* More comments on Functorial injective hulls
@ 2000-03-23 19:12 Peter Freyd
[not found] ` <pjf@saul.cis.upenn.edu>
0 siblings, 1 reply; 2+ messages in thread
From: Peter Freyd @ 2000-03-23 19:12 UTC (permalink / raw)
To: categories
Some comments on:
>Theorem. Let H be a class of morphisms in a category C such that
>1. all H-injective objects form a cogenerating class, and
>2. the class of all H-essential morphisms which are epimorphic
> is precisely the class of isomorphisms of C .
>Then C cannot have natural H-injective hulls (i.e. they cannot
>form an endofunctor together with a natural transformation from Id)
>unless every object in C is H-injective.
Walter wrote "We are able to compensate for the loss of mono through
condition 1". Wouldn't it be simpler just to say that condition 1
implies that everything in H is a monomorphisms?
x y
(Let A --> B be an H-morphism and let X --> A, X --> A be such
x y
that X --> A --> B = X --> A --> B. If x were different from
y then there would be A --> E, E an H-injective object, so that
x y
X --> A --> E were different from X --> A --> E. But there would
have to be B --> E such that A --> E = A --> B --> E and
x y
X --> A --> B would have be different from X --> A --> B.)
So H-morphism is a strengthening of monic and that put's us back to
the situation I outlined:
If the strengthening of monic is such that it becomes an iso
whenever epic then there's an easy proof of the impossibility of
functoriality, with or without a cogenerator.
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: More comments on Functorial injective hulls
[not found] ` <pjf@saul.cis.upenn.edu>
@ 2000-03-24 19:06 ` Walter Tholen
0 siblings, 0 replies; 2+ messages in thread
From: Walter Tholen @ 2000-03-24 19:06 UTC (permalink / raw)
To: categories
Good point, I did not see that! So the whole thing boils down to a very simple
observation on pointed endofunctors (which is certainly well-known for
reflectors): suppose you have an endofunctor F and a natural transformation
u:Id --> F which is pointwise monic; then, if Fu is pointwise epic, u itself is
pointwise epic. Proof:
u_A x,y
A ---> FA ---> B
| | |
u_A | u_FA | | u_B
| | |
FA --> FFA --> FB
Fu_A Fx,Fy
( xu = yu gives Fx.Fu = Fy.Fu, hence Fx = Fy and then ux = uy and x = y.)
Coming back to injectivity: if u was pointwise an H-injective hull, then both
uF and Fu must be isos (independently of H being a class of monos or not!).
Hence, if H is a class of monos, the statement above applies.
> Walter wrote "We are able to compensate for the loss of mono through
> condition 1". Wouldn't it be simpler just to say that condition 1
> implies that everything in H is a monomorphisms?
> x y
> (Let A --> B be an H-morphism and let X --> A, X --> A be such
> x y
> that X --> A --> B = X --> A --> B. If x were different from
> y then there would be A --> E, E an H-injective object, so that
> x y
> X --> A --> E were different from X --> A --> E. But there would
> have to be B --> E such that A --> E = A --> B --> E and
> x y
> X --> A --> B would have be different from X --> A --> B.)
>
>
> So H-morphism is a strengthening of monic and that put's us back to
> the situation I outlined:
>
> If the strengthening of monic is such that it becomes an iso
> whenever epic then there's an easy proof of the impossibility of
> functoriality, with or without a cogenerator.
>
>-- End of excerpt from Peter Freyd
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2000-03-23 19:12 More comments on Functorial injective hulls Peter Freyd
[not found] ` <pjf@saul.cis.upenn.edu>
2000-03-24 19:06 ` Walter Tholen
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