* whoops
@ 2000-12-16 16:22 Peter Freyd
2000-12-17 19:10 ` whoops Michael Barr
0 siblings, 1 reply; 2+ messages in thread
From: Peter Freyd @ 2000-12-16 16:22 UTC (permalink / raw)
To: categories
There was a little trap waiting for Mike and me to fall into. We were
seeking conditions on a category C that force Ab[C] to be abelian
and we came up with the same condition. I wrote, "Note that the
conclusion (abelian) is self-dual but the condition (effective
regular) is not." The trouble is: the definition of Ab[C] is not
self-dual. I suppose my assertion is true as it stands but the
implicit message is wrong. Mike had no such luck; he made it explicit:
"[I]t is sufficient that either C or C^op be exact." (Yes, my
"effective regular" and Mike's "exact" are equivalent -- it follows
just from regularity that the pullback of a cover against itself is
a pushout.)
It took me a while to find a counterexample and I'm not happy with the
one I found. Adjoin to the equational theory of abelian groups a new
constant and no further axioms. Let C be the category of finite
models. As is the case for the finite models of any equational theory,
C is effective regular (and Ab[C] is isomorphic to the category of
finite abelian groups). But Ab[C^op] is not abelian. It's empty.
(C^op doesn't have a terminator.)
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: whoops
2000-12-16 16:22 whoops Peter Freyd
@ 2000-12-17 19:10 ` Michael Barr
0 siblings, 0 replies; 2+ messages in thread
From: Michael Barr @ 2000-12-17 19:10 UTC (permalink / raw)
To: categories
Sigh! Peter is, of course, correct. It actually occurred to me a couple
days after I wrote my reply that Ab[C^op] is not the opposite of Ab[C] but
I never looked into it.
Michael
On Sat, 16 Dec 2000, Peter Freyd wrote:
> There was a little trap waiting for Mike and me to fall into. We were
> seeking conditions on a category C that force Ab[C] to be abelian
> and we came up with the same condition. I wrote, "Note that the
> conclusion (abelian) is self-dual but the condition (effective
> regular) is not." The trouble is: the definition of Ab[C] is not
> self-dual. I suppose my assertion is true as it stands but the
> implicit message is wrong. Mike had no such luck; he made it explicit:
> "[I]t is sufficient that either C or C^op be exact." (Yes, my
> "effective regular" and Mike's "exact" are equivalent -- it follows
> just from regularity that the pullback of a cover against itself is
> a pushout.)
>
> It took me a while to find a counterexample and I'm not happy with the
> one I found. Adjoin to the equational theory of abelian groups a new
> constant and no further axioms. Let C be the category of finite
> models. As is the case for the finite models of any equational theory,
> C is effective regular (and Ab[C] is isomorphic to the category of
> finite abelian groups). But Ab[C^op] is not abelian. It's empty.
> (C^op doesn't have a terminator.)
>
>
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