Scientific articles
Hermite functions and inner product in Sobolev space
M. A. Boudref University of Bouira
Abstract:
Let us consider the orthogonal Hermite system
$\left\{ \varphi_{2n}(x)\right\} _{n\geq 0}$ of even index defined on
$\left( -\infty,\infty \right),$ where
\begin{equation*}
\varphi _{2n}(x)=\frac{e^{-\frac{x^{2}}{2}}}{\sqrt{\left( 2n\right) !}\pi ^{\frac{1}{4}}2^{n}}H_{2n}(x),
\end{equation*}
by
$H_{2n}(x)$ we denote a Hermite polynomial of degree
$2n.$ In this paper, we consider a generalized system
$\left\{ \psi_{r,2n}(x)\right\} $ with
$r>0,$ $n\geq 0$ which is orthogonal with respect to the Sobolev type inner product on
$\left(-\infty ,\infty \right),$ i.e.
\begin{equation*}
\langle f,g \rangle =\lim_{t\rightarrow -\infty }\sum_{k=0}^{r-1}f^{\left(k\right) }(t)g^{\left( k\right) }(t)+\int_{-\infty }^{\infty }f^{\left(r\right) }(x)g^{\left( r\right) }(x)\rho (x)dx
\end{equation*}
with
$\rho (x)=e^{-x^{2}},$ and generated by
$\left\{\varphi_{2n}(x)\right\}_{n\geq 0}.$
The main goal of this work is to study some properties related to the system
$\left\{ \psi_{r,2n}(x)\right\}_{n\geq 0},$
\begin{gather*}
\psi _{r,n}(x)=\frac{(x-a)^{n}}{n!},\quad n=0,1,2,\ldots,r-1,
\\[2pt]
\psi _{r,r+n}(x)=\frac{1}{(r-1)!}\int_{a}^{b}(x-t)^{r-1}\varphi _{n}(t)dt,
\quad n=0,1,2,\ldots\, .
\end{gather*}
We study the conditions on a function
$f(x),$ given in a generalized Hermite orthogonal system, for it to be expandable into a generalized mixed Fourier series as well as the convergence of this Fourier series.
The second result of the paper is the proof of a recurrent formula for the system
$\left\{ \psi _{r,2n}(x)\right\} _{n\geq 0}.$ We also discuss the asymptotic properties of these functions, and this concludes our contribution.
Keywords:
inner product, Sobolev space, Hermite polynomials.
UDC:
517.518.36
MSC: 42C10 Received: 08.02.2023
Accepted: 09.06.2023
Language: English
DOI:
10.20310/2686-9667-2023-28-142-155-168