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El Hassan Youssfi

Publications and source records attributed to El Hassan Youssfi.

6 recordsLinked to original sources
arXiv2026

A Converse Mean-Value Property

We prove a converse mean-value theorem for harmonic functions associated with the invariant Laplace--Beltrami operator on the real unit ball. Under suitable integrability and topological assumptions on the domain and its Green potential, we show that the invariant volume mean-value property characterizes hyperbolic balls centered at the origin. As a consequence, we also answer a question left open by Bruna and Détraz in the setting of invariantly harmonic functions.

math.FA↗
arXiv2020

Sarason Toeplitz product problem for a class of Fock spaces

Sarason Toeplitz product problem asks when the operator TuTv is bounded on various Hilbert spaces of analytic functions, where u and v are analytic. The problem is highly nontrivial for Toeplitz operators on the Hardy space and the Bergman space (even in the case of the unit disk). In this paper, we provide a complete solution to the problem for a class of Fock spaces on the complex plane. In particular, this generalizes an earlier result of Cho, Park, and Zhu.

math.FA↗
arXiv2019

The polyanalytic reproducing kernels

Let $ν$ be a rotation invariant Borel probability measure on the complex plane having moments of all orders. Given a positive integer $q$, it is proved that the space of $ν$-square integrable $q$-analytic functions is the closure of $q$-analytic polynomials, and in particular it is a Hilbert space. We establish a general formula for the corresponding polyanalytic reproducing kernel. New examples are given and all known examples, including those of the analytic case are covered. In particular, weighted Bergman and Fock type spaces of polyanalytic functions are introduced. Our results have a higher dimensional generalization for measure on ${\mathbb C}^p$ which are in rotation invariant with respect to each coordinate.

math.CV↗
arXiv2018

A covariance equation

Let $§$ be a commutative semigroup with identity $e$ and let $Γ$ be a compact subset in the pointwise convergence topology of the space $§'$ of all non-zero multiplicative functions on $§.$ Given a continuous function $F: Γ\to \mathbb C$ and a complex regular Borel measure $μ$ on $Γ$ such that $μ(Γ) \not = 0.$ It is shown that $$ μ(Γ) \int_Γ \varrho(s) \overline{\varrho(t)} |F|^2(\varrho) dμ(\varrho) = \int_Γ \varrho(s) F(\varrho) dμ(\varrho) \int_Γ \overline{\varrho(t) F(\varrho)} dμ(\varrho) $$ for all $(s, t) \in §\times §$ if and only if for some $γ\in Γ, $ the support of $μ$ is contained is contained in $\{ F = 0 \} \cup \{γ\}$. Several applications of this characterization are derived. In particular, the reduction of our theorem to the semigroup of non-negative integers $(\mathbb N_{0}, +)$ solves a problem posed by El Fallah, Klaja, Kellay, Mashregui and Ransford in a more general context. More consequences of our results are given, some of them illustrate the probabilistic flavor behind the problem studied herein and others establish extremal properties of analytic kernels.

math.CV↗
arXiv2011

Hankel operators on Fock spaces and related Bergman kernel estimates

Hankel operators with anti-holomorphic symbols are studied for a large class of weighted Fock spaces on $\cn$. The weights defining these Hilbert spaces are radial and subject to a mild smoothness condition. In addition, it is assumed that the weights decay at least as fast as the classical Gaussian weight. The main result of the paper says that a Hankel operator on such a Fock space is bounded if and only if the symbol belongs to a certain BMOA space, defined via the Berezin transform. The latter space coincides with a corresponding Bloch space which is defined by means of the Bergman metric. This characterization of boundedness relies on certain precise estimates for the Bergman kernel and the Bergman metric. Characterizations of compact Hankel operators and Schatten class Hankel operators are also given. In the latter case, results on Carleson measures and Toeplitz operators along with Hörmander's $L^2$ estimates for the $\bar{\partial}$ operator are key ingredients in the proof.

math.FA↗