### A class of integral operators on mixed norm spaces in the unit ball

This article provided some sufficient or necessary conditions for a class of integral operators to be bounded on mixed norm spaces in the unit ball.

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This article provided some sufficient or necessary conditions for a class of integral operators to be bounded on mixed norm spaces in the unit ball.

In [P] we characterize the pairs of weights for which the fractional integral operator ${I}_{\gamma}$ of order $\gamma $ from a weighted Lebesgue space into a suitable weighted $BMO$ and Lipschitz integral space is bounded. In this paper we consider other weighted Lipschitz integral spaces that contain those defined in [P], and we obtain results on pairs of weights related to the boundedness of ${I}_{\gamma}$ acting from weighted Lebesgue spaces into these spaces. Also, we study the properties of those classes of weights and compare...

In this paper, we present some theorems on weighted approximation by two dimensional nonlinear singular integral operators in the following form: T λ ( f ; x , y ) = ∬ R 2 ( t − x , s − y , f ( t , s ) ) d s d t , ( x , y ) ∈ R 2 , λ ∈ Λ , $${T}_{\lambda}(f;x,y)=\underset{{\mathbb{R}}^{2}}{\phantom{\rule{0.277778em}{0ex}}\int \int \phantom{\rule{0.277778em}{0ex}}}(t-x,s-y,f(t,s))dsdt,\phantom{\rule{0.277778em}{0ex}}(x,y)\in {\mathbb{R}}^{2},\lambda \in \Lambda ,$$ where Λ is a set of non-negative numbers with accumulation point λ0.

We prove a sharp pointwise estimate of the nonincreasing rearrangement of the fractional maximal function of ⨍, ${M}_{\gamma}\u2a0d$, by an expression involving the nonincreasing rearrangement of ⨍. This estimate is used to obtain necessary and sufficient conditions for the boundedness of ${M}_{\gamma}$ between classical Lorentz spaces.

There is a one parameter family of bilinear Hilbert transforms. Recently, some progress has been made to prove Lp estimates for these operators uniformly in the parameter. In the current article we present some of these techniques in a simplified model...

Let $u$ be a weight on $(0,\infty )$. Assume that $u$ is continuous on $(0,\infty )$. Let the operator ${S}_{u}$ be given at measurable non-negative function $\varphi $ on $(0,\infty )$ by $${S}_{u}\varphi \left(t\right)=\underset{0<\tau \le t}{sup}u\left(\tau \right)\varphi \left(\tau \right).$$ We characterize weights $v,w$ on $(0,\infty )$ for which there exists a positive constant $C$ such that the inequality $${\left({\int}_{0}^{\infty}{\left[{S}_{u}\varphi \left(t\right)\right]}^{q}w\left(t\right)\phantom{\rule{0.166667em}{0ex}}dt\right)}^{\frac{1}{q}}\lesssim {\left({\int}_{0}^{\infty}{\left[\varphi \left(t\right)\right]}^{p}v\left(t\right)\phantom{\rule{0.166667em}{0ex}}dt\right)}^{\frac{1}{p}}$$ holds for every $0<p,q<\infty $. Such inequalities have been used in the study of optimal Sobolev embeddings and boundedness of certain operators on classical Lorenz spaces.

We present new formulae providing equivalent quasi-norms on Lorentz-Karamata spaces. Our results are based on properties of certain averaging operators on the cone of non-negative and non-increasing functions in convenient weighted Lebesgue spaces. We also illustrate connections between our results and mapping properties of such classical operators as the fractional maximal operator and the Riesz potential (and their variants) on the Lorentz-Karamata spaces.

In the paper [13] we proved a fixed point theorem for an operator $\mathcal{A}$, which satisfies a generalized Lipschitz condition with respect to a linear bounded operator $A$, that is: $$m(\mathcal{A}x-\mathcal{A}y)\prec Am(x-y).$$ The purpose of this paper is to show that the results obtained in [13], [14] can be extended to a nonlinear operator $A$.

We obtain modular convergence theorems in modular spaces for nets of operators of the form $\left({T}_{w}f\right)\left(s\right)={\int}_{H}{K}_{w}(s-{h}_{w}\left(t\right),f\left({h}_{w}\left(t\right)\right))d{\mu}_{H}\left(t\right)$, w > 0, s ∈ G, where G and H are topological groups and ${{h}_{w}}_{w>0}$ is a family of homeomorphisms ${h}_{w}:H\to {h}_{w}\left(H\right)\subset G.$ Such operators contain, in particular, a nonlinear version of the generalized sampling operators, which have many applications in the theory of signal processing.