Some Calkin algebras have outer automorphisms
We consider various quotients of the C*-algebra of bounded operators on a nonseparable Hilbert space, and prove in some cases that, consistently, there are many outer automorphisms.
math.LO↗
arXiv subjects
Ilijas Farah
Publications and source records attributed to Ilijas Farah.
The isomorphism relation for separable C*-algebras
We prove that the isomorphism relation for separable C$^*$-algebras, and also the relations of complete and $n$-isometry for operator spaces and systems, are Borel reducible to the orbit equivalence relation of a Polish group action on a standard Borel space.
math.OA↗
Amalgamating R^ω-embeddable von Neumann algebras
We observe how a classical model-theoretic fact proves the existence of many strong amalgamation bases for the class of R^ω-embeddable von Neumann algebras, where R is the hyperfinite II_1 factor. In particular, we shows that R itself is a strong amalgamation base.
math.OA↗
Nonseparable UHF algebras II: Classification
For every uncountable cardinal $κ$ there are $2^κ$ nonisomorphic simple AF algebras of density character $κ$ and $2^κ$ nonisomorphic hyperfinite II$_1$ factors of density character $κ$. These estimates are maximal possible. All C*-algebras that we construct have the same Elliott invariant and Cuntz semigroup as the CAR algebra.
math.OA↗
A simple C*-algebra with finite nuclear dimension which is not Z-stable
We construct a simple C*-algebra with nuclear dimension zero that is not isomorphic to its tensor product with the Jiang-Su algebra Z, and a hyperfinite II_1 factor not isomorphic to its tensor product with the separable hyperfinite II_1 factor R. The proofs use a weakening of the Continuum Hypothesis.
math.OA↗
Omitting types and AF algebras
We give model-theoretic characterizations of UHF algebras and of AF algebras as C*-algebras that omit certain sets of types.
math.LO↗
Independence of the existence of Pitowsky spin models
In 1982 I. Pitowsky used Continuum Hypothesis to construct hidden variable models for spin-1/2 and spin-1 particles in quantum mechanics. We show that the existence of Pitowsky models is independent from ZFC.
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Trivial automorphisms
We prove that the statement `For all Borel ideals I and J on $ω$, every isomorphism between Boolean algebras $P(ω)/I$ and $P(ω)/J$ has a continuous representation' is relatively consistent with ZFC. In this model every isomorphism between $P(ω)/I$ and any other quotient $P(ω)/J$ over a Borel ideal is trivial for a number of Borel ideals I on $ω$. We can also assure that the dominating number is equal to $\aleph_1$ and that $2^{\aleph_1}>2^{\aleph_0}$. Therefore the Calkin algebra has outer automorphisms while all automorphisms of $P(ω)/Fin$ are trivial. Proofs rely on delicate analysis of names for reals in a countable support iteration of suslin proper forcings.
math.LO↗
Countable saturation of corona algebras
We present unified proofs of several properties of the corona of $σ$-unital C*-algebras such as AA-CRISP, SAW*, being sub-$σ$-Stonean in the sense of Kirchberg, and the conclusion of Kasparov's Technical Theorem. Although our results were obtained by considering C*-algebras as models of the logic for metric structures, the reader is not required to have any knowledge of model theory of metric structures (or model theory, or logic in general). The proofs involve analysis of the extent of model-theoretic saturation of corona algebras.
math.OA↗
Model theory of operator algebras I: Stability
Several authors have considered whether the ultrapower and the relative commutant of a C*-algebra or II_1 factor depend on the choice of the ultrafilter. We settle each of these questions, extending results of Ge-Hadwin and the first author.
math.OA↗
Turbulence, orbit equivalence, and the classification of nuclear C*-algebras
We bound the Borel cardinality of the isomorphism relation for nuclear simple separable C*-algebras: It is turbulent, yet Borel reducible to the action of the automorphism group of the Cuntz algebra O_2 on its closed subsets. The same bounds are obtained for affine homeomorphism of metrizable Choquet simplexes. As a by-product we recover a result of Kechris and Solecki, namely, that homeomorphism of compacta in the Hilbert cube is Borel reducible to a Polish group action. These results depend intimately on the classification theory of nuclear simple C*-algebras by K-theory and traces. Both of necessity and in order to lay the groundwork for further study on the Borel complexity of C*-algebras, we prove that many standard C*-algebra constructions and relations are Borel, and we prove Borel versions of Kirchberg's O_2-stability and embedding theorems. We also find a C*-algebraic witness for a K_σhard equivalence relation.
math.OA↗
The descriptive set theory of C$^*$-algebra invariants
We establish the Borel computability of various C$^*$-algebra invariants, including the Elliott invariant and the Cuntz semigroup. As applications we deduce that AF algebras are classifiable by countable structures, and that a conjecture of Winter and the second author for nuclear separable simple C*-algebras cannot be disproved by appealing to known standard Borel structures on these algebras.
math.OA↗
Model theory of operator algebras III: Elementary equivalence and II_1 factors
We use continuous model theory to obtain several results concerning isomorphisms and embeddings between II_1 factors and their ultrapowers. Among other things, we show that for any II_1 factor M, there are continuum many nonisomorphic separable II_1 factors that have an ultrapower isomorphic to an ultrapower of M. We also give a poor man's resolution of the Connes Embedding Problem: there exists a separable II_1 factor such that all II_1 factors embed into one of its ultrapowers.
math.OA↗
All automorphisms of all Calkin algebras
The Proper Forcing Axiom implies all automorphisms of every Calkin algebra associated with an infinite-dimensional complex Hilbert space and the ideal of compact operators are inner. As a means of the proof we introduce the notion of Polish $ω_1$-trees and cohrerent families of Polish spaces and prove some uniformization results.
math.LO↗
All automorphisms of the Calkin algebra are inner
We prove that it is relatively consistent with the usual axioms of mathematics that all automorphisms of the Calkin algebra are inner. Together with a 2006 Phillips--Weaver construction of an outer automorphism using the Continuum Hypothesis, this gives a complete solution to a 1977 problem of Brown-Douglas-Fillmore. We also give a simpler and self-contained proof of the Phillips--Weaver result.
math.OA↗
Nonseparable UHF algebras I: Dixmier's problem
There are three natural ways to define UHF (uniformly hyperfinite) C*-algebras, and all three definitions are equivalent for separable algebras. In 1967 Dixmier asked whether the three definitions remain equivalent for not necessarily separable algebras. We give a complete answer to this question. More precisely, we show that in small cardinality two definitions remain equivalent, and give counterexamples in other cases. Our results do not use any additional set-theoretic axioms beyond the usual axioms, namely ZFC.
math.OA↗
A dichotomy for the Mackey Borel structure
We prove that the equivalence of pure states of a separable C*-algebra is either smooth or it continuously reduces $[0,1]^{\bbN}/\ell_2$ and it therefore cannot be classified by countable structures. The latter was independently proved by Kerr--Li--Pichot by using different methods. We also give some remarks on a 1967 problem of Dixmier.
math.OA↗
A dichotomy for the number of ultrapowers
We prove a strong dichotomy for the number of ultrapowers of a given countable model associated with nonprincipal ultrafilters on N. They are either all isomorphic, or else there are $2^{2^{\aleph_0}}$ many nonisomorphic ultrapowers. We prove the analogous result for metric structures, including C*-algebras and II$_1$ factors, as well as their relative commutants and include several applications. We also show that the C*-algebra B(H) always has nonisomorphic relative commutants in its ultrapowers associated with nonprincipal ultrafilters on N.
math.LO↗