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From: Michael Barr <mbarr@math.mcgill.ca>
To: Categories list <categories@mta.ca>
Subject: Name for a concept
Date: Wed, 30 Nov 2005 20:48:52 -0500 (EST)	[thread overview]
Message-ID: <Pine.LNX.4.44.0511302040500.6673-100000@triples.math.mcgill.ca> (raw)

Is there a standard name for a square
A ----> B
|       |
|       |
|       |
v       v
C ----> D
in which the canonical map A ---> B x_D C is epic?  I had always called it
a weak pullback, but Peter Freyd claims that that phrase is reserved for
the case that it satisfies the existence, but not necessarily the
uniqueness of the definition of pullback.  In fact, he claims it means
that Hom(E,-) converts it to the kind of square I am talking about.
What is interesting is that in an abelian category, it satisfies
this condition iff it satisfies the dual condition iff the evident
sequence A ---> B x C ---> D is exact.  Putting a zero at the left end
characterizes a genuine pullback and at the other end a pushout.

Michael





             reply	other threads:[~2005-12-01  1:48 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2005-12-01  1:48 Michael Barr [this message]
2005-12-02 11:19 ` Ronald  Brown
2005-12-02 13:51 ` Marco Grandis
2005-12-05 16:16 ` Eduardo Dubuc
2005-12-07 11:04 ` Marco Grandis
2005-12-05 14:44 Marco Grandis
2005-12-06 10:12 jean benabou
2005-12-07  0:58 ` Toby Bartels
2005-12-07 19:15 ` Eduardo Dubuc
2005-12-07 13:36 Peter Freyd
2005-12-08 11:06 name " Clemens.BERGER
2005-12-08 11:26 Clemens.BERGER

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