From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.io/gmane.science.mathematics.categories/8125 Path: news.gmane.org!not-for-mail From: Colin McLarty Newsgroups: gmane.science.mathematics.categories Subject: Functoriality of pullbacks of sets Date: Sun, 18 May 2014 10:59:45 -0400 Message-ID: Reply-To: Colin McLarty NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 X-Trace: ger.gmane.org 1400458509 23498 80.91.229.3 (19 May 2014 00:15:09 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 19 May 2014 00:15:09 +0000 (UTC) To: "categories@mta.ca" Original-X-From: majordomo@mlist.mta.ca Mon May 19 02:15:04 2014 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 1WmBEN-00061u-3v for gsmc-categories@m.gmane.org; Mon, 19 May 2014 02:15:03 +0200 Original-Received: from mlist.mta.ca ([138.73.1.63]:59276) by smtp3.mta.ca with esmtp (Exim 4.80) (envelope-from ) id 1WmBDu-0005Be-R5; Sun, 18 May 2014 21:14:34 -0300 Original-Received: from majordomo by mlist.mta.ca with local (Exim 4.71) (envelope-from ) id 1WmBDu-0000N2-GC for categories-list@mlist.mta.ca; Sun, 18 May 2014 21:14:34 -0300 Precedence: bulk Xref: news.gmane.org gmane.science.mathematics.categories:8125 Archived-At: It seems to me that Peter Freyd remarked it is easy to define pullbacks in ZF (maybe with with global choice?) so that pullback along one side is functorial, but hard to make it functorial on both sides. In other words we can easily make base change functorial in the bases, but not easily make it functorial in the bases at the same time as in the total spaces. Can anyone direct me to a reference to that work? thanks, Colin [For admin and other information see: http://www.mta.ca/~cat-dist/ ]