From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.io/gmane.science.mathematics.categories/7931 Path: news.gmane.org!not-for-mail From: Jeff Egger Newsgroups: gmane.science.mathematics.categories Subject: Re: subgroupoids of V-categories Date: Wed, 20 Nov 2013 07:53:32 -0800 (PST) Message-ID: References: Reply-To: Jeff Egger NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: ger.gmane.org 1385044791 31185 80.91.229.3 (21 Nov 2013 14:39:51 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 21 Nov 2013 14:39:51 +0000 (UTC) To: Categories , Emily Riehl Original-X-From: majordomo@mlist.mta.ca Thu Nov 21 15:39:54 2013 Return-path: Envelope-to: gsmc-categories@m.gmane.org Original-Received: from smtp3.mta.ca ([138.73.1.186]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VjVQA-0006sF-6M for gsmc-categories@m.gmane.org; Thu, 21 Nov 2013 15:39:54 +0100 Original-Received: from mlist.mta.ca ([138.73.1.63]:43989) by smtp3.mta.ca with esmtp (Exim 4.80) (envelope-from ) id 1VjVP4-0004lP-AJ; Thu, 21 Nov 2013 10:38:46 -0400 Original-Received: from majordomo by mlist.mta.ca with local (Exim 4.71) (envelope-from ) id 1VjVP2-00075Y-OW for categories-list@mlist.mta.ca; Thu, 21 Nov 2013 10:38:44 -0400 In-Reply-To: Precedence: bulk Xref: news.gmane.org gmane.science.mathematics.categories:7931 Archived-At: Hi Emily, I'm a bit worried about the notion of "local monomorphism". If V is, say, a poset, then every arrow in it is (tautologously) monic, and therefore every V-functor is a local monomorphism. Let us consider, for instance, generalised metric spaces. Suppose C has two vertices, p and q, with C(p,q)=0 and C(q,p)=oo (infinity). Then, for any e>0, we can choose G_e with G_e(p,q)=e and G(q,p)=oo, and the identity-on-objects functor G_e --> C has the property you desire. [I'm sure you realise that the definition of "isomorphism" you gave is equivalent to that of "isomorphism in the underlying category"; the underlying category of C is "the arrow", whereas that of G_e is discrete.] So "the maximal subgroupoid" of C is not defined uniquely, except up to "equivalence of underlying category". Moreover, whenever e>d>0, the i-o-o functor G_e --> C factors through G_d, so there is not even an "optimal" choice of e. More precisely, if one were to define a V-groupoid to be a V-category whose underlying category is a groupoid (which is, implicitly, what you seem to be doing), then I've just shown that the forgetful functor from [0,oo]-groupoids to [0,oo]-categories does not have a right adjoint. Of course, I think it would be better to define a V-groupoid in a more Hopf-y kind of way---e.g., in terms of what one might call a co-Maltsev operation---but that might not suit your purposes, and would seem to make things much more difficult, at least in general. Cheers, Jeff. -------------------------------------------- On Tue, 11/19/13, Emily Riehl wrote: Subject: categories: subgroupoids of V-categories To: "Categories" Received: Tuesday, November 19, 2013, 1:51 PM Hi all, For general V (closed symmetric monoidal, bicomplete), is there a general way to construct the maximal subgroupoid of a V-category C? I think I know how to *detect* the maximal subgroupoid. A map in C is an isomorphism iff it is representably so: Writing 1 for the monoidal unit, we say f : 1 -> C(x,y) is an iso iff the induced map f_* : C(z,x) -> C(z,y) is an iso in V for all z. So we might say that a V-category G with the same objects and C and an identity-on-objects local monomorphism G -> C is the maximal subgroupoid provided that a morphism f factors through G(x,y) -> C(x,y) just when f is an isomorphism. In examples, this is probably good enough, but I still would feel better if I had a general construction of the maximal subgroupoid. I feel like this should be some sort of weighted limit, perhaps with some additional structure on V? Thanks, Emily [For admin and other information see: http://www.mta.ca/~cat-dist/ ]