arXiv subjects

Dimitrios Betsakos

Publications and source records attributed to Dimitrios Betsakos.

11 recordsLinked to original sources

arXiv2026

On the Bergman number of hyperbolic domains

We construct a domain $D$ in the plane whose Hardy and Bergman numbers satisfy $0<h(D)<b(D)<+\infty$. We also calculate the Bergman number for certain classes of domains having countable complement in the plane. Finally, we investigate some of the implications of our analysis in the theory of iteration of holomorphic self-maps of the unit disk. Our methods rely on estimates for the hyperbolic metric and a recent related result that connects the Bergman number with the hyperbolic metric.

math.CV↗

arXiv2025

On the Hardy number and the Bergman number of a planar domain

This article deals with functions with a prefixed range and their inclusions in Hardy and weighted Bergman spaces. This idea was originally introduced by Hansen for Hardy spaces, and it was recently taken into weighted Bergman spaces by Karafyllia and Karamanlis. We provide a new characterization for the Hardy number of a domain in terms of its Green function. Based on this, we present a class of domains for which the Hardy number and the Bergman number coincide. However, in general, we show that the Hardy number and the Bergman number of a domain are not equal; even for domains which are regular for the Dirichlet problem.

math.CV↗

arXiv2025

On the rates of convergence of orbits in semigroups of holomorphic functions

Let $(\phi_t)$ be a continuous semigroup of holomorphic self-maps of the unit disk $\mathbb{D}$ with Denjoy-Wolff point $\tau\in\overline{\mathbb{D}}$. We study the rate of convergence of the forward orbits of $(\phi_t)$ to the Denjoy-Wolff point by finding explicit bounds for the quantity $|\phi_t(z)-\tau|$, $z\in\overline{\mathbb{D}}$, $t > 0$. We further discuss the corresponding rate of convergence for the backward orbits of $(\phi_t)$.

math.CV↗

arXiv2025

Symmetrization and the rate of convergence of semigroups of holomorphic functions

Let $(\phi_t)$, $t\ge 0$, be a semigroup of holomorphic self-maps of the unit disk $\mathbb{D}$. Let $\Omega$ be its Koenigs domain and $\tau\in \partial \mathbb{D}$ be its Denjoy-Wolff point. Suppose that $0\in \Omega$ and let $\Omega^\sharp$ be the Steiner symmetrization of $\Omega$ with respect to the real axis. Consider the semigroup $(\phi_t^\sharp)$ with Koenigs domain $\Omega^\sharp$ and let $\tau^\sharp$ be its Denjoy-Wolff point. We show that, up to a multiplicative constant, the rate of convergence of $(\phi_t^\sharp)$ is slower than that of $(\phi_t)$; that is, for every $t>0$, $|\phi_t(0)-\tau|\leq 4\pi\, |\phi_t^\sharp(0)-\tau^\sharp|$. The main tool for the proof is the harmonic measure.

math.CV↗

arXiv2025

Semigroups of holomorphic functions; rectifiability and Lipschitz properties of the orbits

Let $(\phi_t)$ be a semigroup of holomorphic functions in the unit disk. We prove that all its orbits are rectifiable and that its forward orbits are Lipschitz curves. Moreover, we find a necessary and sufficient condition in terms of hyperbolic geometry so that a backward orbit is a Lipschitz curve. We further explore the Lipschitz condition for forward orbits lying on the unit circle and then for semigroups of holomorphic functions in general simply connected domains.

math.CV↗

arXiv2024

The Hardy number and the Bergman number of a planar domain are equal

This article deals with functions with a prefixed range and their inclusion in Hardy and weighted Bergman spaces. This idea was originally introduced by Hansen for Hardy spaces, and it was recently taken into weighted Bergman spaces by Karafyllia and Karamanlis. In particular, we improve a theorem of Karafyllia showing that the Hardy and Bergman numbers of any given domain coincide, that is, the Hardy and weighted Bergman spaces to which a function with prefixed range belongs can be related. The main tools in the proofs are the Green function of the domain and its universal covering map.

math.CV↗

arXiv2022

Temperature on rods with Robin boundary conditions

We consider solutions $u_f$ to the one-dimensional Robin problem with the heat source $f\in L^1[-\pi,\pi]$ and Robin parameter $\alpha>0$. For given $m$, $M$, and $s$, $0\le m<s<M$, we identify the heat sources $f_0$, such that $u_{f_0}$ maximizes the temperature gap $\max_{[-\pi,\pi]}u_f -\min_{[-\pi,\pi]}u_f$ over all heat sources $f$ such that $m\le f\le M$ and $\|f\|_{L^1}=2\pi s$. In particular, this answers a question raised by J.~J.~Langford and P.~McDonald in \cite{LM}. We also identify heat sources, which maximize/minimize $u_f$ at a given point $x_0\in [-\pi,\pi]$ over the same class of heat sources as above and discuss a few related questions.

math.CA↗

arXiv2022

On the monotonicity of the speeds for semigroups of holomorphic self-maps of the unit disk

We study semigroups $(\phi_t)_{t\geq 0}$ of holomorphic self-maps of the unit disk with Denjoy-Wolff point on the boundary. We show that the orthogonal speed of such semigroups is a strictly increasing function. This answers a question raised by F. Bracci, D. Cordella, and M. Kourou, and implies a domain monotonicity property for orthogonal speeds conjectured by Bracci. We give an example of a semigroup such that its total speed is not eventually increasing. We also provide another example of a semigroup having total speed of a certain asymptotic behavior, thus answering another question of Bracci.

math.CV↗

arXiv2022

Conformal capacity of hedgehogs

In this paper we discuss problems concerning the conformal condenser capacity of "hedgehogs", which are compact sets $E$ in the unit disk $\mathbb{D}=\{z:\,|z|<1\}$ consisting of a central body $E_0$ that is typically a smaller disk $\overline{\mathbb{D}}_r=\{z:\,|z|\le r\}$, $0<r<1$, and several spikes $E_k$ that are compact sets lying on radial intervals $I(\alpha_k)=\{te^{i\alpha_k}:\,0\le t<1\}$. The main questions we are concerned with are the following: (1) How does the conformal capacity ${\rm cap}(E)$ of $E=\cup_{k=0}^n E_k$ behave when the spikes $E_k$, $k=1,\ldots,n$, move along the intervals $I(\alpha_k)$ toward the central body if their hyperbolic lengths are preserved during the motion? (2) How does the capacity ${\rm cap}(E)$ depend on the distribution of angles between the spikes $E_k$? We prove several results related to these questions and discuss methods of applying symmetrization type transformations to study the capacity of hedgehogs. Several open problems, including problems on the capacity of hedgehogs in the three-dimensional hyperbolic space, also will be suggested.

math.CV↗

arXiv2021

On the duration of stays of Brownian motion in domains in Euclidean space

Let $T_D$ denote the first exit time of a Brownian motion from a domain $D$ in ${\mathbb R}^n$. Given domains $U,W \subseteq {\mathbb R}^n$ containing the origin, we investigate the cases in which we are more likely to have fast exits from $U$ than $W$, meaning ${\bf P}(T_U {\bf P}(T_W t) > {\bf P}(T_W>t)$ for $t$ large. This result, which applies only in two dimensions, shows that the unit disk has the lowest probability of long stays amongst all Schlicht domains.

math.PR↗

arXiv2020

On the probability of fast exits and long stays of planar Brownian motion in simply connected domains

Let $T^D$ denote the first exit time of a planar Brownian motion from a domain $D$. Given two simply connected planar domains $U,W \neq \SC$ containing $0$, we investigate the cases in which we are more likely to have fast exits (meaning for instance ${\bf P}(T^U {\bf P}(T^W t) > {\bf P}(T^W>t)$ for $t$ large). We prove several results on these questions. In particular, we show that the primary factor in the probability of fast exits is the proximity of the boundary to the origin, while for long stays an important factor is the moments of the exit time. The complex analytic theory that motivated our inquiry is also discussed.

math.PR↗