Weighted holomorphic spaces with trivial closed range multiplication operators

We deal with the space consisting of those analytic functions on the unit disc such that , with . We determine the critical rate of decay of such that the pointwise multiplication operator , and analytic, has closed range in only in the trivial case that is the product of an invertible function in and a finite Blaschke product.

     

Characterization of composition operators with closed range for one-dimensional smooth symbols

We give a full characterization of smooth symbols ψ:R→R$\psi :\mathbb{R}\to \mathbb{R}$ for which the composition operator _Cψ:C^∞(R)→C^∞(R)$C_{\psi }:C^{\infty }(\mathbb{R})\to C^{\infty }(\mathbb{R})$, F?F°ψ$F?F°\psi$ has closed range. This generalizes in a special case the result of Kenessey and Wengenroth who gave such a characterization for smooth injective   symbols ψ:R→R^d$\psi :\mathbb{R}\to {\mathbb{R}}^d$.     

Norms and spectral radii of composition operators acting on the Dirichlet space

We obtain new upper bounds on the norms of univalently induced composition operators acting on the Dirichlet space and compute explicitly the norms for univalent symbols whose range is the disk minus a set of measure zero. As an application, we show that the spectral radius of every univalently induced composition operator on the Dirichlet space is equal to one.     

Level sets and composition operators on the Dirichlet space

We consider composition operators in the Dirichlet space of the unit disc in the plane. Various criteria on boundedness, compactness and Hilbert-Schmidt class membership are established. Some of these criteria are shown to be optimal.     

The spectra of composition operators from linear fractional maps acting upon the Dirichlet space

Let φ:D→D be a non-constant linear fractional transformation (necessarily of the form ). Let D denote the Dirichlet space of analytic functions. We determine the spectrum of the composition operator C_?:D→D defined by C_?(f)=f°φ. Eigenfunctions for the operator C_?:H²→H² frequently do not belong to the space D. However, spectral results for the operator C_?:D→D, much like those that have already been demonstrated for the operator C_?:H²→H², are presented in this paper.     

Some properties of unbounded operators with closed range

Let H ₁, H ₂ be Hilbert spaces and T be a closed linear operator defined on a dense subspace D(T) in H ₁ and taking values in H ₂. In this article we prove the following results:

The norm of a composition operator with linear symbol acting on the Dirichlet space

We obtain a representation for the norm of a composition operator on the Dirichlet space induced by a map of the form φ(z)=az+b. We compare this result to an upper bound for ‖Cφ‖ that is valid whenever φ is univalent. Our work relies heavily on an adjoint formula recently discovered by Gallardo-Gutiérrez and Montes-Rodríguez.     

Extreme points of the closed convex hull of composition operators

We study the extreme points of the closed convex hull of the set of all composition operators on the space of bounded analytic functions and the disk algebra.     

Approximating composition operator norms on the Dirichlet space

Let φ be any univalent self-map of the unit disk D whose image Ω≡φ(D) is compactly contained in D. We provide a method for approximating the norm of the composition operator Cφ on the Dirichlet space to any desired degree of accuracy. The approximation uses a special basis which is orthogonal in both the Bergman space on the disk and the Bergman space on Ω.     

Linear sums of two composition operators on the Fock space

Linear sums of two composition operators of the multi-dimensional Fock space are studied. We show that such an operator is bounded only when both composition operators in the sum are bounded. So, cancelation phenomenon is not possible on the Fock space, in contrast to what have been known on other well-known function spaces over the unit disk. We also show the analogues for compactness and for membership in the Schatten classes. For linear sums of more than two composition operators the investigation is left open.     

Self-commutators of automorphic composition operators on the Dirichlet space

Let be a conformal automorphism on the unit disk and be the composition operator on the Dirichlet space induced by . In this article we completely determine the point spectrum, spectrum, essential spectrum and essential norm of the operators and self-commutators of , which expose that the spectrum and point spectrum coincide. We also find the eigenfunctions of the operators.

     

Bergman空间上复合算子的紧性刻划

研究了C^n中有界强拟凸Ω上Bergman空间A^p(Ω)上的复合算子的有界性、紧性，给出了复合算子Cψ：A^p(Ω)→A^p(Ω)紧性的一个完整刻划。     

Hankel operators on the Dirichlet space

We study (small) Hankel operators on the Dirichlet space D with symbols in a class of function space, and show that such (small) Hankel operators are closely related to the corresponding Hankel operators on the Bergman space and the Hardy space H².     

Compact composition operators on the Smirnov class

We show that a composition operator on the Smirnov class is compact if and only if it is compact on some (equivalently: every) Hardy space for . Along the way we show that for composition operators on both the formally weaker notion of boundedness, and a formally stronger notion we call metric compactness, are equivalent to compactness.

     

Norm-attaining composition operators on the Bloch spaces

We prove that every composition operator C? on the Bloch space (modulo constant functions) attains its norm and characterize the norm-attaining composition operators on the little Bloch space (modulo constant functions). We also identify the extremal functions for ‖C?‖ in both cases.     

Restrictions of bounded linear operators: Closed range

Let be a bounded linear operator on a Banach space and let be a subspace of which is a Banach space and invariant. Denote by the restriction of to This paper explores the questions:

If the range of is closed, under what conditions is the range of closed?

If the range of is closed, under what conditions is the range of closed?

     

Weighted composition operators on the Hilbert space of Dirichlet series

In this paper, we study weighted composition operators on the Hilbert space of Dirichlet series with square summable coefficients. The Hermitianness, Fredholmness and invertibility of such operators are characterized, and the spectra of compact and invertible weighted composition operators are also described.