arXiv subjects
Publications and source records attributed to David Kalaj.
Let $\mathbf H^3$ be the hyperbolic space identified with the unit ball $\mathbf{B}^3 = \{x\in \mathbf{R}^3: |x| < 1\}$ with the Poincaré metric $d_h$ and assume that ${\mathcal{A}}(x_0,p,q):=\{x: p 1+ψ(a,b)$, where $ψ$ is a positive function.
In this paper we extend Rado-Choquet-Kneser theorem for the mappings with Lipschitz boundary data and essentially positive Jacobian at the boundary, without restriction on the convexity of image domain. It is an extension of a recent result of Alessandrini and Nesi \cite{ale}. Some applications for the family of quasiconformal harmonic mappings between Jordan domains are given.
We prove the following theorem: if $w$ is a quasiconformal mapping of the unit disk onto itself satisfying elliptic partial differential inequality $|L[w]|\le \mathcal{B}|\nabla w|^2+Γ$, then $w$ is Lipschitz continuous. This {result} extends some recent results, where instead of an elliptic differential operator is {only} considered {the} Laplace operator.
It is proved the following theorem, if $w$ is a quasiconformal harmonic mappings between two Riemann surfaces with smooth boundary and aproximate analytic metric, then $w$ is a quasi-isometry with respect to Euclidean metric.
Let ${\mathcal H}$ denote the class of all normalized complex-valued harmonic functions $f=h+\bar{g}$ in the unit disk ${\mathbb D}$, and let $K=H+\bar{G}$ denote the harmonic Koebe function. Let $a_n,b_n, A_n, B_n$ denote the Maclaurin coefficients of $h,g,H,G$, and $${\mathcal F}=\{f=h+\bar{g}\in {\mathcal H}:\,|a_n|\leq A_n and |b_n|\leq B_n for n\geq 1}. $$ We show that the radius of univalence of the family ${\mathcal F}$ is $0.112903...$. We also show that this number is also the radius of the starlikeness of ${\mathcal F}$. Analogous results are proved for a subclass of the class of harmonic convex functions in ${\mathcal H}$. These results are obtained as a consequence of a new coefficient inequality for certain class of harmonic close-to-convex functions. Surprisingly, the new coefficient condition helps to improve Bloch-Landau constant for bounded harmonic mappings.
Concrete sharp constants in a pointwise estimate of the gradient of a harmonic function in the unit disk are obtained under the assumption that function belong to Hardy space $h^p$, $p\ge 1$. This generalizes some recent result of Maz'ya & Kresin and a result of Colonna related to the Bloch constant of harmonic mappings of the unit disk into itself.
We study vector functions of ${\mathbb R}^n$ into itself, which are of the form $x \mapsto g(|x|)x\,,$ where $g : (0,\infty) \to (0,\infty) $ is a continuous function and call these radial functions. In the case when $g(t) = t^c$ for some $c \in {\mathbb R}\,,$ we find upper bounds for the distance of image points under such a radial function. Some of our results refine recent results of L. Maligranda and S. Dragomir. In particular, we study quasiconformal mappings of this simple type and obtain norm inequalities for such mappings.
The conformal deformations are contained in two classes of mappings: quasiconformal and harmonic mappings. In this paper we consider the intersection of these classes. We show that, every $K$ quasiconformal harmonic mapping between $\,\mathscr C^{1,μ}$ ($0<α\le 1$) surfaces with $\,\mathscr C^{1,μ}$ boundary is a Lipschitz mapping. This extends some recent results of several authors where the same problem has been considered for plane domains. As an application it is given an explicit Lipschitz constant of normalized isothermal coordinates of a disk-type minimal surface in terms of boundary curve only. It seems that this kind of estimates are new for conformal mappings of the unit disk onto a Jordan domain as well.
We prove that the distortion function of the Gauss map of a harmonic surface coincides with the distortion function of the surface. Consequently, Gauss map of a harmonic surface is ${\mathcal{K}}$ quasiregular if and only if the surface is ${\mathcal{K}}$ quasiregular, provided that the Gauss map is regular or what is shown to be the same, provided that the surface is non-planar.
In this note we determine all numbers $q\in \mathbf R$ such that $|u|^q$ is a subharmonic function, provided that $u$ is a $K-$quasiregular harmonic mappings in an open subset $Ω$ of the Euclidean space $\mathbf R^n$.
We find the sharp constants $C_p$ and the sharp functions $C_p=C_p(x)$ in the inequality $$|u(x)|\leq \frac{C_p}{(1-|x|^2)^{(n-1)/p}}\|u\|_{h^p(B^n)}, u\in h^p(B^n), x\in B^n,$$ in terms of Gauss hypergeometric and Euler functions. This extends and improves some results of Axler, Bourdon and Ramey (\cite{ABR}), where they obtained similar results which are sharp only in the cases $p=2$ and $p=1$.
In this note it is formulated the J. C. C. Nitsche type conjecture for bi-harmonic mappings. The conjecture has been motivated by the radial bi-harmonic mappings between annuli.
We study the Schwarz lemma for harmonic functions and prove sharp versions for the cases of real harmonic functions and the norm of harmonic mappings.
We prove the following theorem: every quasiconformal harmonic mapping between two plane domains with $C^{1,α}$ ($α<1$), respectively $C^{1,1}$ compact boundary is bi-Lipschitz. The distance function with respect to the boundary of the image domain is used. This in turn extends a similar result of the author in \cite{kalajan} for Jordan domains, where stronger boundary conditions for the image domain were needed.
We mainly investigate some properties of quasiconformal mappings between smooth 2-dimensional surfaces with boundary in the Euclidean space, satisfying certain partial differential equations (inequalities) concerning Laplacian, and in particular satisfying Laplace equation and show that that these mappings are Lipschitz. Conformal parametrization of such surfaces and the method developed in our paper \cite{km} have important role in this paper.dan curves and is extended to the case of $C^{2,α}$ surfaces with smooth and compact boundary.
Let $A$ and $A'$ be two circular annuli and let $ρ$ be a radial metric defined in the annulus $A'$. Consider the class $\mathcal H_ρ$ of $ρ-$harmonic mappings between $A$ and $A'$. It is proved recently by Iwaniec, Kovalev and Onninen that, if $ρ=1$ (i.e. if $ρ$ is Euclidean metric) then $\mathcal H_ρ$ is not empty if and only if there holds the Nitsche condition (and thus is proved the J. C. C. Nitsche conjecture). In this paper we formulate an condition (which we call $ρ-$Nitsche conjecture) with corresponds to $\mathcal H_ρ$ and define $ρ-$Nitsche harmonic maps. We determine the extremal mappings with smallest mean distortion for mappings of annuli w.r. to the metric $ρ$. As a corollary, we find that $ρ-$Nitsche harmonic maps are Dirichlet minimizers among all homeomorphisms $h:A\to A'$. However, outside the $ρ$-Nitsche condition of the modulus of the annuli, within the class of homeomorphisms, no such energy minimizers exist. % However, %outside the $ρ-$Nitsche range of the modulus of the annuli, %within the class of homeomorphisms, no such energy minimizers exist. This extends some recent results of Astala, Iwaniec and Martin (ARMA, 2010) where it is considered the case $ρ=1$ and $ρ=1/|z|$.
Let $u\in W^{2,p}_0$, $1\le p\le \infty$ be a solution of the Poisson equation $Δu = h$, $h\in L^p$, in the unit disk. It is proved that $\|\nabla u\|_{L^p} \le a_p\|h\|_{L^p}$ with sharp constant $a_p$ for $p=1$ and $p=\infty$ and that $\|\partial u\|_{L^p} \le b_p\|h\|_{L^p}$ with sharp constant $b_p$ for $p=1$, $p=2$ and $p=\infty$. In addition is proved that for $p>2$ $||\partial u||_{L^\infty}\le c_p\Vert h\Vert_{L^p} $, and $||\nabla u||_{L^\infty}\le C_p\Vert h\Vert_{L^p}, $ with sharp constants $c_p$ and $C_p$. An extension to smooth Jordan domains is given. These problems are equivalent to determining the precise value of $L^p$ norm of {\it Cauchy transform of Dirichlet's problem}.
Let $ρ_Σ=h(|z|^2)$ be a metric in a Riemann surface $Σ$, where $h$ is a positive real function. Let $\mathcal H_{r_1}=\{w=f(z)\}$ be the family of univalent $ρ_Σ$ harmonic mapping of the Euclidean annulus $A(r_1,1):=\{z:r_1< |z| <1\}$ onto a proper annulus $A_Σ$ of the Riemann surface $Σ$, which is subject of some geometric restrictions. It is shown that if $A_Σ$ is fixed, then $\sup\{r_1: \mathcal H_{r_1}\neq \emptyset \}<1$. This generalizes the similar results from Euclidean case. The cases of Riemann and of hyperbolic harmonic mappings are treated in detail. Using the fact that the Gauss map of a surface with constant mean curvature (CMC) is a Riemann harmonic mapping, an application to the CMC surfaces is given (see Corollary \ref{cor}). In addition some new examples of hyperbolic and Riemann radial harmonic diffeomorphisms are given, which have inspired some new J. C. C. Nitsche type conjectures for the class of these mappings.