arXiv subjects
Publications and source records attributed to Omar El-Fallah.
We study the zeros of functions in the Dirichlet space. Using extremal functions, we produce a necessary and sufficient condition for a sequence of points in the unit disk to be a zero set of the classical Dirichlet space. This Shapiro-Shields type condition involves kernels of the harmonic Dirichlet space associated with measures depending on the zero sequence.
We study the local Dirichlet integral of distance functions and their behavior within the harmonic Dirichlet space. We provide estimates for the local Dirichlet integral of distance functions, which allow us to study their membership in the algebra of multipliers of the Dirichlet space. We give sufficient condition for a closed subset of the unit circle to be polar and we also examine cyclicity in the harmonic Dirichlet spaces.
Let $μ$ be a positive finite Borel measure on the unit circle. The associated Dirichlet space $\mathcal{D}(μ)$ consists of holomorphic functions on the unit disc whose derivatives are square integrable when weighted against the Poisson integral of $μ$. We give a sufficient condition on a Borel subset $E$ of the unit circle which ensures that $E$ is a uniqueness set for $\mathcal{D}(μ)$. {We also give somes examples of positive Borel measures $μ$ and uniqueness sets for $\mathcal{D}(μ)$.}
We give a simple proof of the fact that a finite measure $μ$ on the unit disk is a Carleson measure for the Dirichlet space if it satisfies the Carleson one-box condition $μ(S(I))=O(ϕ(|I|))$, where $ϕ:(0,2π]\to(0,\infty)$ is an increasing function such that $\int_0^{2π}(ϕ(x)/x)\,dx<\infty$. We further show that the integral condition on $ϕ$ is sharp.
Let $μ$ be a positive finite measure on the unit circle and $\mathcal{D} (μ)$ the associated Dirichlet space. The generalized Brown-Shields conjecture asserts that an outer function $f \in \mathcal{D} (μ)$ is cyclic if and only if $c\_μ(Z (f))= 0$, where $c\_μ$ is the capacity associated with $\mathcal{D} (μ)$ and $Z(f)$ is the zero set of $f$. In this paper we prove that this conjecture is true for measures with countable support. We also give in this case a complete and explicit characterization of invariant subspaces.
The harmonic Dirichlet space $\cal{D} (\mathbb{T})$ is the Hilbert space of functions $f \in L^2(\mathbb{T})$ such that $$\|f\|_{\cal{D} (\mathbb{T})}^2 := \sum_{n\in\mathbb{Z}} (1+|n|)|\hat{f}(n)|^2 < \infty.$$ We give sufficient conditions for $f$ to be cyclic in $\cal{D} (\mathbb{T})$, in other words, for $\{ζ^nf(ζ):\ n\geq 0\}$ to span a dense subspace of $\cal{D} (\mathbb{T})$.
We consider Dirichlet spaces with superharmonic weights. This class contains both the harmonic weights and the power weights. Our main result is a characterization of the Dirichlet spaces with superharmonic weights that can be identified as de Branges-Rovnyak spaces. As an application, we obtain the dilation inequality \[ {\cal D}_ω(f_r)\le \frac{2r}{1+r}{\cal D}_ω(f) \qquad(0\le r<1), \] where ${\cal D}_ω$ denotes the Dirichlet integral with superharmonic weight $ω$, and $f_r(z):=f(rz)$ is the $r$-dilation of the holomorphic function $f$.
We use the so called resolvent transform method to study the cyclicity of the one point mass singular inner function in weighted Bergman type spaces.
Carleson's corona theorem is used to obtain two results on cyclicity of singular inner functions in weighted Bergman-type spaces on the unit disk. Our method proof requires no regularity conditions on the weights.
We treat the problem of characterizing the cyclic vectors in the weighted Dirichlet spaces, extending some of our earlier results in the classical Dirichlet space. The absence of a Carleson-type formula for weighted Dirichlet integrals necessitates the introduction of new techniques.
Let $\cD$ be the Dirichlet space, namely the space of holomorphic functions on the unit disk whose derivative is square-integrable. We establish a new sufficient condition for a function $f\in\cD$ to be {\em cyclic}, i.e. for $\{pf: p\text{a polynomial}\}$ to be dense in $\cD$. This allows us to prove a special case of the conjecture of Brown and Shields that a function is cyclic in $\cD$ iff it is outer and its zero set (defined appropriately) is of capacity zero.