Wednesday, 14 May 2008

fa.functional analysis - Compact Hausdorff and C^*-algebra "objects" in a category.

Question 1: If I understand you correctly, you're proposing that mathbbC should be a compact Hausdorff object in some category because it represents a functor from that category to the category CH of compact Hausdorff spaces (in something like the sense that the functor Hom(,mathbbC) into Set factors through the forgetful functor from CH to Set). But I don't see why this should be sufficient to make mathbbC a compact Hausdorff object.



That is, presumably, from the approach of functorial semantics, a compact Hausdorff object in a category C should be a product-preserving functor from L to C, where L is the dual of the Kleisli category for the ultrafilter monad on Set (that is, L is the Lawvere theory whose category of (Set-)models is the category of compact Hausdorff spaces). I can see how, more generally, for any Lawvere theory L and category C, every C-model of L (i.e., a product-preserving functor F from L to C) induces a representable functor Hom(-, F(1)) from C to Set which factors through the forgetful functor from Set-models of L to Set. But it's not obvious to me that the converse of this holds as well (that every representable functor from C to Set with this factorization property arises from some C-model of L).



Perhaps I'm missing something and your reasoning for mathbbC being a compact Hausdorff object is something more than this. Perhaps I'm hopelessly confused. But, tentatively, I think the answer to question 1 is "No" or at least "Not necessarily".



(Edit: As seen below, the correspondence does go both ways, so the last line is retracted, leaving the second-to-last line...)

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