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Hans Triebel

Publications and source records attributed to Hans Triebel.

15 recordsLinked to original sources
arXiv2024

Spectral theory for fractal pseudodifferential operators

The paper deals with the distribution of eigenvalues of the compact fractal pseudodifferential operator $T^\mu_\tau$, \[ \big( T^\mu_\tau f\big)(x) = \int_{\mathbb{R}^n} e^{-ix\xi} \, \tau(x,\xi) \, \big( f\mu \big)^\vee (\xi) \, \mathrm{d} \xi, \qquad x\in \mathbb{R}^n, \] in suitable special Besov spaces $B^s_p (\mathbb{R}^n) = B^s_{p,p} (\mathbb{R}^n)$, $s>0$, $1 0, \quad 1<p<\infty. \] We add at the end of the paper a few personal reminiscences illuminating the role of Pietsch in connection with the creation of approximation numbers and entropy numbers.

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arXiv2023

Mapping properties of Fourier transforms, revisited

The paper deals with continuous and compact mappings generated by the Fourier transform between distinguished Besov spaces $B^s_p(\mathbb{R}^n) = B^s_{p,p}(\mathbb{R}^n)$, $1\le p \le \infty$, and between Sobolev spaces $H^s_p(\mathbb{R}^n)$, $1<p< \infty$. In contrast to the paper {\em H. Triebel, Mapping properties of Fourier transforms. Z. Anal. Anwend. 41 (2022), 133--152}, based mainly on embeddings between related weighted spaces, we rely on wavelet expansions, duality and interpolation of corresponding (unweighted) spaces, and (appropriately extended) Hausdorff-Young inequalities. The degree of compactness will be measured in terms of entropy numbers and approximation numbers, now using the symbiotic relationship to weighted spaces.

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arXiv2023

A note on Hausdorff-Young inequalities in function spaces

The classical Hausdorff-Young inequalities for the Fourier transform acting between appropriate $L_p$ spaces are cornerstones of Fourier analysis. Here we extend it to weighted spaces of Besov or Sobolev type where the weight has the form $w(x)=(1+|x|^2)^{\alpha/2}$. This note is not a paper or draft but a sketchy complement to some earlier results where we dealt with mapping properties of the Fourier transform.

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arXiv2022

Quarkonial analysis

The spaces $A^s_{p,q}({\mathbb R}^n)$ with $A \in \{B,F \}$, $s\in {\mathbb R}$ and $0<p,q \le \infty$ are usually introduced in terms of Fourier--analytical decompositions. Related characterizations based on atoms and wavelets are known nowadays in a rather final way. Quarks atomize the atoms into constructive building blocks. It is the main aim of these notes to raise quarkonial decompositions to the same level as related representations of the spaces $A^s_{p,q}({\mathbb R}^n)$ in terms of atoms or wavelets. This will be complemented by some applications. In addition we deal also with quarks in domains and their relations to so--called refined localization spaces.

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arXiv2022

Nuclear Fourier transforms

The paper deals with the problem under which conditions for the parameters $s_1,s_2\in\mathbb{R}$, $1\leq p,q_1,q_2\leq\infty$ the Fourier transform $\mathcal{F}$ is a nuclear mapping from $A^{s_1}_{p,q_1}(\mathbb{R}^n)$ into $A^{s_2}_{p,q_2}(\mathbb{R}^n)$, where $A\in\{B,F\}$ stands for a space of Besov or Triebel-Lizorkin type, and $n\in\mathbb{N}$. It extends the recent paper arXiv:2112.04896 where the compactness of $\mathcal{F}$ acting in the same type of spaces was studied.

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arXiv2022

Mapping properties of Fourier transforms, II

This is the direct continuation of the paper "Mapping properties of Fourier transforms" (arXiv:2112.04896) using the same notation as there without further explanations. It deals with continuous and compact mappings of the Fourier transform $F$ between some weighted function spaces on $\mathbb{R}^n$.

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arXiv2022

Mapping properties of pseudodifferential and Fourier operators

The composition of the Fourier transform in $\mathbb{R}^n$ with a suitable pseudodifferential operator is called a Fourier operator. It is compact in appropriate function spaces. The paper deals with its spectral theory. This is based on mapping properties of the Fourier transform as developed in a preceding paper and related assertions for pseudodifferential operators.

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arXiv2021

Mapping properties of Fourier transforms

The paper deals with continuous and compact mappings generated by the Fourier transform between distinguished function spaces on $\mathbb{R}^n$. The degree of compactness will be measured in terms of related entropy numbers. We are more interested in the interplay of already available ingredients than in generality.

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arXiv2021

Morrey smoothness spaces: A new approach

In the recent years so-called Morrey smoothness spaces attracted a lot of interest. They can (also) be understood as generalisations of the classical spaces $A^s_{p,q} (\mathbb{R}^n)$, $A\in \{B,F\}$, in $\mathbb{R}^n$, where the parameters satisfy $s\in \mathbb{R}$ (smoothness), $0<p \le \infty$ (integrability) and $0<q \le \infty$ (summability). In the case of Morrey smoothness spaces additional parameters are involved. In our opinion, among the various approaches at least two scales enjoy special attention, also in view of applications: the scales $\mathcal{A}^s_{u,p,q} (\mathbb{R}^n)$, with $\mathcal{A}\in \{\mathcal{N}, \mathcal{E}\}$, $u\geq p$, and $A^{s, τ}_{p,q} (\mathbb{R}^n)$, with $τ\geq 0$. We reorganise these two prominent types of Morrey smoothness spaces by adding to $(s,p,q)$ the so--called slope parameter $\varrho$, preferably (but not exclusively) with $-n \le \varrho <0$. It comes out that $|\varrho|$ replaces $n$, and $\min (|\varrho|,1)$ replaces 1 in slopes of (broken) lines in the $( \frac{1}{p}, s)$--diagram characterising distinguished properties of the spaces $A^s_{p,q} (\mathbb{R}^n)$ and their Morrey counterparts. Special attention will be paid to low--slope spaces with $-1 <\varrho <0$, where corresponding properties are quite often independent of $n\in \mathbb{N}$. Our aim is two--fold. On the one hand we reformulate some assertions already available in the literature (many of them are quite recent). On the other hand we establish on this basis new properties, a few of them became visible only in the context of the offered new approach, governed, now, by the four parameters $(s,p,q,\varrho)$.

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arXiv2020

Traces of some weighted function spaces and related non-standard real interpolation of Besov spaces

We study traces of weighted Triebel-Lizorkin spaces $F^s_{p,q}({\mathbb R}^n,w)$ on hyperplanes ${\mathbb R}^{n-k}$, where the weight is of Muckenhoupt type. We concentrate on the example weight $w_α(x) = |x_n|^α$ when $|x_n|\leq 1$, $x\in{\mathbb R}^n$, and $w_α(x)=1$ otherwise, where $α>-1$. Here we use some refined atomic decomposition argument as well as an appropriate wavelet representation in corresponding (unweighted) Besov spaces. The second main outcome is the description of the real interpolation space $(B^{s_1}_{p_1,p_1}({\mathbb R}^{n-k}), B^{s_2}_{p_2,p_2}({\mathbb R}^{n-k}))_{θ,r}$, $0 0$ sufficiently large, $0<θ<1$, $0<r\leq\infty$. Apart from the case $1/r= (1-θ)/{p_1}+ θ/{p_2}$ the question seems to be open for many years. Based on our first result we can now quickly solve this long-standing problem. Here we benefit from some very recent finding of Besoy, Cobos and Triebel.

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arXiv2016

Tempered homogeneous function spaces, II

This paper deals with homogeneous function spaces of Besov-Sobolev type within the framework of tempered distributions in Euclidean $n$-space based on Gauss-Weierstrass semi-groups. Related Fourier-analytical descriptions are incorporated afterwards as so-called domestic norms. This approach avoids the usual ambiguity modulo polynomials when homogeneous function spaces are considered in the context of homogeneous tempered distributions. The motivation to deal with these spaces comes from (nonlinear) heat and Navier-Stokes equations, but also from Keller-Segel sytems and other PDE models of chemotaxis.

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