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On bases from cosines in Lebesgue spaces with variable summability index
 Togrul Muradov^{1}Email author and
 Chingiz Hashimov^{1, 2}
https://doi.org/10.1186/s1366001509516
© Muradov and Hashimov 2016
 Received: 9 July 2015
 Accepted: 21 December 2015
 Published: 4 January 2016
Abstract
In this paper the perturbed system of cosines is considered. Under certain conditions on the summability index \(p (\cdot )\) and perturbation, the basicity of this system in Lebesgue spaces \(L_{p (\cdot )} (0,\pi )\) with variable summability index \(p (\cdot )\) is proved. The obtained results generalize similar results for the case \(p (\cdot )=p=\mbox{const}\).
Keywords
 basicity
 perturbation
 system of cosines
 variable exponent
1 Introduction
Perturbed systems of exponents, cosines and sines play an important role in the theory of spectral theory of differential operators, in the theory of optimal control, in approximation theory and so on. Therefore there are a lot of papers studying the frame properties and also the basis properties (completeness, minimality, basicity, etc.) of the perturbed trigonometric systems in various Banach spaces of functions. For more detailed information see [1–12].
In connection with applications in mechanics and theoretical physics in recent years there is a great interest in studying different problems in Lebesgue spaces with variable summability index. For many results in this direction one can see [13] and also [14, 15]. Basis properties of some trigonometric systems and other systems of functions (Haar system, classical system of Legendre, etc.) were investigated [6, 7, 16, 17].
In this paper a perturbed system of cosines is considered. Stability of the basicity of this system in Lebesgue space with variable summability index is studied. It should be noted that the basicity in generalized Lebesgue space of perturbed systems of exponents was considered earlier in [16, 17].
2 Necessary information
A Banach space will be called a Bspace. A Banach space of sequences of scalars over the field K will be called a Kspace. We give some information on Lebesgue spaces with variable summability index.
Let \(p: [\pi,\pi ]\to [1,+\infty )\) be some Lebesgue measurable function. Denote the class of all functions measurable on \([\pi,\pi ]\) (with respect to the Lebesgue measure) by \({\mathscr {L}}_{0} \).
The following property that we will use is obvious.
Property A
If \(\vert f (t )\vert \le \vert g (t )\vert \) a.e. on \((\pi,\pi )\), then \(\Vert f\Vert _{p_{(\cdot)} } \le \Vert g\Vert _{p_{(\cdot)} } \).
For detailed information on the space \(L_{p (\cdot )}\) one can see [14, 15] and also [13].
We will also use the notion of the space of coefficients. Let us define it. Let \(\vec{x}\equiv \{x_{n} \}_{n\in N} \subset X\) be a nondegenerate system in Bspace X, i.e. \(x_{n} \ne0\), \(\forall n\in N\).
With respect to ordinary operations of addition and multiplication by a complex number, \({\mathscr {K}}_{\vec{x}} \) is a Banach space. We also need some notion and facts from the basis theory.
Definition 1
Let X be some Bspace. We call the system \(\{\varphi_{n} \}_{n\in N} \subset X\) ωlinear independent in X (or simply ωlinear independent) if from \(\sum_{n}c_{n} \varphi_{n} =0\) it follows \(c_{n} =0\), \(\forall n\in N\).
The following theorem is valid.
Theorem 1
 (a)
\(\{\psi_{n} \}_{n\in N} \) is complete in X;
 (b)
\(\{\psi_{n} \}_{n\in N} \) is minimal in X;
 (c)
\(\{\psi_{n} \}_{n\in N} \) is ωlinear independent in X;
 (d)
\(\{\psi_{n} \}_{n\in N} \) is a bases isomorphic to \(\{\varphi_{n} \}_{n\in N} \) in X.
From this theorem we have the following.
Corollary 1
Let \(\{\varphi_{n} \}_{n\in N} \) form a basis for X and \(\operatorname{card} \{n:\psi_{n} \ne\varphi_{n} \}<+\infty\). Then with respect to the system \(\{\psi_{n} \} _{n\in N} \) the statement of Theorem 1 is valid.
For these or other results one can see for example [1–5].
Definition 2
We will also use the following.
Definition 3
The sequence \(\{\lambda_{n} \}_{n\in N }\) is called separated, if \(\inf_{i\neq j} \vert \lambda_{i}\lambda_{j}\vert>0\).
For more details regarding these and other results one can see [1–5].
3 Basic results
 (α):

\(\Vert \{\lambda_{k} \}_{k\in N} \Vert _{{\mathscr {K}}} =\Vert \{\lambda_{\pi (k )} \} _{k\in N} \Vert _{{\mathscr {K}}} \) for an arbitrary permutation \(\pi:N\to N\).
Thus, the following lemma is valid.
Lemma 1
Let the Kspace \({\mathscr {K}}\) satisfy condition (α) and the system of cosines \(\{\cos\lambda _{n} x \}_{n\in N} \) be a \({\mathscr {K}}\)Hilbert in \(L_{p (\cdot )} \) Then the sequence \(\{\lambda_{n} \}_{n\in N} \) is separated.
The analog of Levinson’s wellknown theorem [18] on the completeness of the system of exponents is valid in this case as well.
Theorem 2
Let \(1< p^{} \le p^{+} <+\infty\). If from the system of functions \(\{e^{i\lambda_{k} x} \}\) complete in \(L_{p (\cdot )} (\pi, \pi )\) we reject n arbitrary functions and instead of them add other functions \(e^{i\mu _{j} x} \), \(j=\overline{1, n}\), wherein \(\mu_{k}\), \(k=\overline{1, n}\), are arbitrary pairwise different complex numbers not equal to any of the numbers \(\lambda_{k} \), then the new system will also be complete in \(L_{p (\cdot )} (\pi, \pi )\).
The proof of this theorem is conducted by analogy with the case \(L_{p} (\pi, \pi )\) (i.e. \(p (x )\equiv p=\mathrm{const}\)). As under the conditions of the theorem \((L_{p (\cdot )} (\pi, \pi ) )^{*} =L_{q (\cdot )} (\pi, \pi )\), \(\frac{1}{p (x )} +\frac{1}{q (x )} =1\). The following theorem is also valid.
Theorem 3
Let \(\{\lambda_{n} \}_{n\in N} \subset C\) be an arbitrary sequence of various numbers, and \(1< p^{} \le p^{+} <+\infty\). The system \(\{\cos\lambda_{n} x \} _{n\in N} \) is complete in \(L_{p (\cdot )} (0, \pi )\) if and only if the system \(\{e^{\pm i\lambda_{n} x} \}_{n\in N} \) is complete in \(L_{p (\cdot )} (\pi , \pi )\). If for some \(k_{0} :\lambda_{k_{0} } =0\), then instead of the functions \(e^{i\lambda_{k_{0} } x} \) and \(e^{i\lambda _{k_{0} } x} \) the functions 1 and x should be taken.
From these two theorems we immediately have the following.
Corollary 2
Let \(1< p^{} \le p^{+} <+\infty\). If from the system of functions \(\{\cos\lambda_{k} x \}\) complete in \(L_{p (\cdot )} (0, \pi )\) we reject n arbitrary functions and instead of them add other n functions \(\{\cos\mu_{j} x \}\), \(j=\overline{1, n}\), wherein \(\mu _{k} \), \(k=\overline{1, n}\), are arbitrary complex numbers such that \(\mu_{i} \ne\pm\mu_{j} \) for \(i\ne j\), and not equal to any of the numbers \(\pm\lambda_{k} \), then the obtained system will also be complete in \(L_{p (\cdot )} (0, \pi )\).
Now we cite the basic result of the paper.
Theorem 4
Proof
Theorem 5
Let with respect to the sequences \(\{ \lambda_{n} \}_{n\in N} \) and \(\{\mu_{n} \}_{n\in N} \) all the conditions of Theorem 4 be fulfilled. If the system of sines \(\{\sin\lambda_{n} x \}_{n\in N} \) forms a basis for \(L_{p (\cdot )} (0, \pi )\) equivalent to the basis \(\{\sin nx \}_{n\in N} \), then the system \(\{\sin \mu_{n} x \}_{n\in N} \) also forms a basis for \(L_{p (\cdot )} (0, \pi )\) equivalent to the basis \(\{\sin nx \}_{n\in N} \).
4 Example
With respect to this fact refer to [20].
It is obvious that K is Volterrian and so the operator \(I+K\) is boundedly invertible in \(L_{p (\cdot )} \) (\(I\in L (L_{p (\cdot )} ;L_{p (\cdot )} )\) is a unit operator). Then from the relation \(y_{\lambda} (x )= (I+K )\cos\lambda x\) and from the results of the previous item we see that the system \(\{y_{\lambda_{n} } (x ) \}_{n\in N} \) is a basis in \(L_{p (\cdot )} \) only if the system of cosines \(\{\cos\lambda_{n} x \}_{n\in N} \) forms a basis for \(L_{p (\cdot )} (0, \pi )\).
Thus, the following theorem is valid.
Theorem 6
Let \(p (\cdot )\in WL_{0} \), \(p^{} >1\), \(q\in L_{1} \), and the sequence \(\{\lambda_{n} \}_{n\in N} \subset R\) satisfy the condition \(\sum_{n=0}^{\infty} \vert \lambda_{n} n\vert ^{\alpha} <+\infty\), where \(\alpha=\min \{2; p^{} \}\). Then the system \(\{y_{\lambda_{n} } (x ) \}_{n\in N} \) from the solution of the Cauchy problem (6) is a basis in \(L_{p (\cdot )} \), equivalent to the basis \(\{\cos nx \}_{n\in Z_{+} } \) in \(L_{p (\cdot )} (0, \pi )\).
Declarations
Acknowledgements
This work was supported by the Research Program Competition launched by the National Academy of Sciences of Azerbaijan (Program: Frame theory Applications of Wavelet Analysis to Signal Processing in Seismology and Other Fields). The authors express their deep gratitude to Professor BT Bilalov, corresponding member of the National Academy of Sciences of Azerbaijan, for his inspiring guidance and valuable suggestions during the work.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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