* *-Autonomous Functor Categories, revision
@ 2005-11-26 12:50 Peter Freyd
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From: Peter Freyd @ 2005-11-26 12:50 UTC (permalink / raw)
To: categories
Mike Barr has pointed out that the proof in my last posting of
LEMMA: The object I = H^R is injective in *F*.
doesn't work. (It was actually the fourth proof I had come up with.
I wondered why it was so much simpler). So here's one that does work
(and is just about as simple).
Let
O
|
H^R
|
H^A --> H^B --> T --> O
be exact (all vertical arrows point down). We seek a retraction for
H^R --> T. Since H^R is projective (as is any representable) we may
choose a map H^R --> H^B to yield a commutative triangle. The full
subcategory of representables is closed under finite limits, so let
H^C --> H^R
| |
H^A --> H^B
be a pullback in *F* and let
B --> A
| |
R --> C
be the corresponding pushout in the category of f.p R-modules. The
map from H^C to T is the zero map and we use the hypothesis that
H^R --> T is monic to infer that H^C --> H^R, hence R --> C, are
zero maps. Let O --> K --> B --> A be exact. It is an exercise in
abelian categories that R --> C = 0 implies K --> B --> R is
epi. Now (finally using the projectivity of R) choose a retraction
R --> K. The map H^A --> H^B --> H^K --> H^R is of course, a zero
map and we may factor H^B --> H^K --> H^R as H^B --> T --> H^R.
The map T --> H^R is easily checked to be the retraction we seek.
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