- Research
- Open access
- Published:
Szász-Durrmeyer operators involving Boas-Buck polynomials of blending type
Journal of Inequalities and Applications volume 2017, Article number: 122 (2017)
Abstract
The present paper introduces the Szász-Durrmeyer type operators based on Boas-Buck type polynomials which include Brenke type polynomials, Sheffer polynomials and Appell polynomials considered by Sucu et al. (Abstr. Appl. Anal. 2012:680340, 2012). We establish the moments of the operator and a Voronvskaja type asymptotic theorem and then proceed to studying the convergence of the operators with the help of Lipschitz type space and weighted modulus of continuity. Next, we obtain a direct approximation theorem with the aid of unified Ditzian-Totik modulus of smoothness. Furthermore, we study the approximation of functions whose derivatives are locally of bounded variation.
1 Introduction
For a real-valued bounded function f on \([0,1]\), Bernstein [2] defined a sequence of polynomials given by
to provide a very simple and elegant proof of the Weierstrass approximation theorem. For \(f\in C[0,\infty )\), Szász [3] generalized the Bernstein polynomials to the infinite interval as follows:
provided the infinite series on the right-hand side converges. Recently, the Szász operators, their quantum and post quantum analogues, Szász-Durrmeyer operators and mixed type operators have been intensively studied. We refer the readers to the related papers (cf. [4–9] etc.).
In [1], Sucu et al. introduced the Szász operators involving Boas-Buck type polynomials as follows:
where a generating function of the Boas-Buck type polynomials is given by
and \(A(t)\), \(G(t)\) and \(H(t)\) are analytic functions described as
Motivated by the above work, in the present paper we define Szász-Durrmeyer type operators based on Boas-Buck type polynomials as follows.
For \(\gamma >0\), let \(C_{\gamma }[0,\infty ):= \{ f\in C[0,\infty ): \vert f(t) \vert \leq M (1+t^{\gamma })\mbox{ for some } M>0 \} \) endowed with the norm
Then, for a function \(f\in C_{\gamma }[0,\infty )\), we define
where \(B(k,n+1)\) is the beta function and \(x\geq 0\), \(n\in \mathbb{N}\).
Alternatively, we may write operator (1.3) as
where
and \(\delta (t)\) is the Dirac-delta function.
We study the approximation properties of the operators \(M_{n}\) for functions belonging to different function spaces.
2 Preliminaries
Lemma 1
[1]
For the operators \(B_{n}\), one has
Proof
Since identities (i)-(iii) are proved in [1], we give below the proof of only (iv). Identity (v) follows similarly.
It is easily seen that
and
□
Now, by simple calculations, we obtain identities (iii) and (iv). Hence the details are omitted.
In the following lemma, we obtain the moments for the operators defined by (1.3) utilizing Lemma 1.
Lemma 2
Hence, as a consequence of Lemma 2, we find the following.
Lemma 3
For operator (1.3), we have the following results:
Now, in order to study the approximation properties of the considered operators (1.3), we make the following assumptions on the analytic functions \(A(t)\), \(H(t)\) and \(G(t)\). It is to be noted that the following assumptions are valid pointwise. These assumptions will be needed to prove Theorems 3, 7 and 8 of this paper which are pointwise results.
As a result of the above assumptions, applying Lemma 3, we reach the following important result.
Lemma 4
For operator (1.3), we have
3 Results and discussion
Throughout the paper, we assume \(\delta_{n}(x)=M_{n}((t-x)^{2};x)\).
In the following theorem, we show that the operators defined by (1.3) are an approximation process for \(f\in C_{\gamma }[0, \infty )\), using the Bohman-Korovkin theorem.
Theorem 1
Let \(f\in C_{\gamma }[0,\infty )\). Then
holds uniformly in \(x\in [0,a]\), \(a>0\).
Proof
From Lemma 2, it follows that
uniformly in \(x\in [0,a]\). Hence, by the Bohman-Korovkin theorem, the required result is immediate. □
First, we consider the Lipschitz type space [3] considered by Otto Szász to establish the uniform convergence of the Szász operators for functions in this space. For \(0<\xi \leq 1\), \(x\in (0,\infty )\), \(t\in [0,\infty )\), we define
In the following theorem, we find the rate of convergence of the operators \(M_{n}\) for functions in \(\operatorname{Lip}_{M}^{*}\xi \). We observe that due to the presence of x, in the denominator on the right-hand side, we get only pointwise approximation. In the case of Szász operators [3], this x gets canceled leading to the uniform convergence.
Theorem 2
Let \(f\in \operatorname{Lip}_{M}^{*}(\xi )\) and \(\xi \in (0,1]\). Then, for all \(x\in (0,\infty )\), we have
Proof
By the linearity and positivity of the operators \(M_{n}\), from (1.4) we obtain
Applying Hölder’s inequality with \(p=\frac{2}{\xi } \) and \(q=\frac{2}{2-\xi }\) and Lemma 2, we have
Thus, we reach the desired result. □
In our next result, we establish a Voronovskaja type approximation theorem.
Theorem 3
Let \(f\in C_{\gamma }[0,\infty )\), admitting a derivative of second order at a point \(x\in [0,\infty )\), then there holds
If \(f^{\prime \prime }\) is continuous on \([0,\infty )\), then the limit in (3.1) holds uniformly in \(x\in [0,a]\subset [0,\infty )\), \(a>0\).
Proof
By Taylor’s theorem
where \(\varepsilon (t,x)\in C_{\gamma }[0,\infty )\) and \(\lim_{t\rightarrow x}\varepsilon (t,x)=0\).
Applying the operator \(M_{n}(\cdot,x)\) on both sides of (3.2), we have
Using the Cauchy-Schwarz inequality in the last term of the right-hand side of (3.3), we get
Since \(\varepsilon (t,x)\rightarrow 0\), as \(t\rightarrow x\), applying Theorem 1, for every \(x\in [0,\infty )\), we obtain \(\lim_{n\rightarrow \infty }M_{n}(\varepsilon^{2}(t,x);x)=\varepsilon ^{2}(x,x)=0\). Next applying Lemma 4, for sufficiently large n and every \(x\in [0,\infty )\), we have
Hence,
Now, from (3.3), (3.5) and Lemma 4, the required result follows. □
The uniformity assertion follows from the uniform continuity of \(f^{\prime \prime }\) on \([0,a]\) and the fact that all the other estimates hold uniformly in \(x\in [0,a]\).
In our next theorem, we obtain the degree of approximation of the \(M_{n}\) operators for functions in the space \(C_{2}[0,\infty )\) in terms of the classical modulus of continuity.
Theorem 4
For \(f\in C_{2}[0,\infty )\), we have the following inequality:
where \(\omega (f;\delta_{n}(x))\) is the modulus of continuity of f on \([0,b+1]\).
Proof
From [10], for \(t\in (b+1,\infty )\) and \(x\in [0,b]\), we have
Hence, by applying the Cauchy-Schwarz inequality, we obtain
Choosing \(\delta =\sqrt{\delta_{n}(x)}\), we get the desired result. □
The next section is devoted to the weighted approximation properties of the operators \(M_{n}\).
3.1 Weighted approximation
Let
Next, we study the approximation of functions in the subspace \(C_{2}^{0}[0,\infty )\) of \(C_{2}[0,\infty )\). Such type of function spaces has been considered by several researchers (cf. [11, 12]).
It is well known that the classical modulus of continuity of first order \(\omega (f;\delta ),\delta >0 \) does not tend to zero, as \(\delta \rightarrow 0\), on an infinite interval. A weighted modulus of continuity \(\Omega (f;\delta ) \) which tends to zero as \(\delta \rightarrow 0\) on \([0,\infty ) \) was defined in [13]. For \(f\in C^{0}_{2}[0,\infty )\), the weighted modulus of continuity defined by Yüksel and Ispir [13] is given as follows:
Some properties of \(\Omega (f;\delta )\) are collected in the following lemma.
Lemma 5
[13]
Let \(f\in C^{0}_{2}[0,\infty )\). Then the following results hold:
-
(1)
\(\Omega (f;\delta )\) is a monotonically increasing function of δ;
-
(2)
\(\lim_{\delta \rightarrow 0^{+}}\Omega (f;\delta )=0\);
-
(3)
For each \(m\in \mathbb{N}\), \(\Omega (f;m\delta )\leq m\Omega (f;\delta)\);
-
(4)
For each \(\lambda \in (0,\infty ), \Omega (f;\lambda \delta )\leq (1+\lambda ) \Omega (f;\delta )\).
Firstly, we establish the following basic approximation theorem for functions in the weighted space of continuous functions \(C^{0}_{2}[0, \infty )\) by the operators \(M_{n}\).
Theorem 5
For \(f\in C^{0}_{2}[0,\infty )\) and \(a>0\), we have
Proof
Let \(x_{0}\in [0,\infty )\) be an arbitrary but fixed point. Then
Since \(\vert f(x) \vert \leq \Vert f \Vert _{2}(1+x^{2})\), we have
Let \(\epsilon >0\) be arbitrary. We choose \(x_{0}\) to be so large that
From Theorem 1, there exists \(n_{1}\in \mathbb{N}\) such that
Hence,
Applying Theorem 3, we can find \(n_{2}\in \mathbb{N}\) such that
Let \(n_{0}=\max (n_{1},n_{2})\). Combining (3.8)-(3.11), we obtain
Hence the required result is obtained. □
In our next theorem, we determine the order of approximation for functions in a weighted space of continuous functions on \([0,\infty )\) by \(M_{n}\) operators.
Theorem 6
Let \(f\in C^{0}_{2}[0,\infty )\). Then, for sufficiently large n, we have
where \(C(x)=2(1+x^{2}) (1+C_{1}\vert \eta (x) \vert +\sqrt{C_{1}}\vert \eta (x) \vert ^{1/2} ( 1+\sqrt{C_{2}}\vert \nu (x) \vert ^{1/2} ) ), C_{1}, C_{2}\) are constants independent of x and n and \(\eta (x)\), \(\nu (x)\) are as given in Lemma 4.
Proof
For \(x\in (0,\infty )\) and \(\delta >0\), using (3.7) and Lemma 5, we have
Applying \(M_{n}(\cdot ;x)\) on both sides, we can write
From Lemma 4, for sufficiently large n, it follows
Now, applying the Cauchy-Schwarz inequality in the last term of (3.13), we obtain
Combining the estimates (3.13)-(3.15) and taking
we reach the required result. □
3.2 Unified modulus theorem
We investigate a direct approximation theorem by utilizing the unified Ditzian-Totik modulus of smoothness \(\omega_{\phi^{\tau }}(f,t), 0 \leq \tau \leq 1\). Guo et al. [14] proved the direct, inverse and equivalence approximation theorems with the aid of unified modulus. First, we give the definitions of the Ditzian-Totik modulus of smoothness and the Peetre’s K-functional. Let \(\phi^{2}(x)=x(1+x)\) and \(f\in C_{B}[0,\infty )\), the space of all bounded and continuous functions on \([0,\infty )\) endowed with the norm \(\Vert f \Vert = \sup_{x\in [0,\infty )}\vert f(x) \vert \). The modulus \(\omega_{\phi^{\tau }}(f,t)\), \(0\leq \tau \leq 1\), is defined as
and the appropriate K-functional is given by
where \(W_{\tau }=\{g:g\in A C_{\mathrm{loc}}[0,\infty ):\Vert \phi^{\tau }g^{\prime } \Vert <\infty \}\), \(A C_{\mathrm{loc}}\) denotes the space of locally absolutely continuous functions on \([0,\infty )\).
From [15], there exists a constant \(M>0\) such that
Theorem 7
Let \(f\in C_{B}[0,\infty )\), then for sufficiently large n
where C is independent of f and n.
Proof
By the definition of \(K_{\phi^{\tau }}(f,t)\), for fixed n, x, τ, we can choose \(g=g_{n,x,\tau }\in W_{\tau }\) such that
We may write
Since \(g\in W_{\tau }\), we have
and so
By applying Hölder’s inequality, we get
we may write
Hence, on using the inequality \(\vert a+b \vert ^{r}\leq \vert a \vert ^{r}+\vert b \vert ^{r}\), \(0\leq r\leq 1\).
Thus, from (3.19), (3.20) and the Cauchy-Schwarz inequality, using Theorem 1, we obtain
for sufficiently large n.
Hence, combining (3.17)-(3.19) and (3.21), we find
This completes the proof of the theorem. □
3.3 Rate of convergence of Szász-Durrmeyer operators based on Boas-Buck polynomials
In this section, we discuss the approximation of functions with a derivative of bounded variation. We show that the points x where \(f^{\prime }(x+)\) and \(f^{\prime }(x-)\) exist, the operators \(M_{n}(f;x)\) converge to the function \(f(x)\), as \(n\rightarrow \infty \). In the recent years, several researchers have studied different sequences of linear positive operators. We refer the reader to some of the related papers (cf. [8, 9, 16–20] and [21] etc.). Let \(DBV[0,\infty )\) be the class of all functions in \(C_{2}[0,\infty )\) having a derivative which is locally of bounded variation on \([0,\infty )\). A function \(f\in DBV[0,\infty )\) can be represented as
where g is a function of bounded variation on each finite subinterval of \([0,\infty )\).
Lemma 6
Let \(\alpha =\alpha (n)\rightarrow 0\), as \(n\rightarrow \infty \) and \(\lim_{n\rightarrow \infty }n \alpha (n)=l\in \mathbb{R}\). For all \(x\in (0,\infty )\) and sufficiently large n, we have
where \(\eta (x)\) is as given in Lemma 4.
Proof
Using Lemma 2 and (3.14), we have
when n is large enough. Similarly, we can prove (ii). □
Theorem 8
Let \(f\in DB V[0,\infty )\). Then, for every \(x\in (0,\infty )\) and sufficiently large n, we have
where \(C_{1}\) is a positive constant and \(\bigvee_{a}^{b} f\) denotes the total variation of f on \([a,b]\) and \(f^{\prime }_{x}\) is defined by
Proof
For any \(f\in DBV [0,\infty )\), from (3.22), we may write
where
Since \(M_{n}(e_{0};x)=1\), using (3.23) for every \(x\in (0,\infty )\), we get
Let
Since \(\int_{x}^{t} \delta_{x}(u)\,du=0\), using (3.23), we have
Similarly, we have
Combining relations (3.24)-(3.26), we get
Hence,
Now, assume that
and
Now the problem is reduced to estimate \(C_{n}(f^{\prime }_{x},x)\) and \(D_{n}(f^{\prime }_{x},x)\). Using the definition of \(\xi_{n}(x,t)\) given in Lemma 6 and applying integration by parts, we can write
Thus,
Since \(f^{\prime }_{x}(x)=0\) and \(\xi_{n}(x,t)\leq 1\), we get
Using Lemma 6 and assuming \(t=x-\frac{x}{u}\), we have
Therefore,
Using integration by parts in \(D_{n}(f^{\prime }_{x},x)\) and applying Lemma 6, we have
We have
Since \(f^{\prime }_{x}(x)=0\) and \(1-\xi_{n}(x,t)\leq 1\), we have
Using Lemma 6 and assuming \(t=x+\frac{x}{u}\), we obtain
Putting the values of \(J_{1}\) and \(J_{2}\) in (3.28), we have
Therefore, applying the Cauchy-Schwarz inequality and Lemma 6, we get
Since \(t\leq 2(t-x)\) and \(x\leq t-x\) when \(t\geq 2x\), we have
Using the above inequality, we have
Now from (3.27), (3.29) and (3.31), we reach the required result. □
4 Conclusion
We introduce Szász-Durremeyer type operators involving Boas-Buck type polynomials. Brenke type polynomials, Sheffer polynomials and Appell polynomials turn out to be the special cases of Boas-Buck type polynomials. We obtain the rate of convergence for functions belonging to a Lipschitz type space and also establish a Voronovskaja type theorem for twice continuously differentiable functions. We study the approximation properties of the considered operators for continuous functions in weighted spaces. Lastly, we discuss the rate of approximation of functions having derivatives of bounded variations.
References
Sucu, S, Içöz, G, Varma, S: On some extensions of Szász operators including Boas-Buck-type polynomials. Abstr. Appl. Anal. 2012, Article ID 680340 (2012)
Bernstein, SN: Démonstration du théorém de Weierstrass fondée sur la calcul des probabilitiés. Comm. Soc. Math. Charkow Sér. 2 13, 1-2 (1912)
Szász, O: Generalization of S. Bernstein’s polynomials to the infinite interval. J. Res. Natl. Bur. Stand. 97, 239-245 (1950)
Acar, T: Asymptotic formulas for generalized Szász-Mirkyan operators. Appl. Math. Comput. 263, 223-239 (2015)
Acar, T: \((p,q)\)-generalization of Szász-Mirakyan operators. Math. Methods Appl. Sci. 39(10), 2685-2695 (2016)
Acar, T: Quantitative q-Voronovskaya and q-Grüss-Voronovskaya-type results for q-Szász operators. Georgian Math. J. 23(4), 459-468 (2016)
Acar, T, Ulusoy, G: Approximation properties of generalized Szász-Durrmeyer operators. Period. Math. Hung. 72(1), 64-75 (2016)
Gupta, MK, Beniwal, MS, Goel, P: Rate of convergence of Szász Mirakyan-Durrmeyer operators with derivatives of bounded variation. Appl. Math. Comput. 199(2), 828-832 (2008)
Kajla, A, Acu, AM, Agrawal, PN: Baskakov-Szász-type operators based on inverse Pólya-Eggenberger distribution. Ann. Funct. Anal. 8(1), 106-123 (2017)
İbikli, E, Gadjieva, EA: The order of approximation of some unbounded functions by the sequences of positive linear operators. Turk. J. Math. 19(3), 331-337 (1995)
Acar, T, Aral, A: It weighted approximation by new Bernstein-Chlodowsky-Gadjiev. Filomat 27(2), 371-380 (2013)
Gupta, V, Aral, A: Convergence of the q-analogue of Szász type operators based on Charlier polynomials. Appl. Math. Comput. 216(2), 374-380 (2010)
Yüksel, I, Ispir, N: Weighted approximation by a certain family of summation integral-type operators. Comput. Math. Appl. 52(10-11), 1463-1470 (2006)
Guo, S, Qi, Q, Liu, G: The central approximation theorems for Baskakov-Bézier operators. J. Approx. Theory 51(2), 183-192 (1987)
Ditzian, Z, Totik, V: Moduli of Smoothness. Springer Series in Computational Mathematics, vol. 9. Springer, New York (1987)
Acar, T, Gupta, V, Aral, A: Rate of convergence for generalized Szász operators. Bull. Math. Sci. 1(1), 99-113 (2011)
Dhamija, M, Deo, N: Jain-Durrmeyer operators associated with the inverse Pólya-Eggenberger distribution. Appl. Math. Comput. 286, 15-22 (2016)
Ispir, N: Rate of convergence of generalized rational type Baskakov operators. Math. Comput. Model. 46(5-6), 625-631 (2007)
Karsli, H: Rate of convergence of new Gamma type operators for functions with derivatives of bounded variation. Math. Comput. Model. 45(5-6), 617-624 (2007)
Neer, T, Acu, AM, Agrawal, PN: Bézier variant of genuine-Durrmeyer type operators based on Pólya distribution. Carpath. J. Math. 33(1), 73-86 (2017)
Özarslan, MA, Duman, O, Kaanoǧlu, C: Rates of convergence of certain King-type operators for functions with derivatives of bounded variation. Math. Comput. Model. 52(1-2), 334-345 (2010)
Acknowledgements
The authors are extremely grateful to the reviewers for a critical reading of the manuscript and making valuable suggestions leading to a better presentation of the paper. The authors sincerely thank Prof. M. Mursaleen, Editor, for sending the reports on our paper timely. The first author is grateful to the “Ministry of Human Resource and Development” Govt. of India (Grant no. MHR-02-41-113-429) for providing financial support which made the above work possible. The third author of this paper is also supported by the Research Fund of Hasan Kalyoncu University in 2017.
Author information
Authors and Affiliations
Corresponding author
Additional information
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
All authors contributed equally to the writing of this paper. All authors read and approved the final manuscript.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
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.
About this article
Cite this article
Sidharth, M., Agrawal, P. & Araci, S. Szász-Durrmeyer operators involving Boas-Buck polynomials of blending type. J Inequal Appl 2017, 122 (2017). https://doi.org/10.1186/s13660-017-1396-x
Received:
Accepted:
Published:
DOI: https://doi.org/10.1186/s13660-017-1396-x