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Joachim Toft

Publications and source records attributed to Joachim Toft.

At least 19 recordsLinked to original sources
arXiv2026

Compactness for pseudo-differential and Toeplitz operators on modulation spaces

We deduce various norm equivalences, and convolution estimates for the modulation space $M^{\sharp ,q}_{(\omega )}$ consisting of all $f\in M^{\infty ,q}_{(\omega )}$ such that $|V_\phi f \cdot \omega |$ satisfies a mild vanishing condition at infinity. We prove that $M^{\sharp ,q}_{(\omega )}$ is the completion of the Gelfand-Shilov space $\Sigma _1$ under the $M^{\infty ,q}_{(\omega )}$ norm. We use these results to deduce compactness for $\Psi$DO $\op (\mathfrak a )$, with $\mathfrak a \in M^{\sharp ,q}_{(\omega )}$, $0<q\le 1$, when acting on a broad family of modulation spaces.

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arXiv2026

Orlicz Space Interpolation and Its Applications to Operator Convolution

We prove a strong-type interpolation result for noncommutative Orlicz spaces over semifinite von Neumann algebras. Based on this result, we obtain Young-type convolution estimates for the Weyl pseudodifferential symbols of operators in appropriate Orlicz-Schatten spaces. Equivalently, we prove convolution estimates of Young type for Werner's function-operator convolutions in quantum harmonic analysis.

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arXiv2026

Fourier integral operators on Orlicz modulation spaces

We establish continuity and Schatten-von Neumann properties for Fourier integral operators with amplitudes in Orlicz modulation spaces, when acting on other Orlicz modulation spaces themselves. The phase functions are non smooth and admit second order derivatives in suitable classes of modulation spaces.

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arXiv2025

Quantitative uncertainty principles for time-frequency Gaussian decay

For real symmetric positive definite matrices $A$ and $B$, we characterize when a function $f \in L^2(\mathbb{R}^d)$ satisfies \[ |f(x)| \lesssim e^{-(\frac12 - \lambda) \langle Ax, x\rangle} \quad \text{and} \quad |\widehat{f}(\xi)| \lesssim e^{-(\frac12 - \lambda) \langle B\xi, \xi\rangle} , \qquad \forall \lambda > 0 , \] or even more specified time-frequency decay estimates, in terms of the skewed Hermite series expansion of $f$. We also consider coordinate-wise time-frequency decay and determine when it becomes equivalent to the same bounds on the skewed Hermite coefficients.

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arXiv2025

Convolutions of Orlicz spaces and Orlicz Schatten classes, with applications to Toeplitz operators

Let $\Phi$ be a Young function. We study convolution properties for symbol classes $s_{A,\Phi}$, which consist of all $a$ such that the pseudo-differential operator $\operatorname{Op} _A(a)$ is in the Orlicz Schatten class $\mathscr I _\Phi (L^2(\mathbf R^d))$. Especially we prove Young type results for such classes. We apply the results on Toeplitz operators and prove Orlicz Schatten properties of such operators.

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arXiv2024

Norm estimates for a broad class of modulation spaces, and continuity of Fourier type operators

Let $\mathscr B$ be a normal quasi-Banach function space with respect to $r_0 \in (0,1]$ and $v_0$, $\omega$ be $v$-moderate, and let $r\in [r_0,\infty ]$. Then we prove that $f$ belongs to the modulation space $M(\omega ,\mathscr B )$, iff $V_\phi f$ belongs to the Wiener amalgam space $W ^r(\omega ,\mathscr B )$, and $$ \| f \| _{M(\omega , \mathscr B)} \asymp \| V _\phi f \, \omega \| _{\mathscr B} \asymp \| V _\phi f\| _{W ^r(\omega, \mathscr B)}. $$ We also use the results to deduce continuity for pseudo-differential operators with symbols in weighted $M^{\infty,r_0}$-spaces, with $r_0\le 1$, when acting on $M(\omega ,\mathscr B )$-spaces.

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arXiv2024

Non-isometric translation and modulation invariant Hilbert spaces

Let $\mathcal H$ be a Hilbert space of distributions on $\mathbf R^d$ which contains at least one non-zero element in $\mathscr D '(\mathbf R^d)$. If there is a constant $C_0>0$ such that $$ \nm {e^{i\scal \cdo \xi}f(\cdo -x)}{\mathcal H}\le C_0\nm f{\mathcal H}, \qquad f\in \mathcal H ,\ x,\xi \in \mathbf R^d, $$ then we prove that $\maclH = L^2(\mathbf R^d)$, with equivalent norms.

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arXiv2024

Quasi-Banach Schatten-von Neumann properties in Weyl-H\"ormander calculus

We study structural properties of Wiener-Lebesgue spaces with respect to a slowly varying metrics and certain Lebesgue parameters. For $p\in (0,1]$, we deduce Schatten-$p$ properties for pseudo-differential operators whose symbols, together with their derivatives, obey suitable Wiener-Lebesgue-boundedness conditions. Especially, we perform such investigations for the Weyl-H\"ormander calculus. Finally, we apply our results to global-type SG and Shubin pseudo-differential operators.

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arXiv2024

Hermite expansions for spaces of functions with nearly optimal time-frequency decay

We establish Hermite expansion characterizations for several subspaces of the Fr\'{e}chet space of functions on the real line satisfying \begin{equation*} |f(x)| \lesssim e^{-(\frac{1}{2} - \lambda ) x^{2}} , \qquad | \widehat{f}(\xi )| \lesssim e^{-(\frac{1}{2} - \lambda ) \xi ^{2}} , \qquad \forall \lambda > 0 . \end{equation*} In particular, we extend and improve Fourier characterizations of the so-called proper Pilipovi\'{c} spaces obtained in [J. Funct. Anal. 284 (2023), 109724]. The main ingredients in our proofs are the Bargmann transform and some achieved optimal forms of the Phragm\'{e}n-Lindel\"{o}f principle.

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arXiv2023

Fourier type operators on Orlicz spaces and the role of Orlicz Lebesgue exponents

We deduce continuity and (global) wave-front properties of classes of Fourier multipliers, pseudo-differential, and Fourier integral operators when acting on Orlicz spaces, or more generally, on Orlicz-Sobolev type spaces. In particular, we extend H{\"o}rmander's improvement of Mihlin's Fourier multiplier theorem to the framework of Orlicz spaces. We also show how Young functions $\Phi$ of the Orlicz spaces are linked to properties of certain Lebesgue exponents $p_\Phi$ and $q_\Phi$ emerged from $\Phi$.

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arXiv2023

Factorizations for quasi-Banach time-frequency spaces and Schatten classes

We deduce factorization properties for a quasi-Banach module over a quasi-Banach algebra. Especially we extend a result by Hewitt and prove that if any such algebra which possess a bounded left approximate identity, then any element in the module can be factorized. As applications, we deduce factorization properties for Wiener amalgam spaces, for an extended family of modulation spaces and for Schatten symbol classes in pseudo-differential calculus under multiplications like convolutions, twisted convolutions and symbolic products. For example we show for Wiener amalgam spaces that WL^{1,r}*WL^{p,q}=WL^{p,q} when r in (0,1], and p and q are finite and larger than r. In particular we improve Rudin's identity L^1*L^1=L^1.

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arXiv2023

Pseudo-differential calculi and entropy estimates with Orlicz modulation spaces

We deduce continuity properties for pseudo-differential operators with symbols in Orlicz modulation spaces when acting on other Orlicz modulation spaces. In particular we extend well-known results in the literature. We also show that the entropy functional is continuous on a suitable Orlicz modulation space between $M^p$ and $M^2$ when $p<2$, though the functional is discontinuous on $M^2=L^2$.

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arXiv2023

Schatten-von Neumann properties for H\"ormander classes on compact Lie groups

Let $G$ be a compact Lie group of dimension $n.$ In this work we characterise the membership of classical pseudo-differential operators on $G$ in the trace class ideal $S_{1}(L^2(G)),$ as well as in the setting of the Schatten ideals $S_{r}(L^2(G)),$ for all $r>0.$ In particular, we deduce Schatten characterisations of elliptic pseudo-differential operators of $(\rho,\delta)$-type for the large range $0\leq \delta<\rho\leq 1.$ Additional necessary and sufficient conditions are given in terms of the matrix-valued symbols of the operators, which are global functions on the phase space $G\times \widehat{G},$ with the momentum variables belonging to the unitary dual $\widehat{G}$ of $G$. In terms of the parameters $(\rho,\delta),$ on the torus $\mathbb{T}^n,$ we demonstrate the sharpness of our results showing the existence of atypical operators in the exotic class $\Psi^{-\varkappa}_{0,0}(\mathbb{T}^n),$ $\varkappa>0,$ belonging to all the Schatten ideals. Additional order criteria are given in the setting of classical pseudo-differential operators. We present also some open problems in this setting.

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arXiv2022

Pseudo-differentialoperators on Orlicz modulation spaces

We deduce continuity properties for pseudo-differential operators with symbols in quasi-Banach Orlicz modulation spaces when rely on other quasi-Banach Orlicz modulation spaces. In particular we extend certain results in \cite{GH1,GH2,Toft2,Toft10,Toft16}.

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arXiv2022

Fourier characterizations of Pilipovic spaces

Let $f$ be a function or distribution on $\rr d$. We show that $f$ belongs to a certain Pilipovi{\'c} space, if and only if $f$ and suitable partial fractional Fourier transforms of $f$ satisfy certain types of estimates.

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arXiv2022

An excursion to multiplications and convolutions on modulation spaces

We give a self-contained introduction to (quasi-)Banach modulation spaces of ultradistributions, and review results on boundedness for multiplications and convolutions for elements in such spaces. Furthermore, we use these results to study the Gabor product. As an example, we show how it appears in a phase-space formulation of the nonlinear cubic Schrödinger equation.

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arXiv2022

Wick and anti-Wick characterizations of linear operators on spaces of power series expansions

We study the link between Wick, anti-Wick and analytic kernel operators on the Bargmann transform side. We find classes of kernels, e.g. $\wideparen {\mathcal B} _s$, whose corresponding operators agree with the sets of linear and continuous operators on $\mathcal A _s$, the images of Pilipovi{ć} under the Bargmann transform. We show that in several situations, the sets of Wick, anti-Wick and kernel operators with symbols and kernels in $\wideparen {\mathcal B} _s$ agree. We also show some ring, module and composition properties for $\wideparen {\mathcal B} _s$, and similarly for other spaces related to $\wideparen {\mathcal B} _s$.

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