arXiv subjects
Publications and source records attributed to David Kalaj.
The paper deals with an extremal problem for bounded harmonic functions in the unit ball of $\mathbf{R}^4$. We solve the generalized Khavinson problem in $\mathbf{R}^4$. This precise problem was formulated by G. Kresin and V. Maz'ya for harmonic functions in the unit ball and in the half--space of $\mathbf{R}^n$. We find the optimal pointwise estimates for the norm of the gradient of bounded real--valued harmonic functions.
Given a bounded domain $D \subset {\mathbb R}^n$ strictly starlike with respect to $0 \in D\,,$ we define a quasi-inversion w.r.t. the boundary $\partial D \,.$ We show that the quasi-inversion is bi-Lipschitz w.r.t. the chordal metric if and only if every "tangent line" of $\partial D$ is far away from the origin. Moreover, the bi-Lipschitz constant tends to $1,$ when $\partial D$ approaches the unit sphere in a suitable way. For the formulation of our results we use the concept of the $α$-tangent condition due to F. W. Gehring and J. Väisälä (Acta Math. 1965). This condition is shown to be equivalent to the bi-Lipschitz and quasiconformal extension property of what we call the polar parametrization of $\partial D$. In addition, we show that the polar parametrization, which is a mapping of the unit sphere onto $\partial D\,,$ is bi-Lipschitz if and only if $D$ satisfies the $α$-tangent condition.
We prove the following generalization of Schwarz lemma for harmonic mappings. If $u$ is a harmonic mapping of the unit ball $B_n$ onto itself such that $u(0)=0$ and $\|u\|_p:=\left(\int_S|u(η)|^pdσ(η)\right)^{1/p}<\infty$, $p\ge 1$ then $|u(x)|\le g_p(|x|)\|u\|_p$ for some smooth sharp function $g_p$ vanishing in $0$. Moreover we provide sharp constant $C_p$ in the inequality $\|Du(0)\|\le C_p\|u\|_p$. Those two results extend some known result from harmonic mapping theory (\cite[Chapter~VI]{ABR}).
We first prove the following generalization of Schwarz lemma for harmonic mappings. Let $u$ be a harmonic mapping of the unit ball onto itself. Then we prove the inequality $\|u(x)-(1-\|x\|^2)/(1+\|x\|^2)^{n/2} u(0)\|\le U(|x| N)$. By using the Schwarz lemma for harmonic mappings we derive Heinz inequality on the boundary of the unit ball by providing a sharp constant $C_n$ in the inequality: $\|\partial_r u(rη)\|_{r=1}\ge C_n$, $\|η\|=1$, for every harmonic mapping of the unit ball into itself satisfying the condition $u(0)=0$, $\|u(η)\|=1$.
In this note we establish a Schwarz type inequality for holomorphic mappings between unit balls $B_n$ and $B_m$ in corresponding complex spaces.
In this paper we extend Rado-Choquet-Kneser theorem for the mappings with weak homeomorphic Lipschitz boundary data and Dini's smooth boundary but without restriction on the convexity of image domain, provided that the Jacobian satisfies a certain boundary condition. The proof is based on a recent extension of Rado-Choquet-Kneser theorem by Alessandrini and Nesi \cite{ale} and it is used the approximation principle.
We establish that every $K$-quasiconformal mapping of $w$ of the unit disk $\ID$ onto a $C^2$-Jordan domain $Ω$ is Lipschitz provided that $Δw\in L^p(\ID)$ for some $p>2$. We also prove that if in this situation $K\to 1$ with $\|Δw\|_{L^p(\ID)}\to 0$, and $Ω\to \ID$ in $C^{1,α}$-sense with $α>1/2,$ then the bound for the Lipschitz constant tends to $1$. In addition, we provide a quasiconformal analogue of the Smirnov absolute continuity result over the boundary.
We extend the result of Lavrentiev which asserts that the harmonic measure and the arc-length measure are $A_\infty$ equivalent in a chord-arc Jordan domain. By using this result we extend the classical result of Lindelöf to the class of quasiconformal (q.c.) harmonic mappings by proving the following assertion. Assume that $f$ is a quasiconformal harmonic mapping of the unit disk $\mathbf{U}$ onto a Jordan domain. Then the function $A(z)=\arg(\partial_φ(f(z))/z)$ where $z=re^{iφ}$, is well-defined and smooth in $\mathbf{U}^*=\{z: 0<|z|<1\}$ and has a continuous extension to the boundary of the unit disk if and only if the image domain has $C^1$ boundary.
Let $D$ and $Ω$ be Jordan domains with Dini's smooth boundaries and and let $f:D\mapsto Ω$ be a harmonic homeomorphism. The object of the paper is to prove the following result: If $f$ is quasiconformal, then $f$ is Lipschitz. This extends some recent results, where stronger assumptions on the boundary are imposed, and somehow is optimal, since it coincides with the best condition for Lipschitz behavior of conformal mappings in the plane and conformal parametrization of minimal surfaces.
We consider weighted Bergman projection $P_α: L^{\infty}(\Bbb B) \rightarrow {\cal B} $ where $α>-1$ and $\cal B$ is the Bloch space of the unit ball $\Bbb B$ of the complex space $\Bbb C^n.$ We obtain the exact norm of the operator $P_α$ where the Bloch space is observed as a space with norm (and semi-norm) induced from the Besov space $B_{p},0<p<\infty,(B_{\infty}=\cal B).$ Our work contains, as a special case, the main results from \cite{Kalaj} and \cite{Ant}.
The main result of this paper is related to finding two-sided bounds of norm for the adjoint operator $P^{\ast}$ of the Bergman projection $P,$ where $P$ denotes the Bergman projection wich maps $L^{1}(D,dλ(z))$ onto the Besov space $B_{1}.$ Here $dλ(z)$ is the Möbius invariant measure ${(1-|z|^2)^{-2}}{dA(z)}$. It is shown that $2\leq\| P^{\ast}\|\leq 4.$
In this paper we estimate norms of integral operator induced with Green function related to the Poisson equation in the unit ball with vanishing boundary data.
In this paper we discus the radial extension $w$ of a bi-Lipschitz parameterization $F(e^{it})=f(t)$ of a starlike Jordan curve $γ$ w.r. to 0. We show that, if parameterization is bi-Lipschitz, then the extension is bi-Lipschitz and consequently quasiconformal. If $γ$ is the unit circle, then $\mathrm{Lip}(f)=\mathrm{Lip}(F)=\mathrm{Lip}(w)=K_w$. If $γ$ is not a circle centered at origin, and $F$ is a polar parametrization of $γ$, then we show that $\mathrm{Lip}(f)=\mathrm{Lip}(F)<\mathrm{Lip}(w)$.
Let $n$ be a positive integer. Let $\mathbf U$ be the unit disk, $p\ge 1$ and let $h^p(\mathbf U)$ be the Hardy space of harmonic functions. Kresin and Maz'ya in a recent paper found the representation for the function $H_{n,p}(z)$ in the inequality $$|f^{(n)} (z)|\leq H_{n,p}(z)|\Re(f-\mathcal P_l)|_{h^p(\mathbf U)}, \Re f\in h^p(\mathbf U), z\in \mathbf U,$$ where $\mathcal P_l$ is a polynomial of degree $l\le n-1$. We find or represent the sharp constant $C_{p,n}$ in the inequality $H_{n,p}(z)\le \frac{C_{p,n}}{(1-|z|^2)^{1/p+n}}$. This extends a recent result of the second author and Marković, where it was considered the case $n=1$ only. As a corollary, an inequality for the modulus of the $n-{th}$ derivative of an analytic function defined in a complex domain with the bounded real part is obtained. This result improves some recent result of Kresin and Maz'ya.
We prove a version of Smirnov type theorem and Charatheodory type theorem for a harmonic homeomorphism of the unit disk onto a Jordan surface with rectifiable boundary. Further we establish the classical isoperimetric inequality and Riesz--Zygmund inequality for Jordan harmonic surfaces without any smoothness assumptions of the boundary.
Assume that $(\mathcal{N},\hbar)$ and $(\mathcal{M},\wp)$ are two Riemann surfaces with conformal metrics $\hbar$ and $\wp$. We prove that if there is a harmonic homeomorphism between an annulus $\mathcal{A}\subset \mathcal{N}$ with a conformal modulus $\mathrm{Mod}(\mathcal{A})$ and a geodesic annulus $A_\wp(p,ρ_1,ρ_2)\subset \mathcal{M}$, then we have ${ρ_2}/{ρ_1}\ge Ψ_\wp\mathrm{Mod}(\mathcal{A})^2+1,$ where $Ψ_\wp$ is a certain positive constant depending on the upper bound of Gaussian curvature of the metric $\wp$. An application for the minimal surfaces is given.
Let $N=(Ω,σ)$ and $M=(Ω^*,ρ)$ be doubly connected Riemann surfaces and assume that $ρ$ is a smooth metric with bounded Gauss curvature $\mathcal{K}$ and finite area. The paper establishes the existence of homeomorphisms between $Ω$ and $Ω^*$ that minimize the Dirichlet energy. In the class of all homeomorphisms $f \colon Ω\onto Ω^\ast$ between doubly connected domains such that $\Mod Ω\le \Mod Ω^\ast$ there exists, unique up to conformal authomorphisms of $Ω$, an energy-minimal diffeomorphism which is a harmonic diffeomorphism. The results improve and extend some recent results of Iwaniec, Koh, Kovalev and Onninen (Inven. Math. (2011)), where the authors considered doubly connected domains in the complex plane w.r. to Euclidean metric.
This paper deals with the the norm of the weighted Bergman projection operator $P_α:L^\infty\to \mathcal{B}$ where $α>-1$ and $\mathcal{B}$ is the Bloch space of the unit ball of the complex space $\mathbf{C}^n$. We consider two Bloch norms, the standard Bloch norm and invariant norm w.r.t. automorphisms of the unit ball. Our work contains as a special case the main result of the recent paper \cite{Perala}.