Moment problems in weighted $L^2$ spaces on the real line
Elias Zikkos Khalifa University
Аннотация:
For a class of sets with multiple terms
$$
\{\lambda_n,\mu_n\}_{n=1}^{\infty}:=\{\underbrace{\lambda_1,\lambda_1,\dots,\lambda_1}_{\mu_1 - times},
\underbrace{\lambda_2,\lambda_2,\dots,\lambda_2}_{\mu_2 - times},\dots,
\underbrace{\lambda_k,\lambda_k,\dots,\lambda_k}_{\mu_k - times},\dots\},
$$
having density
$d$ counting multiplicities,
and a doubly-indexed sequence of non-zero complex numbers\linebreak
$\{d_{n,k}:\, n\in\mathbb{N},\, k=0,1,\dots ,\mu_n-1\} $
satisfying certain growth conditions,
we consider a moment problem of the form
$$
\int_{-\infty}^{\infty}e^{-2w(t)}t^k e^{\lambda_n t}f(t)\, dt=d_{n,k},\quad \forall\,\, n\in\mathbb{N}\quad \text{and}\quad k=0,1,2,\dots, \mu_n-1,
$$
in weighted
$L^2 (-\infty, \infty)$ spaces.
We obtain a solution
$f$ which extends analytically as an entire function, admitting a Taylor-Dirichlet series representation
$$
f(z)=\sum_{n=1}^{\infty}\Big(\sum_{k=0}^{\mu_n-1}c_{n,k}
z^k\Big) e^{\lambda_n z},\quad c_{n,k}\in \mathbb{C},\quad\forall\,\, z\in \mathbb{C}.
$$
The proof depends on our previous work where we characterized the closed span of the exponential system
$\{t^k e^{\lambda_n t}:\, n\in\mathbb{N},\,\, k=0,1,2,\dots,\mu_n-1\}$
in weighted
$L^2 (-\infty, \infty)$ spaces,
and also derived a sharp upper bound for the norm of elements of a biorthogonal sequence to the exponential system.
The proof also utilizes notions from Non-Harmonic Fourier series such as Bessel and Riesz–Fischer sequences.
Ключевые слова:
Moment problems, Exponential systems, Biorthogonal families, Weighted Banach spaces, Bessel and Riesz–Fischer sequences.
Язык публикации: английский
DOI:
10.15826/umj.2020.1.014