From: Thomas Streicher <streicher@mathematik.tu-darmstadt.de>
To: categories@mta.ca
Subject: relations on graphs
Date: Fri, 9 Mar 2007 20:10:58 +0100 (CET) [thread overview]
Message-ID: <E1HPm2k-0004lu-Va@mailserv.mta.ca> (raw)
If one allows multiple edges with the same source and target then
they certainly form a topos, namely that of presheaves over the
category with 2 objects and 2 parallel nontrivial arrows.
The \neg\neg-separated objects in this topos are precisely those
graphs where there is at most one edge from one node to another one.
The latter category is not a topos but a quasitopos.
The non-full monos in this category are typical examples of
epic monos which are not isos.
All this can be found in Lawvere's "Qualitative distinctions between
toposes of graphs".
Thomas Streicher
next reply other threads:[~2007-03-09 19:10 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-03-09 19:10 Thomas Streicher [this message]
-- strict thread matches above, loose matches on Subject: below --
2007-03-09 22:40 Richard Garner
2007-03-09 17:49 Ronnie Brown
2007-03-09 17:02 Vaughan Pratt
2007-03-09 15:33 Jamie Vicary
2007-03-09 10:01 Jamie Vicary
2007-03-08 15:15 John Stell
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