Reference request for translating from Top to C*-alg - MathOverflow

Reference request for translating from Top to C*-alg

2011-12-07T13:46:41Z

Some recent questions on MO (for example, http://mathoverflow.net/questions/82708/do-subalgebras-of-cx-admit-a-description-in-terms-of-the-compact-hausdorff-spac) have been about Gelfand duality-- namely, that the categories of compact Hausdorff spaces with continuous maps, and commutative unital C*-algebras with unital $*$ -homomorphisms, are anti-equivalent. Thus one can "translate" properties about compact spaces over to C*-algebras. This can lead to a sort of "dictionary", see for example page 3 of Várilly's book on noncommutative geometry (which is http://books.google.co.uk/books?id=f5cAlaptxd0C&lpg=PP1&dq=non-commutative%20geometry&pg=PA3#v=onepage&q=non-commutative%20geometry&f=false).

Does anyone know a reasonably definitive reference for proofs of such dictionaries, in a self-contained form??

I'm guessing that perhaps such a thing doesn't exist, as these results are folklore (and are easy to prove really-- given the statement, the proofs often form nice exercises). As one is really just studying compact spaces via the category of compact spaces with continuous map, might there be a category theory book which is suitable?

Actually, I am more interested in the non-unital case. Rather than working with proper maps, I instead want to follow Woronowicz. Define a "morphism" between C $^*$ -algebras $A$ and $B$ to be a non-degenerate $*$ -homomorphism $\phi:A\rightarrow M(B)$ from $A$ to the multiplier algebra of $B$, where "non-degenerate" means that $\{ \phi(a)b : a\in A,b\in B \}$ is linearly dense in $B$. Then the category of commutative C $^*$ -algebras and morphisms is anti-equivalent to the category of locally compact spaces and continuous maps. One can then form a similar dictionary-- but here I think the proofs can be a bit trickier (or maybe just they use slightly less standard topology).

Does anyone know a reasonably definitive reference in this more general setting?

Answer by Martin Brandenburg for Reference request for translating from Top to C*-alg

2011-12-08T12:20:51Z

Let's make a list here. Everyone is invited to add and complete the list and the proofs.

List

0) locally compact Hausdorff spaces $\longleftrightarrow$ commutative C*-algebras

0') proper continuous maps $\longleftrightarrow$ non-degenerate C*-homomorphisms

1) compact $\longleftrightarrow$ unital

2) point $\longleftrightarrow$ maximal ideal

3) closed embedding $\longleftrightarrow$ closed ideal

4) surjection/injection $\longleftrightarrow$ injection/surjection

5) homeomorphism $\longleftrightarrow$ automorphism

6) clopen subset $\longleftrightarrow$ projection

7) totally disconnected $\longleftrightarrow$ AF-algebra (AF = approximately finite dimensional)

8) One-point compactification $\longleftrightarrow$ unitalization

9) Stone-Cech compactification $\longleftrightarrow$ multiplier algebra

10) Borel measure $\longleftrightarrow$ positive functional

11) probability measure $\longleftrightarrow$ state

12) disjoint union $\longleftrightarrow$ product

13) product $\longleftrightarrow$ completed tensor product

14) topological K-Theory $K^0$ $\longleftrightarrow$ algebraic K-theory $K_0$

Proofs

0),1),2),3),5) follow directly from Gelfand duality - details can be found, for example, in Murphey's book about C*-algebras. For 0'), see here (I wrote this up because I didn't know any reference). A C*-homomorphism $A \to B$ is nondegenerate if the ideal generated by the image is dense. For 4) see here. 6) is given by characteristic functions. A reference for 7) is Kenneth R. Davidson, C*-Algebras by Example, Theorem III.2.5. It is related to 6) because a commutative C*-algebra is AF iff it is separable and topologically generated by the projections. 8) follows from abstract nonsense and 1). 9) ?. 10) is the Riesz representation Theorem. 11) follows from 10). 12) asserts $C_0(X \coprod Y) = C_0(X) \times C_0(Y)$, which is trivial. 13) asserts that the canonical map $C_0(X) \hat{\otimes} C_0(Y) \to C_0(X \times Y)$ is an isomorphism - this follows from the Theorem of Stone-Weierstraß. 14) is the Theorem of Serre-Swan.

Answer by Amin for Reference request for translating from Top to C*-alg

2011-12-09T01:23:07Z

For 9 I have to say that I have also never seen a direct proof, but I think I remember some hints in Wegge-Olsen (in exercises of 'translation'); I can't garantee that it isn't the usual functorial argument tough...

Answer by Terry Loring for Reference request for translating from Top to C*-alg

2011-12-11T21:23:23Z

Gert Pedersen wrote "In a careless moment a C*-algebraist might be quoted for saying that there is a covariant functor between the categories of commutative C*-algebras with morphisms and the category of locally compact Hausdorff spaces with continuous maps." (Morphisms of Extensions of C*-Algebras: Pushing Forward the Busby Invariant, by Eilers me and Pedersen.)

Either of these correspondences is valid:

List item

proper continuous maps <--> proper *-homomorphisms from A to B

List item

continuous maps <--> nondegenerate *-homomorphisms from A to M(B).

I misunderstood the exercise on Page 44 of Wegge-Olsen is wrong. This set of the discussion below.