arXiv subjects

Mikael Lindström

Publications and source records attributed to Mikael Lindström.

8 recordsLinked to original sources

arXiv2022

Exact essential norm of generalized Hilbert matrix operators on classical analytic function spaces

We compute the exact value of the essential norm of a generalized Hilbert matrix operator acting on weighted Bergman spaces $A^p_v$ and weighted Banach spaces $H^\infty_v$ of analytic functions, where $v$ is a general radial weight. In particular, we obtain the exact value of the essential norm of the classical Hilbert matrix operator on standard weighted Bergman spaces $A^p_α$ for $p>2+α, \, α\ge 0,$ and on Korenblum spaces $H^\infty_α$ for $0 < α< 1.$ We also cover the Hardy space $H^p, \, 1 < p < \infty,$ case. In the weighted Bergman space case, the essential norm of the Hilbert matrix is equal to the conjectured value of its operator norm and similarly in the Hardy space case the essential norm and the operator norm coincide. We also compute the exact value of the norm of the Hilbert matrix on $H^\infty_{w_α}$ with weights $w_α(z)=(1-|z|)^α$ for all $0 < α< 1$. Also in this case, the values of the norm and essential norm coincide.

math.FA↗

arXiv2020

Unified approach to spectral properties of multipliers

Let $\mathbb B_n$ be the open unit ball in $\mathbb C^n$. We characterize the spectra of pointwise multipliers $M_u$ acting on Banach spaces of analytic functions on $\mathbb B_n$ satisfying some general conditions. These spaces include Bergman-Sobolev spaces $A^p_{α,β}$, Bloch-type spaces $\mathcal B_α$, weighted Hardy spaces $H^p_w$ with Muckenhoupt weights and Hardy-Sobolev Hilbert spaces $H^2_β$. Moreover, we describe the essential spectra of multipliers in most of the aforementioned spaces, in particular, in those spaces for which the set of multipliers is a subset of the ball algebra.

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arXiv2020

On the exact value of the norm of the Hilbert matrix operator on weighted Bergman spaces

In this article, the open problem of finding the exact value of the norm of the Hilbert matrix operator on weighted Bergman spaces $A^p_α$ is adressed. The norm was conjectured to be $\fracπ{\sin \frac{(2+α)π}{p}}$ by Karapetrović. We obtain a complete solution to the conjecture for $α\ge 0$ and $2+α+\sqrt{α^2+\frac{7}{2}α+3} \le p < 2(2+α)$ and a partial solution for $2+2α< p < 2+α+\sqrt{α^2+\frac{7}{2}α+3}.$ Moreover, we also show that the conjecture is valid for small values of $α$ when $2+2α< p \le 3+2α.$ Finally, the case $α= 1$ is considered.

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arXiv2018

Rigidity of weighted composition operators on $H^p$

We show that every non-compact weighted composition operator $f \mapsto u\cdot (f\circϕ)$ acting on a Hardy space $H^p$ for $1 \leq p < \infty$ fixes an isomorphic copy of the sequence space $\ell^p$ and therefore fails to be strictly singular. We also characterize those weighted composition operators on $H^p$ which fix a copy of the Hilbert space $\ell^2$. These results extend earlier ones obtained for unweighted composition operators.

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arXiv2018

Norm estimates of weighted composition operators pertaining to the Hilbert Matrix

Very recently, Božin and Karapetrović solved a conjecture by proving that the norm of the Hilbert matrix operator $\mathcal{H}$ on the Bergman space $A^p$ is equal to $\fracπ{\sin(\frac{2π}{p})}$ for $2 < p < 4.$ In this article we present a partly new and simplified proof of this result. Moreover, we calculate the exact value of the norm of $\mathcal{H}$ defined on the Korenblum spaces $H^\infty_α$ for $0 < α\le 2/3$ and an upper bound for the norm on the scale $2/3 < α< 1$.

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arXiv2018

Generalized Volterra operators mapping between Banach spaces of analytic functions

We characterize boundedness and compactness of the classical Volterra operator $T_g \colon H_{v_α}^{\infty} \to H^{\infty}$ induced by a univalent function $g$ for standard weights $v_α$ with $0 \leq α< 1$, partly answering an open problem posed by A. Anderson, M. Jovovic and W. Smith. We also study boundedness, compactness and weak compactness of the generalized Volterra operator $T_g^φ$ mapping between Banach spaces of analytic functions on the unit disc satisfying certain general conditions.

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arXiv2015

Composition Operators on the Bloch space of the Unit Ball of a Hilbert Space

Every analytic self-map of the unit ball of a Hilbert space induces a bounded composition operator on the space of Bloch functions. Necessary and sufficient conditions for compactness of such composition operators are provided, as well as some examples that clarify the connections among such conditions.

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arXiv2013

The essential norm of a weighted composition operator on BMOA

We provide an estimate for the essential norm of a weighted composition operator $W_{ψ,φ}\colon f\mapsto ψ(f\circφ)$ acting on the space $BMOA$ in terms of the weight function $ψ$ and the $n$-th power $φ^n$ of the analytic self-map $φ$ of the open unit disc $\mathbb{D}$. We also provide a new estimate for the norm of the weighted composition operator on $BMOA$.

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