the terminology "z-closed" from a paper by Erik de Vink and Jan Rutten (Theoret Comput Sci, 221:271-293, 1999) but I couldn't find any other references. Have I missed something? I'd be grateful for any observations or suggestions. Sam PS. I'd like to take the opportunity to acknowledge the helpful replies (public and private) to my question about W-types, a few months ago.