Advances in Operator Theory最新文献

This paper considers paired operators in the context of the Lebesgue Hilbert space (L^2) on the unit circle and its subspace, the Hardy space (H^2.) The kernels of such operators, together with their analytic projections, which are generalizations of Toeplitz kernels, are studied. Inclusion relations between such kernels are considered in detail, and the results are applied to describing the kernels of finite-rank asymmetric truncated Toeplitz operators.

Tingley’s problem asks whether every surjective isometry between two unit spheres of Banach spaces can be extended to a surjective real linear isometry between the whole spaces. Let ({A_mu }_{mu in M}) and ({A_{nu }}_{nu in N}) be two collections of uniformly closed extremely C-regular subspaces. In this paper, we prove that if (Delta ) is a surjective isometry between two unit spheres of (ell ^1)-sums of uniformly closed extremely C-regular subspaces ({A_{mu }}_{mu in M}) and ({A_{nu }}_{nu in N}), then (Delta ) admits an extension to a surjective real linear isometry between the whole spaces. Typical examples of such Banach spaces B are (C^1(I)) of all continuously differentiable complex-valued functions on the closed unit interval I equipped with the norm (Vert fVert _{1}=|f(0)|+Vert f'Vert _{infty }) for (fin C^1(I)), (C^{(n)}(I)) of all n-times continuously differentiable complex-valued functions on I with the norm (Vert fVert _{1}=sum _{k=0}^{n-1}|f^{(k)}(0)|+~Vert f^{(n)}Vert _{infty }) for (C^{n}(I)), and (ell ^1(mathbb {N})) of all complex-valued functions on the set (mathbb {N}) of all natural numbers with the norm (Vert aVert _{1}=sum _{nin mathbb {N}}|a(n)|) for (ain ell ^1(mathbb {N})).

The Collatz map (or the (3n{+}1)-map) f is defined on positive integers by setting f(n) equal to (3n+1) when n is odd and n/2 when n is even. The Collatz conjecture states that starting from any positive integer n, some iterate of f takes value 1. In this study, we discuss formulations of the Collatz conjecture by (C^{*})-algebras in the following three ways: (1) single operator, (2) two operators, and (3) Cuntz algebra. For the (C^{*})-algebra generated by each of these, we consider the condition that it has no non-trivial reducing subspaces. For (1), we prove that the condition implies the Collatz conjecture. In the cases (2) and (3), we prove that the condition is equivalent to the Collatz conjecture. For similar maps, we introduce equivalence relations by them and generalize connections between the Collatz conjecture and irreducibility of associated (C^{*})-algebras.

The difficulty for solving ill-posed linear operator equations in Hilbert space is reflected by the strength of ill-posedness of the governing operator, and the inherent solution smoothness. In this study we focus on the ill-posedness of the operator, and we propose a partial ordering for the class of all bounded linear operators which lead to ill-posed operator equations. For compact linear operators, there is a simple characterization in terms of the decay rates of the singular values. In the context of the validity of the spectral theorem the partial ordering can also be understood. We highlight that range inclusions yield partial ordering, and we discuss cases when compositions of compact and non-compact operators occur. Several examples complement the theoretical results.

Many results for Banach spaces also hold for quasi-Banach spaces. One important such example is results depending on the Baire category theorem (BCT). We use the BCT to explore Lions problem for a quasi-Banach couple ((A_0, A_1).) Lions problem, posed in 1960s, is to prove that different parameters ((theta ,p)) produce different interpolation spaces ((A_0, A_1)_{theta , p}.) We first establish conditions on (A_0) and (A_1) so that interpolation spaces of this couple are strictly intermediate spaces between (A_0+A_1) and (A_0cap A_1.) This result, together with a reiteration theorem, gives a partial solution to Lions problem for quasi-Banach couples. We then apply our interpolation result to (partially) answer a question posed by Pietsch. More precisely, we show that if (pne p^*) the operator ideals ({mathcal {L}}^{(a)}_{p,q}(X,Y),) ({mathcal {L}}^{(a)}_{p^*,q^*}(X,Y)) generated by approximation numbers are distinct. Moreover, for any fixed p,  either all operator ideals ({mathcal {L}}^{(a)}_{p,q}(X,Y)) collapse into a unique space or they are pairwise distinct. We cite counterexamples which show that using interpolation spaces is not appropriate to solve Pietsch’s problem for operator ideals based on general s-numbers. However, the BCT can be used to prove a lethargy result for arbitrary s-numbers which guarantees that, under very minimal conditions on X, Y,  the space ({mathcal {L}}^{(s)}_{p,q}(X,Y)) is strictly embedded into ({mathcal {L}}^{mathcal {A}}(X,Y).)

For an alternative (*)-algebra A under some additional hypothesis, we prove that a map from A into itself is a nonlinear mixed (*)-Jordan type derivation if and only is an additive (*)-derivation. As consequence, some results on the complex octonion algebra, associative (*)-algebras, and (W^*)-factor algebras were obtained.

Let (T:Hrightarrow H) be a bounded operator on Hilbert space H. We say that T has a polygonal type if there exists an open convex polygon (Delta subset {mathbb {D}}), with (overline{Delta }cap {mathbb {T}}ne emptyset ), such that the spectrum (sigma (T)) is included in (overline{Delta }) and the resolvent R(z, T) satisfies an estimate (Vert R(z,T)Vert lesssim max {vert z-xi vert ^{-1},:, xi in overline{Delta }cap {mathbb {T}}}) for (zin overline{mathbb {D}}^c). The class of polygonal type operators (which goes back to De Laubenfels and Franks–McIntosh) contains the class of Ritt operators. Let (T_1,ldots ,T_d) be commuting operators on H, with (dge 3). We prove functional calculus properties of the d-tuple ((T_1,ldots ,T_d)) under various assumptions involving poygonal type. The main ones are the following. (1) If the operator (T_k) is a contraction for all (k=1,ldots ,d) and if (T_1,ldots ,T_{d-2}) have a polygonal type, then ((T_1,ldots ,T_d)) satisfies a generalized von Neumann inequality (Vert phi (T_1,ldots ,T_d)Vert le CVert phi Vert _{infty ,{mathbb {D}}^d}) for polynomials (phi ) in d variables; (2) If (T_k) is polynomially bounded with a polygonal type for all (k=1,ldots ,d), then there exists an invertible operator (S:Hrightarrow H) such that (Vert S^{-1}T_kSVert le 1) for all (k=1,ldots ,d).

We consider the universal additive category derived from the category of equivariant separable (C^*)-algebras by introducing homotopy invariance, stability and split-exactness. We show that morphisms in that category permit a particular simple form, thus obtaining the universal property of (KK^G)-theory for G a locally compact group, or a locally compact groupoid with compact base space, or a countable inverse semigroup as a byproduct.

Let G be a locally compact abelian group with Haar measure and (Phi ) be a Young function. In this paper we characterize the space of bilinear Fourier multipliers as a dual space of a certain Banach algebras for Orlicz spaces.

In this paper we continue the investigation of classes of vector-valued sequences that are represented by Banach operator ideals. By a procedure we mean a correspondence (X mapsto X^{textrm{new}}) that assigns a sequence class (X^{textrm{new}}) built upon a given sequence class X. The general question is whether or not (X^{textrm{new}}) is ideal-representable whenever X is. We address this question for three already studied procedures, namely, (X mapsto X^{textrm{u}}), (X mapsto X^{textrm{dual}}) and (X mapsto X^{textrm{fd}}). Applications of the solutions of these problem will provide new concrete examples of ideal-representable sequence classes.