Vestnik Volgogradskogo gosudarstvennogo universiteta. Seriya 1. Mathematica. Physica
RUS  ENG    JOURNALS   PEOPLE   ORGANISATIONS   CONFERENCES   SEMINARS   VIDEO LIBRARY   PACKAGE AMSBIB  
General information
Latest issue
Archive

Search papers
Search references

RSS
Latest issue
Current issues
Archive issues
What is RSS



Mathematical Physics and Computer Simulation:
Year:
Volume:
Issue:
Page:
Find






Personal entry:
Login:
Password:
Save password
Enter
Forgotten password?
Register


Vestnik Volgogradskogo gosudarstvennogo universiteta. Seriya 1. Mathematica. Physica, 2017, Issue 1(38), Pages 22–32
DOI: https://doi.org/10.15688/jvolsu1.2017.1.3
(Mi vvgum160)
 

Mathematics

Harmonic analysis of periodic sequences at infinity

A. A. Ryzhkova

Voronezh State University
References:
Abstract: Let $ X $ be a complex Banach space and $ \mathrm{End} X $ be a Banach algebra. By $ l ^ {\infty} = l ^ {\infty} (Z, X) $ we denote the Banach space of two-sided sequences of vectors in X with the norm
$ \|x\|_ {\infty} =\sup \limits_ {n\in \mathbb{Z}} \|x(n)\| $, $ X: \mathbb{Z} \rightarrow X $, $ x \in l ^ {\infty} $.

By $ c_0 $ we denote the (closed) subspace of sequences of $ l ^ {\infty} $, decreasing at infinity, i.e. $ \lim \limits_ {n \rightarrow \infty} \|x (n)\| = 0 $.
In the space $ l ^ {\infty} $, let us consider the group of operators $ S (n): l ^ {\infty } \rightarrow l ^ {\infty} $, $ n \in \mathbb{Z}$ where $ (S (n) x) (k) = x (k + n) $, $ k \in \mathbb{Z}, x \in l ^ {\infty}$.
The sequence $ x \in\ l^{\infty} $ is called slowly varying at infinity if $S(1) x - x \in c_0$, i.e.

$$ \lim_{N \rightarrow \infty}\|x (n+1)-x (n)\|= 0. $$

The sequence ${x} $ of $ {l} {} ^ {\infty} $ is called periodic at infinity period $ N \geq 1 $, $ N \in \mathbb{N} $, if $ {S} (N) {x} - {x} \in {c}_0$.
An example of a sequence slowly varying at infinity is sequence $ x (n) = \sin (\ln (\alpha + n)) $, $ n \in \mathbb{Z} $, where $\alpha > 0 $.
The set of slowly varying at infinity sequences form a closed subspace of $ l ^ {\infty} $ which is denoted by $ l_ {sl, \infty} ^ {\infty} $.
The set of periodic at infinity period $N$ form a closed subspace of $ l ^ {\infty} $, which is denoted by $ l_ {N, \infty} ^ {\infty} $. Note that $ c_0 \subset {l_ {sl, \infty} ^ {\infty} }\subset l_{N, \infty}^{\infty }$ for any $ N \geq 1 $.
Suppose that $ \gamma_k = e^{\frac {i2 \pi k}{N}} $, $ 0 \leq k \leq N-1 $,—the roots of unity. Note that they form a group, denoted further by $ G_N $.
One of the main results is
Theorem 1. Each periodic at infinity sequence $ x \in l ^{ \infty} $ period $ N \geq 1 $ representation of the form

\begin{equation*} x(n)=\sum\limits_{k=0}^{N-1} x_k(n)\gamma_k ^n, \end{equation*}
where $ x_k \in l_ {sl, \infty} ^{ \infty}, 0 \leq k \leq N-1 $.
In a Banach space $l ^ {\infty} (\mathbb {Z}, X) $, where $ X$—finite-dimensional Banach space, consider the difference equation
\begin{equation} X (n + N) = Bx (n) + y (n),\ n \in \mathbb {Z}, \tag{1} \end{equation}
where $ y \in c_0 (\mathbb {Z}, X), B\in \mathrm{End} X $ with the property $ \Sigma_0 = \sigma (B) \cap \mathbb {T} =$ $\{\gamma_1, \gamma_2 ..., \gamma_m\}$—set of simple eigenvalues, where $\mathbb{T} = \{\lambda \in \mathbb {C}: |\lambda| = 1\} $ and $ \sigma (B) $ denotes the spectrum of the operator $B$.
Theorem 2. Each bounded solution $ x: \mathbb {Z} \rightarrow X $ of the equation (1) is a periodic sequence at infinity, which is a representation of the form
$$ X (n) = \sum \limits_ {k = 1} ^ {N} x_k (n) \gamma ^ n_k, $$
where $ x_k \in l ^ {\infty} _ {sl, \infty} $, $ \gamma_k \in \mathbb {T} $, $ 0 \leq k \leq $ N-1.
Keywords: periodic sequences at infinity, difference equations, eigenvalues, spectral decomposition, projectors.
Document Type: Article
UDC: 517.9
BBC: 22.161
Language: Russian
Citation: A. A. Ryzhkova, “Harmonic analysis of periodic sequences at infinity”, Vestnik Volgogradskogo gosudarstvennogo universiteta. Seriya 1. Mathematica. Physica, 2017, no. 1(38), 22–32
Citation in format AMSBIB
\Bibitem{Ryz17}
\by A.~A.~Ryzhkova
\paper Harmonic analysis of periodic sequences at infinity
\jour Vestnik Volgogradskogo gosudarstvennogo universiteta. Seriya 1. Mathematica. Physica
\yr 2017
\issue 1(38)
\pages 22--32
\mathnet{http://mi.mathnet.ru/vvgum160}
\crossref{https://doi.org/10.15688/jvolsu1.2017.1.3}
Linking options:
  • https://www.mathnet.ru/eng/vvgum160
  • https://www.mathnet.ru/eng/vvgum/y2017/i1/p22
  • Citing articles in Google Scholar: Russian citations, English citations
    Related articles in Google Scholar: Russian articles, English articles
    Mathematical Physics and Computer Simulation
     
      Contact us:
     Terms of Use  Registration to the website  Logotypes © Steklov Mathematical Institute RAS, 2024