1,570 research outputs found

    On Borel equivalence relations related to self-adjoint operators

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    In a recent work, the authors studied various Borel equivalence relations defined on the Polish space SA(H){\rm{SA}}(H) of all (not necessarily bounded) self-adjoint operators on a separable infinite-dimensional Hilbert space HH. In this paper we study the domain equivalence relation EdomSA(H)E_{\rm{dom}}^{{\rm{SA}}(H)} given by AEdomSA(H)BdomA=domBAE_{\rm{dom}}^{{\rm{SA}}(H)}B\Leftrightarrow {\rm{dom}}{A}={\rm{dom}}{B} and determine its exact Borel complexity: EdomSA(H)E_{\rm{dom}}^{{\rm{SA}}(H)} is an FσF_{\sigma} (but not KσK_{\sigma}) equivalence relation which is continuously bireducible with the orbit equivalence relation ERNE_{\ell^{\infty}}^{\mathbb{R}^{\mathbb{N}}} of the standard Borel group =(N,R)\ell^{\infty}=\ell^{\infty}(\mathbb{N},\mathbb{R}) on RN\mathbb{R}^{\mathbb{N}}. This, by Rosendal's Theorem, shows that EdomSA(H)E_{\rm{dom}}^{{\rm{SA}}(H)} is universal for KσK_{\sigma} equivalence relations. Moreover, we show that generic self-adjoint operators have purely singular continuous spectrum equal to R\mathbb{R}.Comment: 10 pages, added more detail of the proof of Proposition 3.8 after the referee's suggestio

    On Polish Groups of Finite Type

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    Sorin Popa initiated the study of Polish groups which are embeddable into the unitary group of a separable finite von Neumann algebra. Such groups are called of finite type. We give necessary and sufficient conditions for Polish groups to be of finite type, and construct exmaples of such groups from semifinite von Neumann algebras. We also discuss permanence properties of finite type groups under various algebraic operations. Finally we close the paper with some questions concerning Polish groups of finite type.Comment: 20 page

    Weyl-von Neumann Theorem and Borel Complexity of Unitary Equivalence Modulo Compacts of Self-Adjoint Operators

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    Weyl-von Neumann Theorem asserts that two bounded self-adjoint operators A,BA,B on a Hilbert space HH are unitarily equivalent modulo compacts, i.e., uAu+K=BuAu^*+K=B for some unitary uU(H)u\in \mathcal{U}(H) and compact self-adjoint operator KK, if and only if AA and BB have the same essential spectra: σess(A)=σess(B)\sigma_{\rm{ess}}(A)=\sigma_{\rm{ess}}(B). In this paper we consider to what extent the above Weyl-von Neumann's result can(not) be extended to unbounded operators using descriptive set theory. We show that if HH is separable infinite-dimensional, this equivalence relation for bounded self-adjoin operators is smooth, while the same equivalence relation for general self-adjoint operators contains a dense GδG_{\delta}-orbit but does not admit classification by countable structures. On the other hand, apparently related equivalence relation $A\sim B\Leftrightarrow \exists u\in \mathcal{U}(H)\ [u(A-i)^{-1}u^*-(B-i)^{-1}$ is compact], is shown to be smooth. Various Borel or co-analytic equivalence relations related to self-adjoint operators are also presented.Comment: 36 page

    Spectral Properties of Schr\"odinger Operators on Perturbed Lattices

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    We study the spectral properties of Schr\"{o}dinger operators on perturbed lattices. We shall prove the non-existence or the discreteness of embedded eigenvalues, the limiting absorption principle for the resolvent, construct a spectral representation, and define the S-matrix. Our theory covers the square, triangular, diamond, Kagome lattices, as well as the ladder, the graphite and the subdivision of square lattice

    Notes on the Krupa-Zawisza Ultrapower of Self-Adjoint Operators

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    It is known that there is a difficulty in constructing the ultrapower of unbounded operators. Krupa and Zawisza gave a rigorous definition of the ultrapower A^{omega} of a selfadjoint operator A. In this note, we give alternative description of A^{omega} and the Hilbert space H(A) on which A^{omega} is densely defined, which provides a criterion to determine to which representing sequence (\xi_n)n of a given vector \xi in dom(A^{omega}) has the property that A^{omega}\xi = (A\xi_n)_{omega} holds.Comment: 13page

    The Cauchy problem for Schrodinger type equation with degeneracy

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    Structure of bicentralizer algebras and inclusions of type III factors (Mathematical aspects of quantum fields and related topics)

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    We investigate the structure of the relative bicentralizer algebra B(N ⊂ M, φ) for inclusions of von Neumann algebras with normal expectation where N is a type Ill₁ subfactor and φ ∈ N* is a faithful state. We first construct a canonical flow β[φ] : R*₊ ↷ B(N C M, ip) on the relative bicentralizer algebra and we show that the W*-dynamical system (B(N ⊂ M, φ), β[φ]) is independent of the choice of ip up to a canonical isomorphism. In the case when N = M, we deduce new results on the structure of the automorphism group of B(M, φ) and we relate the period of the flow β[φ] to the tensorial absorption of Powers factors. For general irreducible inclusions N ⊂ M, we relate the ergodicity of the flow β[φ] to the existence of irreducible hyperfinite subfactors in M that sit with normal expectation in N. When the inclusion N ⊂ M is discrete, we prove a relative bicentralizer theorem and we use it to solve Kadison's problem when N is amenable
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