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On trigonometric approximation of () function by product means of its Fourier series
Journal of Inequalities and Applications volume 2013, Article number: 300 (2013)
Abstract
In the present paper, we generalize a theorem of Lal and Singh (Indian J. Pure Appl. Math. 33(9):1443-1449, 2002) on the degree of approximation of a function belonging to the weighted ()-class using product means of its Fourier series. We have used here the modified definition of the weighted ()-class of functions in view of Khan (Commun. Fac. Sci. Univ. Ank. Ser. A1 Math. Stat. 31:123-127, 1982) and rectified some errors appearing in the paper of Lal and Singh (Indian J. Pure Appl. Math. 33(9):1443-1449, 2002). A few applications of approximation of functions will also be highlighted.
MSC: 40C99, 40G99, 41A10, 42B05, 42B08.
Dedication
Dedicated to Professor Hari M Srivastava
1 Introduction
Approximation by trigonometric polynomials is at the heart of approximation theory. The most important trigonometric polynomials used in the approximation theory are obtained by linear summation methods of Fourier series of 2π-periodic functions on the real line (i.e. Cesàro means, Nörlund means, Euler means and Product Cesàro-Nörlund means, Cesàro-Euler means etc.). Much of the advance in the theory of trigonometric approximation is due to the periodicity of the functions. Various investigators such as Khan [1–3], Qureshi [4], Chandra [5], Leindler [6], Mittal et al. [7], Mittal, Rhoades and Mishra [8], Mishra [9], Rhoades et al. [10] have determined the degree of approximation of 2π-periodic functions belonging to different classes Lipα, , and of functions through trigonometric Fourier approximation (TFA) using different summability matrices. Recently, Mittal et al. [11] have obtained the degree of approximation of functions belonging to the -class by a general summability matrix, which generalizes the results of Chandra [5]. In this paper, we determine the degree of approximation of functions belonging to the ()-class by using means of its Fourier series, which in turn generalizes the result of Lal and Singh [12]. We also note some errors appearing in the paper of Lal and Singh [12] and rectify them in the light of observations of Khan [13].
Let be a 2π-periodic and Lebesgue integrable function. The Fourier series of is given by
with n th partial sum called trigonometric polynomial of degree (order) n of the first terms of the Fourier series of f.
A function if
A function if
, for , if
A function for if
(Definition 5.38 of Mc Fadden [14]). Given a positive increasing function and an integer , if
A function [13] if
We note that if , then the weighted class coincides with the class and if , then the class coincides with the class . The class for .
Also, we observe that
The -norm of a function is defined by
The -norm of a function is defined by
and the degree of approximation is given by Zygmund ([15], p.114)
in terms of n, where is a trigonometric polynomial of degree n. This method of approximation is called trigonometric Fourier approximation (tfa).
Let be a given infinite series with the sequence of n th partial sums . If
then an infinite series with the partial sums is said to be summable to the definite number s (Hardy [16]).
An infinite series is said to be summable to s if
The transform of the transform defines the transform of the partial sums of the series , i.e., the product summability is obtained by superimposing summability on summability.
Thus, if
where denotes the transform of , then the series with the partial sums is said to be summable to the definite number s, and we can write
Therefore, we have
We note that and are also trigonometric polynomials of degree (or order) n.
The product transform plays an important role in signal theory as a double digital filter.
The Riemann-Lebesgue theorem states that if is integrable over , then as , we have and .
We shall use the following notation throughout the paper:
2 Known theorem
Lal and Singh [12] have obtained a theorem on the degree of approximation of a function belonging to the class by means of its Fourier series. They proved the following theorem.
Theorem 2.1 is a 2π-periodic function belonging to , then the degree of approximation of f by means of its Fourier series satisfies
provided satisfy the following conditions:
where δ is an arbitrary number such that , conditions (2.1) and (2.2) hold uniformly in x and are means of series (1.1).
Remark 1 The proof proceeds by estimating , which is represented in terms of an integral. The domain of integration is divided into two parts - from and . Referring to the second integral as and using Hölder’s inequality, the authors [12] obtain
The authors then make the substitution to obtain
In the next step is removed from the integrand by replacing it with . While is an increasing function, is now a decreasing function. Therefore, from the second mean value theorem of integrals, this step is invalid.
Remark 2 There is a fatal error in the proof of the main theorem of Lal and Singh [[12], p.1447]. In the calculation of , the authors [12] obtain
note that . Therefore one has , which need not be since ϵ might be for some .
3 Main result
The observation of Remark 1 motivated us to determine a proper set of conditions to extend Theorem 2.1 on the degree of approximation of functions f of the weighted ()-class by product means of its Fourier series. More precisely, we prove the following.
Theorem 3.1 If is 2π-periodic, Lebesgue integrable and belonging to the weighted ()-class, then the degree of approximation of by means of its Fourier series is given by
provided a positive increasing function satisfies the following conditions:
and
where δ is an arbitrary number such that , q is the conjugate index of p, , , conditions (3.2), (3.3) hold uniformly in x, and are means of Fourier series (1.1).
Note 1 Using condition (3.4), we get for .
Note 2 Conditions (2.1) and (2.2) of Theorem 2.1 and (3.2) and (3.3) of Theorem 3.1 are derived from the theorem of Khan [17].
4 Lemma
For the proof of our theorem, we need the following lemma.
Lemma 4.1 For , we have
Proof of Lemma 4.1 For , we have
This completes the proof of Lemma 4.1. □
5 Proof of Theorem 3.1
Following Titchmarsh [[18], p.403] and using the Riemann-Lebesgue theorem, the n th partial sum of Fourier series (1.1) at may be written as
so that the transform of is given by
Now, the transform of is given by
Therefore, we have
Clearly,
Hence, by Minkowski’s inequality,
Then .
Using Hölder’s inequality, the fact that , condition (3.2), , for , , , Lemma 4.1, Note 1 and the second mean value theorem for integrals, we have
Again applying Hölder’s inequality, , , for , conditions (3.3), (3.4), Note 1 and the second mean value theorem for integrals, we have
in view of increasing nature of , , , where lie in .
Collecting (5.1)-(5.3), we get
Now, using the -norm of a function, we get
This completes the proof of Theorem 3.1.
6 Corollaries and example
The following corollaries can be derived from Theorem 3.1.
Corollary 1 If , then the generalized weighted ()-class reduces to the class , and the degree of approximation of a function is given by
Proof The result follows by setting in (3.1), we have
Thus, we get
This completes the proof of Corollary 1. □
Corollary 2 If , , , then the weighted ()-class reduces to the class , and the degree of approximation of a 2π-periodic function f belonging to the class is given by
Proof Putting , , in Theorem 3.1, we have
or
or
since otherwise the right-hand side of the above equation will not be .
Hence
This completes the proof of Corollary 2. □
Corollary 3 If , for and in (3.1), then . In this case, the degree of approximation of the function () class is given by
Proof For in Corollary 2, we get
Thus, we have
This completes the proof of Corollary 3. □
Examples (i) Consider the infinite series
The n th partial sum of (6.1) is given by
Since does not exist, therefore the series (6.1) is not convergent and so
Also, does not exist. Therefore the series (6.1) is not summable.
Now,
Here does not exist, the series (6.1) is not summable.
Finally,
the series (6.1) is summable.
Therefore the series (6.1) is neither summable nor summable. But it is summable to 0. Therefore the product summability is more powerful than and . Consequently, gives better approximation than the individual methods and .
(ii) If we take the sequence for , then there are two cases:
(a) If , then the sequence is already convergent and so it is summable.
(b) If , then the sequence is not summable but summable.
(iii) It is a well-known result that a Hausdorff matrix is a Nörlund matrix if and only if it is a Cesàro matrix. Cesàro means and Euler means both are Hausdorff means, but they are not comparable. Two matrices are called comparable if either they are equivalent (that is, they sum the same set of sequences), or one method is stronger than the other (that is, it has the larger convergence domain). Hardy [16], in his book on Divergent Series, showed that and methods are not comparable.
Note that the sequence is summable but not bounded, whereas the sequence given by , and
is bounded but not summable.
7 Conclusion
Several results concerning the degree of approximation of periodic functions belonging to the generalized weighted ()-class by product means of its Fourier series have been reviewed. Further, a proper set of conditions have been discussed to rectify the errors pointed out in Remarks 1 and 2. The theorem of this paper is an attempt to formulate the problem of approximation of the function () through trigonometric polynomials generated by the product summability transform of the Fourier series of f in a simpler manner. The product summability used in this paper plays an important role in signal theory as a double digital filter and the theory of machines in mechanical engineering.
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Acknowledgements
The authors would like to thank the anonymous referees for several useful interesting comments and suggestions about the paper. Special thanks are to Professor Hari Mohan Srivastava for kind cooperation, smooth behavior during communication and for his efforts to send the reports of the manuscript timely. This research work is supported by CPDA, SVNIT, Surat (Gujarat), India.
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Authors’ contributions
LNM computed lemmas and established the main theorem in this direction with appropriate Examples 1 and 2 as well as interesting corollaries. LNM and VNM conceived of the study and participated in its design and coordination. VNM, VS and LNM contributed equally and significantly to this work. All the authors drafted the manuscript, read and approved the final version of manuscript.
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Mishra, V.N., Sonavane, V. & Mishra, L.N. On trigonometric approximation of () function by product means of its Fourier series. J Inequal Appl 2013, 300 (2013). https://doi.org/10.1186/1029-242X-2013-300
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DOI: https://doi.org/10.1186/1029-242X-2013-300