Finding questions between functional analysis and set theory Are there some good questions on functional analysis whose solution depends on tools in set theory? My major is mathematical logic, I think tools in set theory, especially infinity combinatorics and forcing, should be used to solve some questions in functional analysis. For functional analysis, I just have read the main part of Conway's textbook. In this book, I have not found such questions.
 A: One interesting example is A discontinuous homomorphism from C(X) without CH, by W. Hugh Woodin, which begins with the following introduction.


Suppose that X is an infinite compact Hausdorff space and let C(X) be the algebra
of continuous real-valued functions on X. Then C(X) is a commutative Banach algebra
relative to the supnorm: || f || = sup{ f(p) | p ∈ X }. A well-known question of I.
Kaplansky posed around 1947 asks whether every algebra homomorphism of C{X)
into a Banach algebra B is necessarily continuous.




There is a discontinuous homomorphism of C(X) if and only if there is a
discontinuous homomorphism of C(X, C), the C*-algebra of continuous complexvalued
functions on X. We prefer to deal with the real case; some of the references
adopt the complex view. The question is now known to be independent of the axioms
of set theory, ZFC. H. G. Dales [1] and J. Esterle [3] independently constructed
discontinuous homomorphisms of C{X) for any infinite space X assuming the
Continuum Hypothesis, CH. About the same time R. Solovay [7] proved that it is
relatively consistent with ZFC that every homomorphism of C(X) for any space X is
necessarily continuous. Solovay's result was improved [8] fairly soon thereafter, to
obtain the relative consistency with ZFC + Martin's Axiom (ZFC + MA) that every
homomorphism of C(X) for any space X is continuous. We refer the reader to [2] for
an exposition of the latter result concerning MA, historical points and related results.
After these results several questions remained. This paper is concerned with the
question of whether the existence of a discontinuous homomorphism of C{X) is
possible given the failure of the Continuum Hypothesis.




A standard method to obtain the consistency of a proposition with the negation
of the Continuum Hypothesis (¬CH) when a proof of the proposition assuming CH
is known is to attempt to use MA + ¬CH in place of CH and prove the proposition.
However by the result indicated above, this approach will not succeed.
The main theorem of this paper, formulated using the terminology of forcing, is
the following.




THEOREM. Assume CH. Let P be the Cohen partial order for adding ω2 Cohen
reals. Then in VP there exists a discontinuous homomorphism of C(X)for every infinite compact Hausdorff space X.


A: Chris Phillips and Nik Weaver wrote a paper called The Calkin Algebra has Outer Automorphisms, where they showed that the Continuum Hypothesis implies that the Calkin algebra $\mathcal{B(H)/K(H)}$ has outer automorphisms.  See also this paper of Farah, McKenney, and Schimmerling, and references therein; they show that it is relatively consistent with ZFC that the Calkin algebra has only inner automorphisms, and hence the question of existence of outer automorphisms is independent of ZFC.
A: Although the construction of Tsirelson's space doesn't use set theory per se, in this short essay Tsirelson recounts (among other things) how his construction was inspired by forcing.
A: Though this is a little more advanced, there is actually some very exciting research right now at the intersection of descriptive set theory, ergodic theory, and von Neumann algebras. It is quite striking that the three areas have powerful tools for looking at similar problems, and yet tend to be applicable in different cases. For a nice introduction to some of these ideas  from a more set-theoretical point of view I would say check out
"Topics in Orbit Equivalence" by Kechris and Miller.
doi:10.1007/b99421
Here is a link where you can download it (though you might need a subscription but many universities will have it so it should work on a department computer.) It is actually quite elementary, you need some basic descriptive set theory and measure theory, but arrives at quite deep theorems.
A: One of the most elementary examples is the use of the infinite version of Ramsey's theorem to prove Rosenthals $\ell_1$ theorem. See Chapter 10 in Albiac & Kalton, Topics in Banach Space Theory. You'll find ultraproducts in Chapter 11.  More model theory than set theory, but still logic.  Deeper set theory (large cardinals, for example) have also been used.  Check out, e.g., Todorcevic, S* on MathSci Net for recent things.
A: The survey article ``Set theory and C*-algebras'' by Nik Weaver might have some things along the lines you are looking for ( Bull. Symbolic Logic Volume 13, Issue 1 (2007), 1-20; see also math/0604198 on the arXiv). For a particular example of Weaver's recent work in this area, see Akemann & Weaver's paper
Consistency of a counterexample to Naimark's problem

We construct a C*-algebra that has only one irreducible representation up to unitary equivalence but is not isomorphic to the algebra of compact operators on any Hilbert space. This answers an old question of Naimark. Our construction uses a combinatorial statement called the diamond principle, which is known to be consistent with but not provable from the standard axioms of set theory (assuming those axioms are consistent). We prove that the statement ``there exists a counterexample to Naimark's problem which is generated by $\aleph_1$ elements'' is undecidable in standard set theory.

There has also been some work by I. Farah on applying set-theoretical techniques to operator-algebraic problems.
