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Alexei Yu. Karlovich

Publications and source records attributed to Alexei Yu. Karlovich.

At least 19 recordsLinked to original sources
arXiv2020

Calkin images of Fourier convolution operators with slowly oscillating symbols

Let $Φ$ be a $C^*$-subalgebra of $L^\infty(\mathbb{R})$ and $SO_{X(\mathbb{R})}^\diamond$ be the Banach algebra of slowly oscillating Fourier multipliers on a Banach function space $X(\mathbb{R})$. We show that the intersection of the Calkin image of the algebra generated by the operators of multiplication $aI$ by functions $a\inΦ$ and the Calkin image of the algebra generated by the Fourier convolution operators $W^0(b)$ with symbols in $SO_{X(\mathbb{R})}^\diamond$ coincides with the Calkin image of the algebra generated by the operators of multiplication by constants.

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arXiv2019

Noncompactness of Fourier Convolution Operators on Banach Function Spaces

Let $X(\mathbb{R})$ be a separable Banach function space such that the Hardy-Littlewood maximal operator $M$ is bounded on $X(\mathbb{R})$ and on its associate space $X'(\mathbb{R})$. Suppose $a$ is a Fourier multiplier on the space $X(\mathbb{R})$. We show that the Fourier convolution operator $W^0(a)$ with symbol $a$ is compact on the space $X(\mathbb{R})$ if and only if $a=0$. This result implies that nontrivial Fourier convolution operators on Lebesgue spaces with Muckenhoupt weights are never compact.

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arXiv2019

Hardy-Littlewood maximal operator on reflexive variable Lebesgue spaces over spaces of homogeneous type

We show that the Hardy-Littlewood maximal operator is bounded on a reflexive variable Lebesgue space $L^{p(\cdot)}$ over a space of homogeneous type $(X,d,μ)$ if and only if it is bounded on its dual space $L^{p'(\cdot)}$, where $1/p(x)+1/p'(x)=1$ for $x\in X$. This result extends the corresponding result of Lars Diening from the Euclidean setting of $\mathbb{R}^n$ to the setting of spaces of homogeneous type $(X,d,μ)$.

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arXiv2019

Algebra of convolution type operators with continuous data on Banach function spaces

We show that if the Hardy-Littlewood maximal operator is bounded on a reflexive Banach function space $X(\mathbb{R})$ and on its associate space $X'(\mathbb{R})$, then the space $X(\mathbb{R})$ has an unconditional wavelet basis. As a consequence of the existence of a Schauder basis in $X(\mathbb{R})$, we prove that the ideal of compact operators $\mathcal{K}(X(\mathbb{R}))$ on the space $X(\mathbb{R})$ is contained in the Banach algebra generated by all operators of multiplication $aI$ by functions $a\in C(\dot{\mathbb{R}})$, where $\dot{\mathbb{R}}=\mathbb{R}\cup\{\infty\}$, and by all Fourier convolution operators $W^0(b)$ with symbols $b\in C_X(\dot{\mathbb{R}})$, the Fourier multiplier analogue of $C(\dot{\mathbb{R}})$.

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arXiv2018

Hardy-Littlewood maximal operator on the associate space of a Banach function space

Let $\mathcal{E}(X,d,μ)$ be a Banach function space over a space of homogeneous type $(X,d,μ)$. We show that if the Hardy-Littlewood maximal operator $M$ is bounded on the space $\mathcal{E}(X,d,μ)$, then its boundedness on the associate space $\mathcal{E}'(X,d,μ)$ is equivalent to a certain condition $\mathcal{A}_\infty$. This result extends a theorem by Andrei Lerner from the Euclidean setting of $\mathbb{R}^n$ to the setting of spaces of homogeneous type.

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arXiv2017

Density of Analytic Polynomials in Abstract Hardy Spaces

Let $X$ be a separable Banach function space on the unit circle $\mathbb{T}$ and $H[X]$ be the abstract Hardy space built upon $X$. We show that the set of analytic polynomials is dense in $H[X]$ if the Hardy-Littlewood maximal operator is bounded on the associate space $X'$. This result is specified to the case of variable Lebesgue spaces.

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arXiv2017

The Coburn-Simonenko theorem for Toeplitz operators acting between Hardy type subspaces of different Banach function spaces

Let $Γ$ be a rectifiable Jordan curve, let $X$ and $Y$ be two reflexive Banach function spaces over $Γ$ such that the Cauchy singular integral operator $S$ is bounded on each of them, and let $M(X,Y)$ denote the space of pointwise multipliers from $X$ to $Y$. Consider the Riesz projection $P=(I+S)/2$, the corresponding Hardy type subspaces $PX$ and $PY$, and the Toeplitz operator $T(a):PX\to PY$ defined by $T(a)f=P(af)$ for a symbol $a\in M(X,Y)$. We show that if $X\hookrightarrow Y$ and $a\in M(X,Y)\setminus\{0\}$, then $T(a)\in\mathcal{L}(PX,PY)$ has a trivial kernel in $PX$ or a dense image in $PY$. In particular, if $1<q\le p<\infty$, $1/r=1/q-1/p$, and $a\in L^{r}\equiv M(L^p,L^q)$ is a nonzero function, then the Toeplitz operator $T(a)$, acting from the Hardy space $H^p$ to the Hardy space $H^q$, has a trivial kernel in $H^p$ or a dense image in $H^q$.

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arXiv2017

Semi-Fredholmness of weighted singular integral operators with shifts and slowly oscillating data

Let $α,β$ be orientation-preserving homeomorphisms of $[0,\infty]$ onto itself, which have only two fixed points at $0$ and $\infty$, and whose restrictions to $\mathbb{R}_+=(0,\infty)$ are diffeomorphisms, and let $U_α,U_β$ be the corresponding isometric shift operators on the space $L^p(\mathbb{R}_+)$ given by $U_μf=(μ')^{1/p}(f\circμ)$ for $μ\in\{α,β\}$. We prove sufficient conditions for the right and left Fredholmness on $L^p(\mathbb{R}_+)$ of singular integral operators of the form $A_+P_γ^++A_-P_γ^-$, where $P_γ^\pm=(I\pm S_γ)/2$, $S_γ$ is a weighted Cauchy singular integral operator, $A_+=\sum_{k\in\mathbb{Z}}a_kU_α^k$ and $A_-=\sum_{k\in\mathbb{Z}}b_kU_β^k$ are operators in the Wiener algebras of functional operators with shifts. We assume that the coefficients $a_k,b_k$ for $k\in\mathbb{Z}$ and the derivatives of the shifts $α',β'$ are bounded continuous functions on $\mathbb{R}_+$ which may have slowly oscillating discontinuities at $0$ and $\infty$.

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arXiv2015

On a Weighted Singular Integral Operator with Shifts and Slowly Oscillating Data

Let $α,β$ be orientation-preserving diffeomorphism (shifts) of $\mathbb{R}_+=(0,\infty)$ onto itself with the only fixed points $0$ and $\infty$ and $U_α,U_β$ be the isometric shift operators on $L^p(\mathbb{R}_+)$ given by $U_αf=(α')^{1/p}(f\circα)$, $U_βf=(β')^{1/p}(f\circβ)$, and $P_2^\pm=(I\pm S_2)/2$ where \[ (S_2 f)(t):=\frac{1}{πi}\int\limits_0^\infty \left(\frac{t}τ\right)^{1/2-1/p}\frac{f(τ)}{τ-t}\,dτ, \quad t\in\mathbb{R}_+, \] is the weighted Cauchy singular integral operator. We prove that if $α',β'$ and $c,d$ are continuous on $\mathbb{R}_+$ and slowly oscillating at $0$ and $\infty$, and \[ \limsup_{t\to s}|c(t)|<1, \quad \limsup_{t\to s}|d(t)|<1, \quad s\in\{0,\infty\}, \] then the operator $(I-cU_α)P_2^++(I-dU_β)P_2^-$ is Fredholm on $L^p(\mathbb{R}_+)$ and its index is equal to zero. Moreover, its regularizers are described.

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arXiv2014

Fredholmness and Index of Simplest Weighted Singular Integral Operators with Two Slowly Oscillating Shifts

Let $α$ and $β$ be orientation-preserving diffeomorphisms (shifts) of $\mathbb{R}_+=(0,\infty)$ onto itself with the only fixed points $0$ and $\infty$, where the derivatives $α'$ and $β'$ may have discontinuities of slowly oscillating type at $0$ and $\infty$. For $p\in(1,\infty)$, we consider the weighted shift operators $U_α$ and $U_β$ given on the Lebesgue space $L^p(\mathbb{R}_+)$ by $U_αf=(α')^{1/p}(f\circα)$ and $U_βf= (β')^{1/p}(f\circβ)$. For $i,j\in\mathbb{Z}$ we study the simplest weighted singular integral operators with two shifts $A_{ij}=U_α^i P_γ^++U_β^j P_γ^-$ on $L^p(\mathbb{R}_+)$, where $P_γ^\pm=(I\pm S_γ)/2$ are operators associated to the weighted Cauchy singular integral operator $$ (S_γf)(t)=\frac{1}{πi}\int_{\mathbb{R}_+} \left(\frac{t}τ\right)^γ\frac{f(τ)}{τ-t}dτ$$ with $γ\in\mathbb{C}$ satisfying $0<1/p+\Reγ<1$. We prove that the operator $A_{ij}$ is a Fredholm operator on $L^p(\mathbb{R}_+)$ and has zero index if \[ 0<\frac{1}{p}+\Reγ+\frac{1}{2π}\inf_{t\in\mathbb{R}_+}(ω_{ij}(t)\Imγ), \quad \frac{1}{p}+\Reγ+\frac{1}{2π}\sup_{t\in\mathbb{R}_+}(ω_{ij}(t)\Imγ)<1, \] where $ω_{ij}(t)=\log[α_i(β_{-j}(t))/t]$ and $α_i$, $β_{-j}$ are iterations of $α$, $β$. This statement extends an earlier result obtained by the author, Yuri Karlovich, and Amarino Lebre for $γ=0$.

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arXiv2014

Maximally Modulated Singular Integral Operators and their Applications to Pseudodifferential Operators on Banach Function Spaces

We prove that if the Hardy-Littlewood maximal operator is bounded on a separable Banach function space $X(\mathbb{R}^n)$ and on its associate space $X'(\mathbb{R}^n)$ and a maximally modulated Calderón-Zygmund singular integral operator $T^Φ$ is of weak type $(r,r)$ for all $r\in(1,\infty)$, then $T^Φ$ extends to a bounded operator on $X(\mathbb{R}^n)$. This theorem implies the boundedness of the maximally modulated Hilbert transform on variable Lebesgue spaces $L^{p(\cdot)}(\mathbb{R})$ under natural assumptions on the variable exponent $p:\mathbb{R}\to(1,\infty)$. Applications of the above result to the boundedness and compactness of pseudodifferential operators with $L^\infty(\mathbb{R},V(\mathbb{R}))$-symbols on variable Lebesgue spaces $L^{p(\cdot)}(\mathbb{R})$ are considered. Here the Banach algebra $L^\infty(\mathbb{R},V(\mathbb{R}))$ consists of all bounded measurable $V(\mathbb{R})$-valued functions on $\mathbb{R}$ where $V(\mathbb{R})$ is the Banach algebra of all functions of bounded total variation.

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arXiv2014

On regularization of Mellin PDO's with slowly oscillating symbols of limited smoothness

We study Mellin pseudodifferential operators (shortly, Mellin PDO's) with symbols in the algebra $\widetilde{\mathcal{E}}(\mathbb{R}_+,V(\mathbb{R}))$ of slowly oscillating functions of limited smoothness introduced in \cite{K09}. We show that if $\mathfrak{a}\in\widetilde{\mathcal{E}}(\mathbb{R}_+,V(\mathbb{R}))$ does not degenerate on the "boundary" of $\mathbb{R}_+\times\mathbb{R}$ in a certain sense, then the Mellin PDO ${\rm Op}(\mathfrak{a})$ is Fredholm on the space $L^p$ for $p\in(1,\infty)$ and each its regularizer is of the form ${\rm Op}(\mathfrak{b})+K$ where $K$ is a compact operator on $L^p$ and $\mathfrak{b}$ is a certain explicitly constructed function in the same algebra $\widetilde{\mathcal{E}}(\mathbb{R}_+,V(\mathbb{R}))$ such that $\mathfrak{b}=1/\mathfrak{a}$ on the "boundary" of $\mathbb{R}_+\times\mathbb{R}$. This result complements a known Fredholm criterion from \cite{K09} for Mellin PDO's with symbols in the closure of $\widetilde{\mathcal{E}}(\mathbb{R}_+,V(\mathbb{R}))$.

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arXiv2013

Boundedness of Pseudodifferential Operators on Banach Function Spaces

We show that if the Hardy-Littlewood maximal operator is bounded on a separable Banach function space $X(\mathbb{R}^n)$ and on its associate space $X'(\mathbb{R}^n)$, then a pseudodifferential operator $\operatorname{Op}(a)$ is bounded on $X(\mathbb{R}^n)$ whenever the symbol $a$ belongs to the Hörmander class $S_{ρ,δ}^{n(ρ-1)}$ with $0<ρ\le 1$, $0\leδ<1$ or to the the Miyachi class $S_{ρ,δ}^{n(ρ-1)}(\varkappa,n)$ with $0\leδ\leρ\le 1$, $0\leδ<1$, and $\varkappa>0$. This result is applied to the case of variable Lebesgue spaces $L^{p(\cdot)}(\mathbb{R}^n)$.

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arXiv2012

The Cauchy Singular Integral Operator on Weighted Variable Lebesgue Spaces

Let $p:\R\to(1,\infty)$ be a globally log-Hölder continuous variable exponent and $w:\R\to[0,\infty]$ be a weight. We prove that the Cauchy singular integral operator $S$ is bounded on the weighted variable Lebesgue space $L^{p(\cdot)}(\R,w)=\{f:fw\in L^{p(\cdot)}(\R)\}$ if and only if the weight $w$ satisfies \[ \sup_{-\infty<a<b<\infty} \frac{1}{b-a}\|wχ_{(a,b)}\|_{p(\cdot)}\|w^{-1}χ_{(a,b)}\|_{p'(\cdot)}<\infty \quad (1/p(x)+1/p'(x)=1). \]

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arXiv2011

Pseudodifferential Operators on Variable Lebesgue Spaces

Let $\mathcal{M}(\mathbb{R}^n)$ be the class of bounded away from one and infinity functions $p:\mathbb{R}^n\to[1,\infty]$ such that the Hardy-Littlewood maximal operator is bounded on the variable Lebesgue space $L^{p(\cdot)}(\mathbb{R}^n)$. We show that if $a$ belongs to the Hörmander class $S_{ρ,δ}^{n(ρ-1)}$ with $0<ρ\le 1$, $0\leδ<1$, then the pseudodifferential operator $\Op(a)$ is bounded on the variable Lebesgue space $L^{p(\cdot)}(\R^n)$ provided that $p\in\cM(\R^n)$. Let $\mathcal{M}^*(\mathbb{R}^n)$ be the class of variable exponents $p\in\mathcal{M}(\mathbb{R}^n)$ represented as $1/p(x)=θ/p_0+(1-θ)/p_1(x)$ where $p_0\in(1,\infty)$, $θ\in(0,1)$, and $p_1\in\mathcal{M}(\mathbb{R}^n)$. We prove that if $a\in S_{1,0}^0$ slowly oscillates at infinity in the first variable, then the condition \[ \lim_{R\to\infty}\inf_{|x|+|ξ|\ge R}|a(x,ξ)|>0 \] is sufficient for the Fredholmness of $\Op(a)$ on $L^{p(\cdot)}(\R^n)$ whenever $p\in\cM^*(\R^n)$. Both theorems generalize pioneering results by Rabinovich and Samko \cite{RS08} obtained for globally log-Hölder continuous exponents $p$, constituting a proper subset of $\mathcal{M}^*(\mathbb{R}^n)$.

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arXiv2011

On an Interesting Class of Variable Exponents

Let $\mathcal{M}(\mathbb{R}^n)$ be the class of functions $p:\mathbb{R}^n\to[1,\infty]$ bounded away from one and infinity and such that the Hardy-Littlewood maximal function is bounded on the variable Lebesgue space $L^{p(\cdot)}(\mathbb{R}^n)$. We denote by $\mathcal{M}^*(\mathbb{R}^n)$ the class of variable exponents $p\in\mathcal{M}(\mathbb{R}^n)$ for which $1/p(x)=θ/p_0+(1-θ)/p_1(x)$ with some $p_0\in(1,\infty)$, $θ\in(0,1)$, and $p_1\in\mathcal{M}(\mathbb{R}^n)$. Rabinovich and Samko \cite{RS08} observed that each globally log-Hölder continuous exponent belongs to $\mathcal{M}^*(\mathbb{R}^n)$. We show that the class $\mathcal{M}^*(\mathbb{R}^n)$ contains many interesting exponents beyond the class of globally log-Hölder continuous exponents.

math.CA↗
arXiv2011

On singular integral operators with semi-almost periodic coefficients on variable Lebesgue spaces

Let $a$ be a semi-almost periodic matrix function with the almost periodic representatives $a_l$ and $a_r$ at $-\infty$ and $+\infty$, respectively. Suppose $p:\mathbb{R}\to(1,\infty)$ is a slowly oscillating exponent such that the Cauchy singular integral operator $S$ is bounded on the variable Lebesgue space $L^{p(\cdot)}(\mathbb{R})$. We prove that if the operator $aP+Q$ with $P=(I+S)/2$ and $Q=(I-S)/2$ is Fredholm on the variable Lebesgue space $L_N^{p(\cdot)}(\mathbb{R})$, then the operators $a_lP+Q$ and $a_rP+Q$ are invertible on standard Lebesgue spaces $L_N^{q_l}(\mathbb{R})$ and $L_N^{q_r}(\mathbb{R})$ with some exponents $q_l$ and $q_r$ lying in the segments between the lower and the upper limits of $p$ at $-\infty$ and $+\infty$, respectively.

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