## Publications

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In Press
Fernandes, Cláudio A., Alexei Yu. Karlovich, and Yuri I. Karlovich. "Fourier convolution operators with symbols equivalent to zero at infinity on Banach function spaces." Proceedings of ISAAC 2019. In Press.
Fernandes, Cláudio A., and Alexei Yu. Karlovich. "Semi-almost periodic Fourier multipliers on rearrangement-invariant spaces with suitable Muckenhoupt weights." Boletín de la Sociedad Matemática Mexicana (In Press).
2019
Fernandes, Cláudio A., Alexei Yu. Karlovich, and Yuri. I. Karlovich. "Algebra of convolution type operators with continuous data on Banach function spaces." Banach Center Publications. 119 (2019): 157-171.Website
Karlovich, Alexei, and Eugene Shargorodsky. "The Brown-Halmos theorem for a pair of abstract Hardy spaces." Journal of Mathematical Analysis and Applications. 472 (2019): 246-265.Website
Karlovich, Alexei Yu. "Hardy-Littlewood maximal operator on the associate space of a Banach function space." Real Analysis Exchange. 44.1 (2019): 119-140.Website
Fernandes, Cláudio A., Alexei Yu. Karlovich, and Yuri I. Karlovich. "Noncompactness of Fourier convolution operators on Banach function spaces." Annals of Functional Analysis. 10.4 (2019): 553-561.
Karlovich, Alexei Yu., and Eugene Shargorodsky. "When does the norm of a Fourier multiplier dominate its L-infinfty norm?" Proceedings of the London Mathematical Society. 118 (2019): 901-941.Website
2018
Karlovich, Alexei Yu., Yuri I. Karlovich, and Amarino B. Lebre. "Criteria for n(d)-normality of weighted singular integral operators with shifts and slowly oscillating data." Proceedings of the London Mathematical Society. 116.4 (2018): 997-1027 .Website
Karlovich, Alexei Yu., and Eugene Shargorodsky. "More on the density of analytic polynomials in abstract Hardy spaces." The Diversity and Beauty of Applied Operator Theory. Operator Theory: Advances and Applications, vol. 268. Eds. Albrecht Böttcher, Daniel Potts, Peter Stollman, and David Wenzel. Basel: Birkhäuser, 2018. 319-329.
André, Carlos, Maria Amélia Bastos, Alexei Yu. Karlovich, Bernd Silbermann, and Ion Zaballa(Eds.) Operator Theory, Operator Algebras, and Matrix Theory. Basel: Birkhäuser, 2018.Website
Karlovich, Alexei Yu., Yuri I. Karlovich, and Amarino B. Lebre. "Semi-Fredholmness of weighted singular integral operators with shifts and slowly oscillating data." Operator Theory, Operator Algebras, and Matrix Theory. Operator Theory: Advances and Applications, vol. 267. Eds. Carlos André, Maria Amélia Bastos, Alexei Yu. Karlovich, Bernd Silbermann, and Ion Zaballa. Basel: Birkhäuser, 2018. 221-246.
2017
Karlovich, Alexei Yu. "Density of analytic polynomials in abstract Hardy spaces." Commentationes Mathematicae. 57.2 (2017): 131-141.Website
Karlovich, Alexei Yu., Yuri I. Karlovich, and Amarino B. Lebre. "The index of weighted singular integral operators with shifts and slowly oscillating data." Journal of Mathematical Analysis and Applications. 450 (2017): 606-630.Website
Bini, Dario, Torsten Ehrhardt, Alexei Yu. Karlovich, and Ilya M. Spitkovsky(eds.) Large Truncated Toeplitz Matrices, Toeplitz Operators, and Related Topics. The Albrecht Böttcher Anniversary Volume. Basel: Birkhäuser Basel, 2017.Website
Karlovich, Alexei Yu., Yuri I. Karlovich, and Amarino B. Lebre. "Necessary Fredholm conditions for weighted singular integral operators with shifts and slowly oscillating data." Journal of Integral Equations and Applications. 29.3 (2017): 365-399.
Karlovich, Alexei Yu. "Toeplitz operators on abstract Hardy spaces built upon Banach function spaces." Journal of Function Spaces. 2017 (2017): Article ID 9768210, 8 pages.Website
2016
Karlovich, Alexei Yu., Yuri I. Karlovich, and Amarino B. Lebre. "The generalized Cauchy index of some semi-almost periodic functions." Boletín de la Sociedad Matemática Mexicana. 22.2 (2016): 473-485. AbstractWebsite

We compute the generalized Cauchy index of some semi-almost periodic functions, which are important
in the study of the Fredholm index of singular integral operators with shifts and slowly oscillating data.

Karlovich, Alexei Yu., Yuri I. Karlovich, and Amarino B. Lebre. "On a weighted singular integral operator with shifts and slowly oscillating data." Complex Analysis and Operator Theory. 10.6 (2016): 1101-1131. AbstractWebsite

Let $$\alpha,\beta$$ be orientation-preserving diffeomorphism (shifts) of $$\mathbb{R}_+=(0,\infty)$$ onto itself with the only fixed points $$0$$ and $$\infty$$ and $$U_\alpha,U_\beta$$ be the isometric shift operators on $$L^p(\mathbb{R}_+)$$ given by $$U_\alpha f=(\alpha')^{1/p}(f\circ\alpha)$$, $$U_\beta f=(\beta')^{1/p}(f\circ\beta)$$, and $$P_2^\pm=(I\pm S_2)/2$$ where $(S_2 f)(t):=\frac{1}{\pi i}\int\limits_0^\infty \left(\frac{t}{\tau}\right)^{1/2-1/p}\frac{f(\tau)}{\tau-t}\,d\tau, \quad t\in\mathbb{R}_+,$
is the weighted Cauchy singular integral operator. We prove that if $$\alpha',\beta'$$ 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 $$(I-cU_\alpha)P_2^++(I-dU_\beta)P_2^-$$ is Fredholm on $$L^p(\mathbb{R}_+)$$ and its index is equal to zero. Moreover, its regularizers are described.

Karlovich, Alexei Yu., Yuri I. Karlovich, and Amarino B. Lebre. "One-sided invertibility criteria for binomial functional operators with shift and slowly oscillating data." Mediterranean Journal of Mathematics. 13.6 (2016): 4413-4435.Website
2015
Karlovich, Alexei Yu. "Banach algebra of the Fourier multipliers on weighted Banach function spaces." Concrete Operators. 2.1 (2015): 27-36. AbstractWebsite

Let $$\mathcal{M}_{X,w}(\mathbb{R})$$ denote the algebra of the Fourier multipliers on a separable weighted Banach function space $$X(\mathbb{R},w)$$. We prove that if the Cauchy singular integral operator $$S$$ is bounded on $$X(\mathbb{R},w)$$, then $$\mathcal{M}_{X,w}(\mathbb{R})$$ is continuously embedded into $$L^\infty(\mathbb{R})$$. An important consequence of the continuous embedding $$\mathcal{M}_{X,w}(\mathbb{R})\subset L^\infty(\mathbb{R})$$ is that $$\mathcal{M}_{X,w}(\mathbb{R})$$ is a Banach algebra.

Karlovich, Alexei Yu. "Commutators of convolution type operators on some Banach function spaces." Annals of Functional Analysis. 6.4 (2015): 191-205. AbstractWebsite

We study the boundedness of Fourier convolution operators $$W^0(b)$$ and the compactness of commutators of $$W^0(b)$$ with multiplication operators $$aI$$ on some Banach function spaces $$X(\mathbb{R})$$ for certain classes of piecewise quasicontinuous functions $$a\in PQC$$ and piecewise slowly oscillating Fourier multipliers $$b\in PSO_{X,1}^\diamond$$. We suppose that $$X(\mathbb{R})$$ is a separable rearrangement-invariant space with nontrivial Boyd indices or a reflexive variable Lebesgue space, in which the Hardy-Littlewood maximal operator is bounded. Our results complement those of Isaac De La Cruz-Rodríguez, Yuri Karlovich, and Iván Loreto Hernández obtained for Lebesgue spaces with Muckenhoupt weights.

Karlovich, Alexei Yu. "Fredholmness and index of simplest weighted singular integral operators with two slowly oscillating shifts." Banach Journal of Mathematical Analysis. 9.3 (2015): 24-42. AbstractWebsite

Let $$\alpha$$ and $$\beta$$ be orientation-preserving diffeomorphism (shifts) of $$\mathbb{R}_+=(0,\infty)$$ onto itself with the only fixed points $$0$$ and $$\infty$$, where the derivatives $$\alpha'$$ and $$\beta'$$ may have discontinuities of slowly oscillating type at $$0$$ and $$\infty$$. For $$p\in(1,\infty)$$, we consider the weighted shift operators $$U_\alpha$$ and $$U_\beta$$ given on the Lebesgue space $$L^p(\mathbb{R}_+)$$ by $$U_\alpha f=(\alpha')^{1/p}(f\circ\alpha)$$ and $$U_\beta f=(\beta')^{1/p}(f\circ\beta)$$. For $$i,j\in\mathbb{Z}$$ we study the simplest weighted singular integral operators with two shifts $$A_{ij}=U_\alpha^i P_\gamma^++U_\beta^j P_\gamma^-$$ on $$L^p(\mathbb{R}_+)$$, where $$P_\gamma^\pm=(I\pm S_\gamma)/2$$ are operators associated to the weighted Cauchy singular integral operator $(S_\gamma f)(t)=\frac{1}{\pi i}\int_{\mathbb{R}_+} \left(\frac{t}{\tau}\right)^\gamma\frac{f(\tau)}{\tau-t}d\tau$ with $$\gamma\in\mathbb{C}$$ satisfying $$0<1/p+\Re\gamma<1$$. We prove that the operator $$A_{ij}$$ is a Fredholm operator on $$L^p(\mathbb{R}_+)$$ and has zero index if $0<\frac{1}{p}+\Re\gamma+\frac{1}{2\pi}\inf_{t\in\mathbb{R}_+}(\omega_{ij}(t)\Im\gamma), \quad \frac{1}{p}+\Re\gamma+\frac{1}{2\pi}\sup_{t\in\mathbb{R}_+}(\omega_{ij}(t)\Im\gamma)<1,$ where $$\omega_{ij}(t)=\log[\alpha_i(\beta_{-j}(t))/t]$$ and $$\alpha_i$$, $$\beta_{-j}$$ are iterations of $$\alpha$$, $$\beta$$. This statement extends an earlier result obtained by the author, Yuri Karlovich, and Amarino Lebre for $$\gamma=0$$.

Karlovich, Alexei Yu. "Maximally modulated singular integral operators and their applications to pseudodifferential operators on Banach function spaces." Function Spaces in Analysis. Contemporary Mathematics, 645. Ed. Krzysztof Jarosz. Providence, Rhode Island: American Mathematical Society, 2015. 165-178. Abstract

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 $$T^{\Phi}$$ is of weak type $$(r,r)$$ for all $$r\in(1,\infty)$$, then $$T^{\Phi}$$ 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.