From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.io/gmane.science.mathematics.categories/4342 Path: news.gmane.org!not-for-mail From: Vaughan Pratt Newsgroups: gmane.science.mathematics.categories Subject: Re: A question on adjoints Date: Wed, 19 Mar 2008 16:41:07 -0700 Message-ID: NNTP-Posting-Host: main.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Trace: ger.gmane.org 1241019883 12831 80.91.229.2 (29 Apr 2009 15:44:43 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 29 Apr 2009 15:44:43 +0000 (UTC) To: Categories list Original-X-From: rrosebru@mta.ca Thu Mar 20 11:16:51 2008 -0300 Return-path: Envelope-to: categories-list@mta.ca Delivery-date: Thu, 20 Mar 2008 11:16:51 -0300 Original-Received: from Majordom by mailserv.mta.ca with local (Exim 4.61) (envelope-from ) id 1JcLUI-0004tm-7j for categories-list@mta.ca; Thu, 20 Mar 2008 11:11:22 -0300 Original-Sender: cat-dist@mta.ca Precedence: bulk X-Keywords: X-UID: 106 Original-Lines: 14 Xref: news.gmane.org gmane.science.mathematics.categories:4342 Archived-At: So it *is* a counterexample---to the notion that any old graph theorist can do category theory. Focusing on the naturality, I forgot about functoriality (done that before). More embarrassing is not thinking to perform the easiest test of all category theory, F(0) = 0. And most embarrassing is thinking that Mike could have overlooked such an easy example. Sorry, Mike! Vaughan Michael Barr wrote: > Actually, F isn't even a functor. The unique arrow 0 --> Ub has to give > a canonical arrow F0 = 1 --> b, which there isn't.