Józef Burzyk
Necessary and sufficient conditions are given for density of shift-invariant subspaces of the space $\mathcal{L}$ of integrable functions of bounded support with the inductive limit topology.
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We construct a countable inductive limit of weighted Banach spaces of holomorphic functions, which is not a topological subspace of the corresponding weighted inductive limit of spaces of continuous functions. The main step of our construction, using a special sequence of outer holomorphic functions, shows that a certain sequence space is isomorphic to a complemented subspace of a weighted space of holomorphic functions in two complex variables. This example solves in the negative a well-known open problem raised by Bierstedt, Meise and Summers.
Inductive algebras for the irreducible unitary representations of the universal cover of the group of unimodular two by two matrices are classified. The classification of homogeneous shift operators is obtained as a direct consequence. This gives a new approach to the results of Bagchi and Misra.
In this article, we characterize nearly invariant subspaces of finite defect for the backward shift operator acting on the vector-valued Hardy space which is a vectorial generalization of a result of Chalendar-Gallardo-Partington (C-G-P). Using this characterization of nearly invariant subspace under the backward shift we completely describe the almost invariant subspaces for the shift and its adjoint acting on the vector valued Hardy space.
Almost invariant subspaces of shift operators and products of Toeplitz and Hankel operators
本文通过Toeplitz与Hankel算子乘积的值域或核，给出了后向移位算子几乎不变子空间的更显式刻画，并证明其与前向移位算子的几乎不变子空间相同。
Spectra and invariant subspaces of compressed shifts on nearly invariant subspaces
本文研究压缩位移在近不变子空间上的谱性质和不变子空间结构，通过单位等价性与变换理论揭示S*不变性放松对谱结构的影响，拓展了经典模型空间理论的应用范围。
Following Beurling's theorem and a study of the topology of invariant subspaces by R. Douglas and C. Pearcy description of path connected components of invariant subspace lattice for shift of multiplicity one has been given by R.Yang. This paper generalizes result to arbitrary finite multiplicity. We show that there exists one to one correspondence between the invariant subspace lattice of shift of arbitrary finite multiplicity and the space of inner functions.
Our aim in this paper is to obtain necessary and sufficient conditions for weighted shift operators on the Hilbert spaces $\ell^{2}(\mathbb Z)$ and $\ell^{2}(\mathbb N)$ to be subspace-transitive, consequently, we show that the Herrero question (D. A. Herrero. Limits of hypercyclic and supercyclic operators, J. Funct. Anal., 99 (1991)179-190) holds true even on a subspace of a Hilbert space, i.e. there exists an operator $T$ such that both $T$ and $T^*$ are subspace-hypercyclic operators for some subspaces. We display the conditions on the direct sum of two invertable bilateral forward weighte…
In this paper, we discuss to the nonuniform sampling problem in principal shift-invariant subspaces of mixed Lebesgue spaces. We proposed a fast reconstruction algorithm which allows to exactly reconstruct the functions in the principal shift-invariant subspaces as long as the sampling set $X=\{(x_{j},y_{k}):k,j\in \mathbb{J}\}$ is sufficiently dense. Our results improve the result in \cite{LiLiu}.
In this note, we describe the backward shift invariant subspaces for a large class of reproducing kernel Hilbert spaces. This class includes in particular de Branges-Rovnyak spaces (the non-extreme case) and the range space of co-analytic Toeplitz operators.
The translation of an operator is defined by using conjugation with time-frequency shifts. Thus, one can define $Λ$-shift-invariant subspaces of Hilbert-Schmidt operators, finitely generated, with respect to a lattice $Λ$ in $\mathbb{R}^{2d}$. These spaces can be seen as a generalization of classical shift-invariant subspaces of square integrable functions. Obtaining sampling results for these subspaces appears as a natural question that can be motivated by the problem of channel estimation in wireless communications. These sampling results are obtained in the light of the frame theory in a se…
It is well known that the structure of nontrival invariant subspaces for the shift operator on the Bergman space is extremely complicated and very little is known about their specific structures, and that a complete description of the structure seems unlikely. In this paper, we find that any invariant subspace $M(\neq \{0\})$ for the shift operator $M_z$ on the Bergman space $A^{^p}_α(\D)~(1\leq p< \infty,~~-1<α<\infty)$ contains a nonempty subset that lies in $A^{^p}_α(\D)\backslash H^{^p}(\D)$. To a certain extent, this result describes the specific structure of every nonzero invariant subsp…
Subspaces of $L^2(\mathbb{R}^n)$ Invariant Under Shifts by a Crystal Group
本文研究了$L^2(\mathbb{R}^n)$中不变于晶格群位移的子空间的特征。
We construct a nuclear space $Φ$ as an inductive limit of finite-dimensional subspaces of a Hilbert space $H$ in such a way that $(Φ,H,Φ')$ becomes a rigged Hilbert space, thus simplifying the construction by Bellomonte and Trapani.
We obtaine the full characterization of proper closed invariant subspaces of a generalized backward shift operator (Pommiez operator) in the Frechet space of all holomorphic functions on a simply connected domain $Ω$ of the complex plane, containing the origin. In the case when the function, which defines this operator, is not has zeros in $Ω$ all such subspaces are finite-dimensional. If additionally $Ω$ coincides with the complex plane, then the considered operator is unicellular. If this function has zeros in $Ω$, then the family of mentioned invariant subspaces splits into two classes: the…
We give an intrinsic characterisation of the separable reflexive Banach spaces that embed into separable reflexive spaces with an unconditional basis all of whose normalised block sequences with the same growth rate are equivalent. This uses methods of E. Odell and T. Schlumprecht.
Non-commutative multivariable versions of weighted shift operators arise naturally as `weighted' left creation operators acting on the Fock space Hilbert space. We identify a natural notion of periodicity for these $N$-tuples, and then find a family of inductive limit algebras determined by the periodic weighted shifts which can be regarded as non-commutative multivariable generalizations of the Bunce-Deddens C*-algebras. We establish this by proving that the C*-algebras generated by shifts of a given period are isomorphic to full matrix algebras over Cuntz-Toeplitz algebras. This leads to an …
Let $\{T(t)\}_{t\geq0}$ be a $C_0$-semigroup on an infinite dimensional separable Hilbert space; a suitable definition of near $\{T(t)^*\}_{t\geq0}$ invariance of a subspace is presented in this paper. A series of prototypical examples for minimal nearly $\{S(t)^*\}_{t\geq0}$ invariant subspaces for the shift semigroup $\{S(t)\}_{t\geq0}$ on $L^2(0,\infty)$ are demonstrated, which have close links with nearly $T_θ^*$ invariance on Hardy spaces of the unit disk for Toeplitz operators associated with an inner function $θ$. Especially, the corresponding subspaces on Hardy spaces of the right half…
It is an English translation of the paper originally published in Russian and Ukrainian in 1987. In the appendix of his book S.Banach introduced the following definition Let $X$ be a Banach space and $Γ$ be a subspace of the dual space $X^*$. The set of all limits of $w^{*}$-convergent sequences in $Γ$ is called the $w^*${\it -derived set} of $Γ$ and is denoted by $Γ_{(1)}$. For an ordinal $α$ the $w^{*}$-{\it derived set of order} $α$ is defined inductively by the equality: $$ Γ_{(α)}=\bigcup _{β<α}((Γ_{(β)})_{(1)}. $$
A particular case of results from [K2] is as follows. Let the unitary asymptote of a contraction $T$ contain the bilateral shift (of finite or infinite multiplicity). Then there exists an invariant subspace $\mathcal M$ of $T$ such that $T|_{\mathcal M}$ is similar to the unilateral shift of the same multiplicity. The proof is based on the Sz.-Nagy--Foias functional model for contractions. In the present paper this result is generalized to polynomially bounded operators, but in the simplest case. Namely, it is proved that if the unitary asymptote of a polynomially bounded operator $T$ contains…
Noncommutative commutant lifting, interpolation, and shift-invariant subspaces
本文在非交换Drury-Arveson空间中建立Sarason型交换子提升定理，解决非交换Nevanlinna-Pick插值问题，并给出移位不变子空间的Beurling-Lax-Halmos型刻画。
In this paper, the problem of reconstruction of signals in mixed Lebesgue spaces from their random average samples has been studied. Probabilistic sampling inequalities for certain subsets of shift-invariant spaces have been derived. It is shown that the probabilities increase to one when the sample size increases. Further, explicit reconstruction formulae for signals in these subsets have been obtained for which the numerical simulations have also been performed.
In 1998, John B. Conway and Liming Yang wrote a paper in which they posed a number of open questions regarding the shift on $P^t(μ)$ spaces. A few of these have been completely resolved, while at least one remains wide open. In this paper, we review some of the solutions, mention some alternate approaches and discuss further the problem that remains unsolved.
A particular case of [07] was generalized from contractions to polynomially bounded operators in [G19]. Namely, it is proved in [G19] that if the unitary asymptote of a polynomially bounded operator $T$ contains the bilateral shift of multiplicity $1$, then there exists an invariant subspace $\mathcal M$ of $T$ such that $T|_{\mathcal M}$ is similar to the unilateral shift of multiplicity $1$. In the present paper, some corollaries of this result are given. In particular, reflexivity of polynomially bounded operators described above is proved.
Invariant subspaces of the direct sum of forward and backward shifts on vector-valued Hardy spaces
研究向量值Hardy空间上前向与后向移位直和算子的不变子空间，建立其与双边移位一类不变子空间的一一对应，并将其表示为混合Toeplitz与Hankel算子的核或值域，推广了一维情形下的已有结果。
We define the local trace function for subspaces of $\ltworn$ which are invariant under integer translation. Our trace function contains the dimension function and the spectral function defined by Bownik and Rzeszotnik and completely characterizes the given translation invariant subspace. It has properties such as positivity, additivity, monotony and some form of continuity. It behaves nicely under dilations and modulations. We use the local trace function to deduce, using short and simple arguments, some fundamental facts about wavelets such as the characterizing equations, the equality betwe…
Subspaces of $L^2(\mathbb{R}^n)$ Invariant Under Crystallographic Shifts
本文研究了$L^2(\mathbb{R}^n)$中晶格平移不变子空间的结构，通过酉表示理论和直接积分方法，揭示了晶格群作用下的不变子空间特性及中心分解。
Invariant subspaces of compressions of the Hardy shift on some parametric spaces
本文研究 Hardy 移位在参数空间上的压缩算子 $S_{\alpha,\beta}$，证明其为再生核 Hilbert 空间上的移位，并完整分类其不变子空间，指出它们虽循环但可能不由游荡子空间生成，并给出充要条件。
Invariance Preserving Conjugations on the Hardy Space
研究\(H^2\)上共轭算子\(C\)，刻画其将平移不变子空间映射到平移不变子空间的情况，还探讨了映射到余不变子空间时共轭的存在与否。
On hyperinvariant subspaces of operators containing unilateral shifts
研究含单边移位的希尔伯特空间算子\(T\)的超不变子空间问题，基于\(T\)的相似模型，通过特定技术建立相关算子，分析特定线性流形截面，得出\(T\)有非平凡超不变子空间的条件及相关结论。
Let $X$ be a separable nonquasireflexive Banach space. Let $Y$ be a Banach space isomorphic to a subspace of $X^*$. The paper is devoted to the following questions: 1. Under what conditions does there exist an isomorphic embedding $T:Y\to X^*$ such that subspace $T(Y)\subset X^*$ is total? 2. If such embeddings exist, what are the possible orders of $T(Y)$? Here we need to recall some definitions. For a subset $M\subset X^*$ we denote the set of all limits of weak$^*$ convergent sequences in $M$ by $M_{(1)}$. Inductively, for ordinal number $α$ we let $$M_{(α)}=\cup_{β<α}(M_{(β)})_{(1)}.$$ The…
Nearly invariant subspaces in the real Hardy space
本文研究实 Hardy 空间中 backward shift 算子的近不变子空间，探讨有限缺陷近不变子空间，并为 backward shift 算子的近似不变子空间提供特征描述。
Shift-invariant subspaces of Sobolev type
本文综述了Sobolev型移位不变子空间的结构，并利用傅里叶乘子将相关定理推广至Sobolev型空间。
We extend the Balian-Low theorem to Gabor subspaces of $L^2(\mathbb R)$ by involving the concept of additional time-frequency shift invariance. We prove that if a Gabor system on a lattice of rational density is a Riesz sequence generating a subspace which is invariant under an additional time-frequency shift, then its generator cannot decay fast simultaneously in time and frequency.
Invariant Subspaces and the $C_{00}$-Property of $3$-Brownian Shifts
本文研究3布朗移位在特定不变子空间上的单位ary等价性及$C_{00}$-性质，基于布朗扩展理论，探讨其渐进行为。
We prove that if T is an operator on an infinite-dimensional Hilbert space whose spectrum and essential spectrum are both connected and whose Fredholm index is only 0 or 1, then the only nontrivial norm-stable invariant subspaces of T are the finite-dimensional ones. We also characterize norm-stable invariant subspaces of any weighted unilateral shift operator. We show that quasianalytic shift operators are points of norm continuity of the lattice of the invariant subspaces. We also provide a necessary condition for strongly stable invariant subspaces for certain operators.
A complete characterization of nearly-invariant subspaces of finite defect for the backward shift operator acting on the Hardy space is provided in the spirit of Hitt and Sarason's theorem. As a corollary we describe the almost-invariant subspaces for the shift and its adjoint.
The Lifting Problem for Commuting Subnormals (LPCS) asks for necessary and sufficient conditions for a pair of subnormal operators on Hilbert space to admit commuting normal extensions. \ We study LPCS within the class of commuting 2-variable weighted shifts $\mathbf{T} \equiv (T_1,T_2)$ with subnormal components $T_1$ and $T_2$, acting on the Hilbert space $\ell ^2(\mathbb{Z}^2_+)$ with canonical orthonormal basis $\{e_{(k_1,k_2)}\}_{k_1,k_2 \geq 0}$ . \ The \textit{core} of a commuting 2-variable weighted shift $\mathbf{T}$, $c(\mathbf{T})$, is the restriction of $\mathbf{T}$ to the invarian…
In this article, we briefly describe nearly $T^{-1}$ invariant subspaces with finite defect for a shift operator $T$ having finite multiplicity acting on a separable Hilbert space $\mathcal{H}$ as a generalization of nearly $T^{-1}$ invariant subspaces introduced by Liang and Partington in \cite{YP}. In other words we characterize nearly $T^{-1}$ invariant subspaces with finite defect in terms of backward shift invariant subspaces in vector-valued Hardy spaces by using Theorem 3.5 in \cite{CDP}. Furthermore, we also provide a concrete representation of the nearly $T_B^{-1}$ invariant subspaces…
The lattice of closed invariant subspaces of the Volterra operator acting on $L^2(0,1)$ was completely described by Sarason. On the other hand, he explicitly found the lattice of closed invariant subspaces of the shift plus Volterra operator on $L^2(0,1)$. Inspired by Sarason's results, we find the lattice of closed invariant subspaces of the shift plus complex Volterra operator acting on the Hardy space
Let $\mathcal{W}$ be the corresponding wandering subspace of an invariant subspace of the Bergman shift. By identifying the Bergman space with $H^2(\mathbb{D}^2)\ominus[z-w]$, a sufficient and necessary conditions of a closed subspace of $H^2(\mathbb{D}^2)\ominus[z-w]$ to be a wandering subspace of an invariant subspace is given also, and a functional charaterization and a coefficient characterization for a function in a wandering subspace are given. As a byproduct, we proved that for two invariant subspaces $\mathcal{M}$, $\mathcal{N}$ with $\mathcal{M}\supsetneq\mathcal{N}$ and $dim(\mathcal…
We study the structure of the backward shift invariant and nearly invariant subspaces in weighted Fock-type spaces $\mathcal{F}_W^p$, whose weight $W$ is not necessarily radial. We show that in the spaces $\mathcal{F}_W^p$ which contain the polynomials as a dense subspace (in particular, in the radial case) all nontrivial backward shift invariant subspaces are of the form $\mathcal{P}_n$, i.e., finite dimensional subspaces consisting of polynomials of degree at most $n$. In general, the structure of the nearly invariant subspaces is more complicated. In the case of spaces of slow growth (up to…
Invariant subspaces of perturbed backward shift
本文研究 Hardy 空间中后向移位有限秩扰动的不变闭子空间，并据此分类几乎不变子空间、细化近不变子空间概念。
We extend the existing theory of approximation orders provided by shift-invariant subspaces of $L_2$ to the setting of Sobolev spaces, provide treatment of $L_2$ cases that have not been covered before, and apply our results to determine approximation order of solutions to a refinement equation with a higher-dimensional solution space.
In this paper we study subspaces which are invariant under squares and cubes (separately as well as jointly) of unicellular backward weighted shift operators on a separable Hilbert space. The finite-dimensional subspaces are characterized for all weights and the infinite-dimensional subspaces are characterized for two classes of weights.
It is well known that the kernel of a Toeplitz operator is nearly invariant under the backward shift $S^*$. This paper shows that kernels of finite-rank perturbations of Toeplitz operators are nearly $S^*$-invariant with finite defect. This enables us to apply a recent theorem by Chalendar--Gallardo--Partington to represent the kernel in terms of backward shift-invariant subspaces, which we identify in several important cases.
We first generalize the results of León and Müller [Studia Math. 175(1) 2006] on hypercyclic subspaces to sequences of operators on Fréchet spaces with a continuous norm. Then we study the particular case of iterates of an operator T and show a simple criterion for having no hypercyclic subspace. Finally we deduce from this criterion a characterization of weighted shifts with hypercyclic subspaces on the spaces lp or c0, on the space of entire functions and on certain Köthe sequence spaces. We also prove that if P is a non-constant polynomial and D is the differentiation operator on the space …
Invariant subspaces of Brownian shifts on vector-valued Hardy spaces
本文研究了向量值Hardy空间上Brownian移位的不变子空间，刻画了其结构并解决了相应的酉等价问题。
We investigate in detail the connection between harmonic maps from Riemann surfaces into the unitary group $\U(n)$ and their Grassmannian models: these are families of shift-invariant subspaces of $L^2(S^1,\C^n)$. With the help of operator-theoretic methods we derive a criterion for finiteness of the uniton number which has a large number of applications discussed in the paper.
Reducing and Invariant subspaces under two commuting shift operators
针对单位圆上取值于带重数 Hardy 空间的平方可积函数空间，刻画在双边移位与局部单边移位下的约化及不变子空间，条件推广了 Helson 与 Beurling-Lax-Halmos 定理。
Shift invariant subspaces of large index in the Bloch space
本文研究Bloch空间与小Bloch空间上移位算子的不变子空间格，构造大指标闭不变子空间，并利用二次对偶中弱星闭包的指标稳定性证明任意指标弱星闭不变子空间的存在性。
Using the tools of Sz.-Nagy--Foias theory of contractions, we describe in detail the invariant subspaces of the operator $ S\oplus S^* $, where $ S $ is the unilateral shift on a Hilbert space. This answers a question of Câmara and Ross.
The Grassmannian model represents harmonic maps from Riemann surfaces by families of shift-invariant subspaces of a Hilbert space. We impose a natural symmetry condition on the shift-invariant subspaces that corresponds to considering an important class of harmonic maps into symmetric and $k$-symmetric spaces. In particular, we obtain new general forms for such symmetric shift-invariant subspaces and for the corresponding extended solutions.
Two problems are posed that involve the star-invariant subspace $K^p_θ$ (in the Hardy space $H^p$) associated with an inner function $θ$. One of these asks for a characterization of the extreme points of the unit ball in $K^\infty_θ$, while the other concerns the Fermat equation $f^n+g^n=h^n$ in $K^p_θ$.
Shift invariant subspaces in the Bloch space
本文研究Bloch空间中平移算子的弱星闭不变子空间，证明有界解析函数可分解为循环因子与满足持久性性质的真不变子空间生成因子，奇异内函数起关键作用，并指出与Bergman空间描述的显著差异，同时给出非循环的可逆函数，回答了九十年代初的开放问题。
Invariant subspaces for finite index shifts in Hardy spaces
本文研究了有限索引位移在Hardy空间中的不变子空间特征化及闭子空间的存在性。
The invariant subspace problem and Rosenblum operators I
本文通过估计$\\|T^n+\lambda T^{-n}\\|$的增长研究可逆算子的不变子空间，引入基于Rosenblum算子的移位表示算子，并利用Hankel算子的单射性给出不变子空间问题的等价条件。
A closed subspace is invariant under the Cesàro operator $\mathcal{C}$ on the classical Hardy space $H^2(\mathbb D)$ if and only if its orthogonal complement is invariant under the $C_0$-semigroup of composition operators induced by the affine maps $φ_t(z)= e^{-t}z + 1 - e^{-t}$ for $t\geq 0$ and $z\in \mathbb D$. The corresponding result also holds in the Hardy spaces $H^p(\mathbb D)$ for $1<p<\infty$. Moreover, in the Hilbert space setting, by linking the invariant subspaces of $\mathcal{C}$ to the lattice of the closed invariant subspaces of the standard right-shift semigroup acting on a pa…
In our recent work, the sampling and reconstruction of non-decaying signals, modeled as members of weighted-$L_p$ spaces, were shown to be stable with an appropriate choice of the generating kernel for the shift-invariant reconstruction space. In this paper, we extend the Strang-Fix theory to show that, for $d$-dimensional signals whose derivatives up to order $L$ are all in some weighted-$L_p$ space, the weighted norm of the approximation error can be made to go down as $O(h^L)$ when the sampling step $h$ tends to $0$. The sufficient condition for this decay rate is that the generating kernel…
Shift invariant subspaces in growth spaces and sets of finite entropy
本文完整刻画了增长空间中由 Nevanlinna 类函数生成的平移不变子空间，证明其具有 Beurling 型结构，并推广了 Korenblum-Roberts 定理。
We study reducing subspaces for an analytic multiplication operator M_{z^{n}} on the Bergman space L_{a}^{2}(A_{r}) of the annulus A_{r}, and we prove that M_{z^{n}} has exactly 2^n reducing subspaces. Furthermore, in contrast to what happens for the disk, the same is true for the Hardy space on the annulus. Finally, we extend the results to certain bilateral weighted shifts, and interpret the results in the context of complex geometry.
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