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Yuxia Liang

Publications and source records attributed to Yuxia Liang.

8 recordsLinked to original sources
arXiv2026

Invariance and near invariance for non-cyclic shift semigroups

We characterise the subspaces of $H^2(\mathbb D)$ that are invariant under the semigroups generated by two higher-order shifts $S^m $ and $S^{n}$. Complete descriptions are obtained for both invariant and nearly invariant subspaces associated with these operators and their adjoints. The approach is based on vector-valued Hardy spaces, the Beurling--Lax theorem, and matrix-valued inner functions. Finally we apply these results to non-cyclic shift semigroups and to Toeplitz operators induced by finite Blaschke products.

math.FA↗
arXiv2025

Composition operators between Toeplitz kernels

Recently, it was shown that the image of a Toeplitz kernel of dimension greater than $1$ under composition by an inner function is nearly $S^*$-invariant if and only if the inner function is an automorphism. Building on this, we determine the minimal Toeplitz kernel containing the image of a Toeplitz kernel under a composition operator with a general inner symbol, and extend this to weighted composition operators. Specifically, the corresponding cases for minimal model spaces are also given, thereby extending known work on the action of composition operators on model spaces. Finally, we use the equivalences between Toeplitz kernels to derive the explicit maximal vectors for several Toeplitz kernels, with symbols expressed in terms of composition operators and inner functions.

math.FA↗
arXiv2025

Nearly invariant subspaces and kernels of Toeplitz operators on the Hardy space over the bidisk

In this paper, the analysis of nearly invariant subspaces and kernels of Toeplitz operators on the Hardy space over the bidisk is developed. Firstly, we transcribe Chalendar, Chevrot and Partington's result to vector-valued Hardy space $H^{2}_{\ma{H}}(\mathbb{D})$ when $\ma{H}$ is an infinite dimensional separable complex Hilbert space. Secondly, we explore the definition of nearly invariant subspaces on Hardy space over the bidisk, and apply it to characterize kernels of Toeplitz operators. Finally, we define the nearly invariant subspaces for commutative isometric tuples, which allows us to show that the kernel of general Toeplitz operators is also nearly invariant.

math.FA↗
arXiv2024

Cyclic nearly invariant subspaces for semigroups of isometries

In this paper, the structure of the nearly invariant subspaces for discrete semigroups generated by several (even infinitely many) automorphisms of the unit disc is described. As part of this work, the near $S^*$-invariance property of the image space $C_φ({\rm ker\, } T)$ is explored for composition operators $C_φ$, induced by inner functions $φ$, and Toeplitz operators $T$. After that, the analysis of nearly invariant subspaces for strongly continuous multiplication semigroups of isometries is developed with a study of cyclic subspaces generated by a single Hardy class function. These are characterised in terms of model spaces in all cases when the outer factor is a product of an invertible function and a rational (not necessarily invertible) function. Techniques used include the theory of Toeplitz kernels and reproducing kernels.

math.FA↗
arXiv2020

Nearly invariant subspaces for shift semigroups

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-plane and the unit disk are related to model spaces. This work further includes a discussion on the structure of the closure of certain subspaces related to model spaces in Hardy spaces.

math.FA↗
arXiv2020

Nearly invariant subspaces for operators in Hilbert spaces

For a shift operator $T$ with finite multiplicity acting on a separable infinite dimensional Hilbert space we represent its nearly $T^{-1}$ invariant subspaces in terms of invariant subspaces under the backward shift. Going further, given any finite Blaschke product $B$, we give a description of the nearly $T_{B}^{-1}$ invariant subspaces for the operator $T_B$ of multiplication by $B$ in a scale of Dirichlet-type spaces.

math.FA↗
arXiv2020

Representing kernels of perturbations of Toeplitz operators by backward shift-invariant subspaces

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.

math.FA↗