From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.io/gmane.science.mathematics.categories/3079 Path: news.gmane.org!not-for-mail From: Robert Pare Newsgroups: gmane.science.mathematics.categories Subject: Undirected graphs Date: Wed, 8 Mar 2006 11:07:20 -0400 (AST) Message-ID: <20060308150720.9D5D07375B@chase.mathstat.dal.ca> NNTP-Posting-Host: main.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Trace: ger.gmane.org 1241019083 7155 80.91.229.2 (29 Apr 2009 15:31:23 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 29 Apr 2009 15:31:23 +0000 (UTC) To: categories@mta.ca Original-X-From: rrosebru@mta.ca Wed Mar 8 23:27:28 2006 -0400 Return-path: Envelope-to: categories-list@mta.ca Delivery-date: Wed, 08 Mar 2006 23:27:28 -0400 Original-Received: from Majordom by mailserv.mta.ca with local (Exim 4.52) id 1FHBmk-0002uW-MM for categories-list@mta.ca; Wed, 08 Mar 2006 23:25:54 -0400 Original-Sender: cat-dist@mta.ca Precedence: bulk X-Keywords: X-UID: 25 Original-Lines: 15 Xref: news.gmane.org gmane.science.mathematics.categories:3079 Archived-At: [note from moderator: resent due to faulty From: ] I've been following the recent posts on undirected graphs with interest. But I have a question. I think it's being said that undirected graphs are the same as directed graphs with involution. (Presheaves on the full subcategory of SET determined by 1 and 2, or just 2.) Which is nice but what about loops? The involution might fix a loop or not. So wouldn't we be getting undirected graphs with two kinds of loops, whole loops and semiloops? What am I missing? Bob