252 research outputs found
Generic absoluteness and boolean names for elements of a Polish space
It is common knowledge in the set theory community that there exists a
duality relating the commutative -algebras with the family of -names
for complex numbers in a boolean valued model for set theory . Several
aspects of this correlation have been considered in works of the late 's
and early 's, for example by Takeuti, and by Jech. Generalizing Jech's
results, we extend this duality so as to be able to describe the family of
boolean names for elements of any given Polish space (such as the complex
numbers) in a boolean valued model for set theory as a space
consisting of functions whose domain is the Stone space of , and
whose range is contained in modulo a meager set. We also outline how this
duality can be combined with generic absoluteness results in order to analyze,
by means of forcing arguments, the theory of .Comment: 27 page
Hyperfiniteness and Borel asymptotic dimension of boundary actions of hyperbolic groups
We give a new short proof of the theorem due to Marquis and Sabok, which
states that the orbit equivalence relation induced by the action of a finitely
generated hyperbolic group on its Gromov boundary is hyperfinite. Our methods
permit moreover to show that every such action has finite Borel asymptotic
dimension.Comment: 10 pages, comments welcom
Strongly outer actions of amenable groups on Z-stable nuclear C*-algebras
Let A be a separable, unital, simple, Z-stable, nuclear Cā-algebra, and let Ī±:GāAut(A) be an action of a discrete, countable, amenable group. Suppose that the orbits of the action of G on T(A) are finite and that their cardinality is bounded. We show that the following are equivalent: (1) Ī± is strongly outer; (2) Ī±āidZ has the weak tracial Rokhlin property. If G is moreover residually finite, the above conditions are also equivalent to (3) Ī±āidZ has finite Rokhlin dimension (in fact, at most 2). If āeT(A) is furthermore compact, has finite covering dimension, and the orbit space āeT(A)/G is Hausdorff, we generalize results by Matui and Sato to show that Ī± is cocycle conjugate to Ī±āidZ, even if Ī± is not strongly outer. In particular, in this case the equivalences above hold for Ī± in place of Ī±āidZ. In the course of the proof, we develop equivariant versions of complemented partitions of unity and uniform property Ī as technical tools of independent interest
C+ - Algebras and the Uncountable: A Systematic Study of the Combinatorics of the Uncountable in the Noncommutative Framework
In this dissertation we investigate nonseparable C-algebras using methods coming from logic, specifically from set theory. The material is divided into three main parts.
In the first part we study algebras known as counterexamples to Naimarks problem, namely C-algebras that are not isomorphic to the algebra of compact operators on some Hilbert space, yet still have only one irreducible representation up to unitary equivalence.
Such algebras have to be simple, nonseparable and non-type I, and they are known to exist if the diamond principle (a strengthening of the continuum hypothesis) is assumed. With the motivation of finding further characterizations for these counterexamples, we undertake the study of their trace spaces, led by some elementary observations about the unitary action on the state space of these algebras, which seem to suggest that a counterexample to Naimarks problem could have at most one trace. We show that this is not the case and, assuming diamond, we prove that every Choquet simplex with countably many extreme points occurs as the trace space of a counterexample to Naimarks problem and that, moreover, there exists a counterexample whose tracial simplex is nonseparable. The second part of this dissertation revolves around the Calkin algebra (H) and the
general problem of what nonseparable C-algebras embed into it. We prove that, under Martins axiom, all C-algebras of density character less than 20 embed into the Calkin algebra. Moving to larger C-algebras, we show that (within ZFC alone) Cred(F20 ) and Cm ax(F20 ), where F20 is the free group on 20 generators, and every nonseparable UHF algebra with density character at most 20 , embed into the Calkin algebra. On the other hand, we prove that it is consistent with ZFC + 20 , for every ordinal 2, that the abelian C-algebra generated by an increasing chain of 2 projections does not embed into Q(H). Hence, the statement Every C-algebra of density character strictly less than 20 embeds into the Calkin algebra is independent from ZFC+ 20 , for every ordinal > 2. Finally, we show that the proof of Voiculescus noncommutative version of the Weyl-
von Neumann theorem consists, when looked from the right perspective, of a sequence of applications of the Baire category theorem to certain ccc posets. This allows us, assuming
Martins axiom, to generalize Voiculescus results to nonseparable C-algebras of density character less than 20 .
The last part of this manuscript concerns lifting of abelian subalgebras of coronas of non-unital C-algebras. Given a subset of commuting elements in a corona algebra, we study what could prevent the existence of a commutative lifting of such subset to the
multiplier algebra. While for finite and countable families the only issues arising are of K-theoretic nature, for larger families the size itself becomes an obstruction. We prove in fact, for a primitive, non-unital, -unital C-algebra A, that there exists a set of 1
orthogonal positive elements in the corona of A which cannot be lifted to a collection of
commuting elements in the multiplier algebra of A
The Role of MRI-TRUS Fusion Biopsy in the Diagnosis of Clinical Significant Prostate Cancer (CsPca)
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