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Dunkl generalization of Phillips operators and approximation in weighted spaces
Advances in Difference Equations volume 2020, Article number: 365 (2020)
Abstract
The purpose of this article is to introduce a modification of Phillips operators on the interval \([ \frac{1}{2},\infty ) \) via a Dunkl generalization. We further define the Stancu type generalization of these operators as \(\mathcal{S}_{n, \upsilon }^{\ast }(f;x)=\frac{n^{2}}{e_{\upsilon }(n\chi _{n}(x))}\sum_{\ell =0}^{\infty } \frac{(n\chi _{n}(x))^{\ell }}{\gamma _{\upsilon }(\ell )}\int _{0}^{\infty } \frac{e^{-nt}n^{\ell +2\upsilon \theta _{\ell }-1}t^{\ell +2\upsilon \theta _{\ell }}}{\gamma _{\upsilon }(\ell )}f ( \frac{nt+\alpha }{n+\beta } ) \,\mathrm{d}t\), \(f\in C_{\zeta }(R^{+})\), and calculate their moments and central moments. We discuss the convergence results via Korovkin type and weighted Korovkin type theorems. Furthermore, we calculate the rate of convergence by means of the modulus of continuity, Lipschitz type maximal functions, Peetre’s K-functional and the second order modulus of continuity.
1 Introduction and preliminaries
The theory of special functions has a long and rich history and a key tool in the study of special functions with reflection symmetries is the Dunkl operator. Various other classes of Dunkl operators have become important, in the first place the trigonometric Dunkl operators of Heckman, Opdam and the Cherednik operators [11]. The aim of this paper is to study a Dunkl type generalization of some approximating operators.
Szász operators [24] provide an extension to Bernstein operators [8] on the interval \([0,\infty )\). In recent years, several authors have studied the Dunkl type generalization of Szász operators (see [13, 14, 16–18, 22, 26]). Recently, several research papers have appeared on Dunkl analogues of different operators (see [2, 3, 5, 9, 19, 25]).
Sucu [23] introduced a Dunkl analogue of Szász operators. That is, for \(f\in C[0,\infty )\), \(x\geqq 0\), \(\upsilon \geqq 0\) and \(n\in \mathbb{N}\),
where \(\mathbb{N}\) is the set of all natural numbers and
The coefficients \(\gamma _{\upsilon }\) are given as
with the recursion
where
Studies on Dunkl type generalizations [20] demonstrate an error estimation to the operators which allow us to have a much faster approximation to the function which is being approximated. Like Bernstein operators which are related to Dunkle type generalization, possibly it can be used for approximate solution of dynamical systems, like [1, 6, 7, 15, 21].
In Sect. 2, we modify the Phillips operators [20] to (2.4) via a Dunkl generalization and further define their Stancu type generalization (2.5). We obtain moments and central moments of these operators. In Sect. 3, we prove some Korovkin type and weighted Korovkin type theorems for operators (2.5). Section 4 is devoted to a study of the rate of convergence by means of the modulus of continuity, Lipschitz type maximal functions, Peetre’s K-functional and the second order modulus of continuity.
2 New operators and their moments
Let \(\{\chi _{n}(x)\}\) be a sequence of nonnegative continuous functions on \([0,\infty )\) as
where
Moreover, suppose
For \(f\in C_{\zeta }(\mathbb{R}^{+})=\{f\in C[0,\infty ):f(t)=O(t^{\zeta }), \zeta >n, n\in \mathbb{N}\}\), we define
where \(e_{\upsilon }(x)\), \(\gamma _{\upsilon }\) and \(\theta _{\ell }\) are defined as in [23] by (1.2), (1.3) and (1.5), respectively.
Lemma 2.1
Let\(e_{\ell }=t^{\ell -1}\), \(\ell =1,2,3,4,5\)and\(\mathcal{J}_{n,\upsilon }(x)\)defined by (2.3). Then for\(x\geqq 0\), \(\mathcal{P}_{n,\upsilon }(e_{1};x)=1\)and for any\(x\geqq \frac{1}{2n}\)we have
Remark 2.2
For any \(0\leqq x\leqq \frac{1}{2n}\), we have \(\mathcal{P}_{n,\upsilon }(e_{2};x)=\frac{1}{n}\); \(\mathcal{P}_{n,\upsilon }(e_{3};x)= \frac{2}{n^{2}}\); \(\mathcal{P}_{n,\upsilon }(e_{4};x)=\frac{6}{n^{3}}\); \(\mathcal{P}_{n,\upsilon }(e_{5};x)=\frac{24}{n^{4}}\).
Here we also introduce the Stancu type generalization to the operators defined by (2.4). Thus, for each \(f\in C_{\zeta }(\mathbb{R}^{+})\) the modified version of the operators (2.4) is defined as
where \(0\leq \alpha \leq \beta \). Note that if we take \(\alpha =\beta =0\) in (2.5), then the operators \(\mathcal{S}_{n,\upsilon }^{\ast }\) reduce to operators defined by (2.4) and if take \(\chi _{n}(x)=x\) in \(\mathcal{P}_{n,\upsilon }\), then we get the operators defined and studied in [20].
Lemma 2.3
For\(x\geqq 0\), \(\mathcal{S}_{n,\upsilon }^{\ast }(e_{1};x)=1\)and for\(x\geqq \frac{1}{2n}\), we have
Lemma 2.4
For\(0\leqq x\leqq \frac{1}{2n}\), we have
Lemma 2.5
Suppose\(\eta _{j}=(e_{2}-x)^{j}\)for\(j=1,2,3,4\), where\(e_{2}=t\). Then for\(x\geqq \frac{1}{2n}\)we have
Lemma 2.6
Suppose\(\eta _{j}=(e_{2}-x)^{j}\)for\(j=1,2,3,4\), where\(e_{2}\)defined in Lemma2.3. Then for any\(0\leqq x\leqq \frac{1}{2n}\)we have
As we can see from Lemmas 2.3 and 2.5, in our analysis of linear operators \(\mathcal{S}_{n,\upsilon }^{\ast }(\cdot ;\cdot )\) we have to know the behavior of the function \(x\mapsto {e_{\upsilon }(-n \chi _{n}(x))}/{e_{\upsilon }(n \chi _{n}(x))}\) on \([0,\infty )\). By using (1.2) to (1.5), here we consider the properties of the following ratio:
when \(|\upsilon |<1/2\). Because of \(\varPhi (0,1,z)=1\), we have \(\mathcal{J}_{n,0}(x)=e^{-2x}\), and the ratio (2.6) can be expressed in the following form:
According to (1.2) and (1.3) we get the values of the successive derivatives of the function \(e_{\upsilon }\) at the origin,
i.e.,
3 Korovkin type approximation
In the present section the results related to uniform convergence of the operators defined by (2.5) are given via the well-known Korovkin and weighted Korovkin type theorems.
Let \(\mathbb{R^{+}}=[0,\infty )\) and \(C_{B}(\mathbb{R^{+}})\) denote the linear normed space with the norm
Let
Theorem 3.1
Let the function\(f\in C[0,\infty )\cap \mathcal{H}\)and the operators\(\mathcal{S}_{n,\upsilon }^{\ast }(\cdot ;\cdot )\)be defined by (2.5). Then
uniformly onU, whereUis any compact subset of\([0,\infty )\).
Proof
We apply the well-known Korovkin’s theorem to prove the uniform convergence of the operators \(\mathcal{S}_{n,\upsilon }^{\ast }(\cdot ;\cdot )\). Therefore, for \(\ell =1,2,3\), we see \(\lim_{n\rightarrow \infty }\mathcal{S}_{n,\upsilon }^{\ast }(e_{\ell };x)=x^{\ell -1}\) uniformly. Therefore, we have
Hence the result is proved. □
We recall the weighted spaces defined by Gadžiev [12]. We write \(B_{\sigma }(\mathbb{R}^{+})\) for the set of all functions such that
with \(\|f\|_{\sigma }=\sup_{x\geqq 0}\frac{|f(x)|}{\sigma (x)}\), where \(m_{f}\) is a constant depending on f, and \(x\rightarrow \phi (x)\) is a continuous and strictly increasing function such as \(\sigma (x)=1+\phi ^{2}(x)\) with \(\lim_{x\rightarrow \infty }\sigma (x)=\infty \). Let \(C_{\sigma }(\mathbb{R}^{+})=B_{\sigma }(\mathbb{R}^{+})\cap C( \mathbb{R}^{+}) \). Note that [12] the sequence of positive linear operators \(\{L_{n}\}_{n\geqq 1}\) maps \(C_{\sigma }(\mathbb{R}^{+})\) into \(B_{\sigma }(\mathbb{R}^{+})\) if and only if
with \(\sigma (x)=1+\phi ^{2}(x)\), \(x\in \mathbb{R}^{+}\) and K is a positive constant. Let \(C_{\sigma }^{0}(\mathbb{R}^{+})\) be a subset of \(C_{\sigma }(\mathbb{R}^{+})\) such that
Theorem 3.2
Let\(\mathcal{S}_{n,\upsilon }^{\ast }\)be the sequence of positive linear operators acting from\(C_{\sigma }(\mathbb{R}^{+})\)into\(B_{\sigma }(\mathbb{R}^{+})\)such that
Then, for all\(f\in C_{\sigma }^{0}(\mathbb{R}^{+})\), we have
Proof
Consider \(\varphi (x)=x\), \(\sigma (x)=1+x^{2}\) and
Then, by Korovkin’s theorem, it is easily proved that \(\lim_{n\rightarrow \infty } \vert \vert \mathcal{S}_{n, \upsilon }^{\ast } ( e_{\ell };x ) -x^{\ell -1} \vert \vert _{\sigma }=0\), for \(\ell =1,2,3\). Hence, for any \(f\in C_{\sigma }^{0}(\mathbb{R}^{+})\), we get
□
Theorem 3.3
Let\(\mathcal{S}_{n,\upsilon }^{\ast }(\cdot ;\cdot )\)be the operators defined by (2.5). Then for every\(f \in C^{0}_{\sigma }(\mathbb{R}^{+})\), we have
Proof
We prove this theorem in the light of 3.2. Take \(f(t)=e_{\ell }\) defined by Lemma 2.3. Then, for any \(f(t)\in C_{\sigma }^{0}(\mathbb{R}^{+})\), \(\mathcal{S}_{n,\upsilon }^{\ast }(e_{\ell };x)\rightarrow x^{\ell -1}\) (\(\ell =1,2,3\)) uniformly as \(n\rightarrow \infty \). For \(\ell =1\), by applying Lemma 2.3, we get \(\mathcal{S}_{n,\upsilon }^{\ast }(e_{1};x)=1\), so that
Take \(\ell =2\) and \(x\geqq \frac{1}{2n}\), we get
In the case of \(0\leqq x\leqq \frac{1}{2n}\), we get
Then
In a similar way if take \(\ell =3\) and \(x\geqq \frac{1}{2n}\), we get
In the case of \(0\leqq x\leqq \frac{1}{2n}\), we get
This proves the theorem. □
4 Rate of convergence
We denote the set of all uniformly continuous functions by \(\tilde{C}[0,\infty )\). Let \(\tilde{\omega }(f;\tilde{\delta })\) denote the modulus of continuity of \(f\in \tilde{C}[0,\infty )\), i.e.
which satisfies \(\lim_{\tilde{\delta }\rightarrow 0+}\tilde{\omega }(f;\tilde{\delta })=0\), and
In the light of Lemmas 2.5 and 2.6 we use the notation
where
and
Theorem 4.1
For any\(f\in \tilde{C}[0,\infty )\),
where\(\tilde{\delta }_{n,\upsilon }(x)\)is defined by (4.4).
Proof
Since \(\mathcal{S}_{n,\upsilon }^{\ast }(e_{1};x)=1\), by (4.2) we get
From the Cauchy–Schwarz inequality we conclude that
Therefore,
Choose \(\tilde{\delta }=\tilde{\delta }_{n,\upsilon }(x)=\sqrt{\mathcal{S}_{n,\upsilon }^{\ast } ( \eta _{2};x ) }\), then we get the result. □
Here we use the usual class of Lipschitz functions and obtain the rate of convergence of the sequence of positive linear operators \(\mathcal{S}_{n,\upsilon }^{\ast }(\cdot ;\cdot )\) (2.5). For \(\mathcal{L}>0\), \(0<\varrho \leqq 1\) and for the continuous functions f on \([0,\infty ) \), the class of Lipschitz functions \(\mathrm{Lip}_{\mathcal{L},\varrho }(f)\) is
Theorem 4.2
For any\(f\in \mathrm{Lip}_{\mathcal{L},\varrho }\), we have
where\(\tilde{\delta }_{n,\upsilon }(x)\)is defined by (4.4).
Proof
By the Hölder inequality and (4.6), we get
□
The space of all continuous and bounded functions on \(\mathbb{R}^{+}\) is denoted by \(C_{B}(\mathbb{R}^{+})\) and
The norm on \(C_{B}^{2}(\mathbb{R}^{+})\) is given by
where the norm for \(C_{B}(\mathbb{R}^{+})\) is
Theorem 4.3
Let\(\psi \in C_{B}^{2}(\mathbb{R}^{+})\). Then
where\(\mu _{n,\upsilon }(x)=\tilde{\delta }_{n,\upsilon }(x)+ \frac{(\tilde{\delta }_{n,\upsilon }(x))^{2}}{2}\).
Proof
By a Taylor series expansion for \(\psi \in C_{B}^{2}(\mathbb{R}^{+})\) we obtain
where
Therefore,
By using the linearity of \(\mathcal{S}_{n,\upsilon }^{\ast }\) we get
Therefore
From the Cauchy–Schwarz inequality
Thus, we have
□
The Peetre’s K-functional is a result of potential research work on the approximation process presented by Peetre in 1968. Peetre was able to investigate the interpolation spaces between two Banach spaces and interactions with the real interpolation on the K-functional. For any \(f\in C_{B}(\mathbb{R}^{+})\), Peetre’s well-known K-functional property is defined as
where
For any \(\breve{\delta }>0\) and a positive constant \(\mathfrak{C}\) one has \(\mathcal{K}_{2}(f;\breve{\delta })\leqq \mathfrak{C}\omega _{2}(f; \breve{\delta }^{\frac{1}{2}})\), where
Theorem 4.4
Let\(f\in C_{B}(\mathbb{R}^{+})\). Then there exists a positive constant\(\mathfrak{D}\)such as
where\(\lambda _{n,\upsilon }(x)\)is given by4.3and\(\omega _{2} ( f;\frac{\lambda _{n,\upsilon }(x)}{2} ) \)is given by (4.4).
Proof
Take \(\psi \in C_{B}^{2}(\mathbb{R}^{+})\). Thus we get
By taking the infimum over all \(\psi \in C_{B}^{2}(\mathbb{R}^{+})\) and by using (4.10), we get
Now, for an absolute constant \(\mathfrak{D}>0\) in [10], we use the following relation:
where \(\breve{\delta }=\frac{\lambda _{n,\upsilon }(x)}{2}\). This completes the proof. □
For an arbitrary \(f\in C_{\sigma }^{0}(\mathbb{R}^{+})\) the following weighted modulus of continuity was defined in [4]:
satisfying
and
Theorem 4.5
For any\(f \in C^{0}_{\sigma }(\mathbb{R}^{+})\), we have
where\(\mathcal{A}\)is a positive constant and for\(x\geqq \frac{1}{2n}\)
and, for\(0\leqq x\leqq \frac{1}{2n}\),
Proof
We prove it by using (4.13), (4.15) and the Cauchy–Schwarz inequality. We have
From Lemmas 2.5, 2.6 we easily conclude that
where \(\mathcal{A}_{1}\) and \(\mathcal{A}_{2}\) are positive constants, and for \(x\geqq \frac{1}{2n}\)
and, for \(0\leqq x\leqq \frac{1}{2n}\),
By applying the Cauchy–Schwarz inequality we easily see that
Similarly for the constants \(\mathcal{A}_{3}>0\) and \(\mathcal{A}_{4}>0\), we have
Finally, in view of (4.16), (4.18)–(4.20) and choosing \(\hat{\delta }=O (\sqrt{\mathfrak{A}_{n,\upsilon }} )\), and \(\mathcal{A}=2(1+\mathcal{A}_{2}+2\mathcal{A}_{3}\mathcal{A}_{4})\), we easily are led to the desired result. □
5 Conclusion
In the present article, we have defined the operators (2.4) via a Dunkl generalization and further defined their Stancu type generalization by (2.5) and obtained their moments and central momemts of these operators. We have proved some Korovkin type and weighted Korovkin type theorems for the operators (2.5). Furthermore, we have studied the rate of convergence by means of the modulus of continuity, Lipschitz type maximal functions, Peetre’s K-functional and the second order modulus of continuity. It is to be noted that, if we take \(\alpha =\beta =0\) in (2.5), then the operators \(\mathcal{S}_{n,\upsilon }^{\ast }\) reduce to operators defined by (2.4) and if we take \(\chi _{n}(x)=x\) in \(\mathcal{P}_{n,\upsilon }\), then we get the operators defined and studied in [20].
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Mursaleen, M., Nasiruzzaman, M., Kılıçman, A. et al. Dunkl generalization of Phillips operators and approximation in weighted spaces. Adv Differ Equ 2020, 365 (2020). https://doi.org/10.1186/s13662-020-02820-9
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DOI: https://doi.org/10.1186/s13662-020-02820-9