From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.io/gmane.science.mathematics.categories/8559 Path: news.gmane.org!not-for-mail From: Robin Cockett Newsgroups: gmane.science.mathematics.categories Subject: Partial functors .. Date: Mon, 16 Mar 2015 17:12:51 -0600 Message-ID: Reply-To: Robin Cockett NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Trace: ger.gmane.org 1426591315 23780 80.91.229.3 (17 Mar 2015 11:21:55 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 17 Mar 2015 11:21:55 +0000 (UTC) To: Categories list Original-X-From: majordomo@mlist.mta.ca Tue Mar 17 12:21:45 2015 Return-path: Envelope-to: gsmc-categories@m.gmane.org Original-Received: from smtp3.mta.ca ([138.73.1.127]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YXpZA-0007HJ-LU for gsmc-categories@m.gmane.org; Tue, 17 Mar 2015 12:21:44 +0100 Original-Received: from mlist.mta.ca ([138.73.1.63]:42402) by smtp3.mta.ca with esmtp (Exim 4.80) (envelope-from ) id 1YXpYK-0007tA-PJ; Tue, 17 Mar 2015 08:20:52 -0300 Original-Received: from majordomo by mlist.mta.ca with local (Exim 4.71) (envelope-from ) id 1YXpYI-0003sU-Sx for categories-list@mlist.mta.ca; Tue, 17 Mar 2015 08:20:50 -0300 Precedence: bulk Xref: news.gmane.org gmane.science.mathematics.categories:8559 Archived-At: David Leduc googlemail.com> writes: > A partial functor from C to D is given by a subcategory S of C and a > functor from S to D. What is the appropriate notion of natural > transformation between partial functors that would allow to turn small > categories, partial functors and those "natural transformations" into > a bicategory? The difficulty is that two partial functors from C to D > might not have the same definition domain. Here is a basic and quite natural interpretation (if someone has not already pointed this out): One can have a n.t F => G iff F is less defined than G and on their common domain (which is just the domain of F) there is a natural transformation from F => \rst{F} G. Partial functors, of course, form a restriction category so they are naturally partial order enriched (by restriction). This 2-cell structure must simply respect this partial order ... This is certainly not the only possibility, unfortunately ... for example why not also allow partial natural transformations ... which are less defined than the functor. Here one does have to be a bit careful: a natural transformation must "know" the subcategory it is working with ... thus defining the natural transformation as a function on arrows (rather than just objects) is worthwhile adjustment (see MacLane page 19, Excercise 5). This then works too .... I hope this helps. -robin [For admin and other information see: http://www.mta.ca/~cat-dist/ ]