E. Ostrovsky, L. Sirota
In this article we investigate the Fourier series and transforms for the functions defined on the $ [0, 2 π]^ d $ or $ R^d $ and belonging to the exponential Orlicz and some other rearrangement invariant (r.i.) spaces.
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The method of using rearrangements to give sufficient conditions for Fourier inequalities between weighted Lebesgue spaces is revisited. New results in the case q < p are established and a comparison between two known sufficient conditions is completed. In addition, examples are given to show that a simple weight condition that is sufficient for the weighted Fourier inequality in the cases 2 < q < p and 1 < q < p < 2 is no longer sufficient in the case 1 < q < 2 < p, contrary to statements in Theorems 1 and 4 of, "Weighted Fourier inequalities: new proofs and generalizations", J. Fourier Anal.…
The Fourier transform in weighted rearrangement invariant spaces
该研究探讨加权重排不变空间上傅里叶变换的有界性，推导得出权函数有界性、空间等价性及指数关系等结论，并应用于证明薛定谔方程相关算子的无界性。
We study inequalities of the form \begin{equation*} ρ( \lvert \hat{f} \rvert) \leq C σ(f) < \infty, \end{equation*} with $f \in L_{1}(\mathbb{R}^n)$, the Lebesgue-integrable functions on $\mathbb{R}^n$ and \begin{equation*} \hat{f}(ξ) := \int_{\mathbb{R}^n} f(x) \, e^{- 2 πi ξ\cdot x} dx, \ \ \ ξ\in \mathbb{R}^n. \end{equation*} The functionals $ρ$ and $σ$ are so-called rearrangement-invariant (r.i.) norms on $M_{+}(\mathbb{R}^n)$, the nonnegative measurable functions on $\mathbb{R}^n$. Results first proved in the general context of r.i. spaces are then both specialized and expanded on in the …
In this article we introduce and investigate some new Banach spaces, so - called moment spaces, and consider applications to the Fourier series, singular integral operators, theory of martingales.
Fourier rearrangements on discrete groups: $L^p$ and operator-norm subsequence convergence
本文针对双曲群或多项式增长群，构造傅里叶项重排使子序列在算子范数及$L^p$范数下收敛，证实Révész性质与多项式增长猜想，否证自由群反例猜想。
We prove sharp inequalities for the symmetric-decreasing rearrangement in Fourier space of functions in $\mathbb{R}^d$. Our main result can be applied to a general class of (pseudo-)differential operators in $\mathbb{R}^d$ of arbitrary order with radial Fourier multipliers. For example, we can take any positive power of the Laplacian $(-Δ)^s$ with $s> 0$ and, in particular, any polyharmonic operator $(-Δ)^m$ with integer $m \geq 1$. As applications, we prove radial symmetry and real-valuedness (up to trivial symmetries) of optimizers for: i) Gagliardo-Nirenberg inequalities with derivatives of…
In this paper non-asymptotic exact rearrangement invariant norm estimates are derived for the maximum distribution of the family elements of some rearrangement invariant (r.i.) space over unbounded measure in the entropy terms and in the terms of generic chaining. We consider some applications in the martingale theory and in the theory of Fourier series.
This article studies the rearrangement problem for Fourier series introduced by P.L. Ulyanov, who conjectured that every continuous function on the torus admits a rearrangement of its Fourier coefficients such that the rearranged partial sums of the Fourier series converge uniformly to the function. The main theorem here gives several new equivalences to this conjecture in terms of the convergence of the rearranged Fourier series in the strong (equivalently in this case, weak) operator topologies on $B(L_2(T))$. Additionally, a new framework for further investigation is introduced by consideri…
We show that the decreasing rearrangement of the Fourier series with respect to the Jacobi polynomials for functions in $L_p$ does not converge unless $p=2$. As a by-product of our work on quasi-greedy bases in $L_{p}(μ)$, we show that no normalized unconditional basis in $L_p$, $p\not=2$, can be semi-normalized in $L_q$ for $q\not=p$, thus extending a classical theorem of Kadets and Pełczy{ń}ski from 1968.
We study divergence properties of Fourier series on Cantor-type fractal measures, also called mock Fourier series. We show that in some cases the $L^1$-norm of the corresponding Dirichlet kernel grows exponentially fast, and therefore the Fourier series are not even pointwise convergent. We apply these results to the Lebesgue measure to show that a certain rearrangement of the exponential functions, which we call scrambled Fourier series, have a corresponding Dirichlet kernel whose $L^1$-norm grows exponentially fast, which is much worse than the known logarithmic bound. The divergence propert…
A new class of function spaces generalizing the Arias-de-Reyna space
本文提出了一类推广Arias-de-Reyna空间的函数空间QA_{φ,ψ}，研究其结构和与其他空间的关系。
The Fourier transform is an extremizer of a class of bounded operators
证明Fourier算子在有界性上等价于联合强型算子，借此完全刻画径向单调权下的加权Fourier不等式，解决了Benedetto-Heinig等提出的长期问题。
Weighted Norm Inequalities for the Strichartz Fourier transform on the Heisenberg Group
本文研究了Heisenberg群上Strichartz傅里叶变换的加权范数不等式，推导了Pitt类型的不等式，并探讨了其在不确定性原理和Paley不等式中的应用。
Phase symmetrization and a no-aliasing concentration principle in Paley-Wiener spaces
该研究在无混叠条件下构造性分析Donoho-Stark集中问题，通过相位对齐函数推导不等式，结合傅里叶相位对齐与几何估计证明，还推广至单位圆多项式并明确模式化集中方法的范围与局限。
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