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Approximation results on Dunkl generalization of Phillips operators via q-calculus
Advances in Difference Equations volume 2019, Article number: 244 (2019)
Abstract
The main purpose of this paper is to construct q-Phillips operators generated by Dunkl generalization. We prove several results of Korovkin type and estimate the order of convergence in terms of several moduli of continuity.
1 Introduction and auxiliary results
In 1950, Szász [27] defined the following operators for a continuous function \(f\in C[0, \infty )\):
provided that the series is convergent. In [26], Sucu approximated the Szász-operators defined by (1.1) by Dunkl generalization with an exponential function (see [24]). For \(\upsilon >-\frac{1}{2}\), Cheikh et al. [6] studied q-Hermite type polynomials and gave definitions of q-Dunkl analogues of exponential functions and recursion formula as follows:
The q-integer \([ n ] _{q}\) and q-factorial \([ n ] _{q}!\), respectively, are defined by
The q-calculus appeared as a new area in approximation theory and has a lot of applications in different mathematical areas and physics such as number theory, combinatorics, orthogonal polynomials, basic hypergeometric functions, quantum theory, mechanics, and the theory of relativity (see [13,14,15]).
Içöz [11] generalized the Dunkl–Szász operators defined by (1.1) via q-integers as follows:
for \(\upsilon >\frac{1}{2}\), \(x\geq 0\), \(0< q<1\) and \(f\in C[0,\infty )\).
Recent improvements of Szász type operators generated by exponential function via Dunkl generalization are given in [1,2,3, 12, 16,17,18, 20, 23, 25, 28].
The main purpose of this article is to construct the q-Phillips operators generated by Dunkl generalization via q-calculus. For more details on the approximation of classical Phillips operators via Dunkl type version, we refer to the recent article [21]. We obtain a Korovkin type result, as well as local and weighted approximations. We also study convergence properties by using the modulus of continuity and investigate the rate of convergence for functions belonging to the Lipschitz class. For further details and more information on approximation, we refer to [9, 10, 19].
For every \(f\in C_{\zeta }[0,\infty )=\{f\in C[0,\infty ): f(t)=O(t ^{\zeta }), t\rightarrow \infty \}\) and \(x\in [0,\infty )\), \(\zeta >n\), \(n \in \mathbb{N}\cup \{0\}\), \(\upsilon \geq - \frac{1}{2}\), we define
where
For the proof of a basic estimate, we use the generalized q-gamma function.
Definition 1.1
The generalized q-gamma function is defined by
where \(\varGamma _{q}(t)=K(A;t)\gamma _{q}^{A}(t)\) and \(K(A;t)= \frac{1}{1+A}A^{t} (1+\frac{1}{A} )_{q}^{t} (1+A ) _{q}^{t-1}\). Moreover, for any positive integer n, we have \(K(A;n)=q^{\frac{n(n-1)}{2}}\) and \(\varGamma _{q}(n)=q^{\frac{n(n-1)}{2}} \gamma _{q}^{A}(n)\), which also satisfy the following equation:
For more details, see [8].
2 Estimation of moments
Lemma 2.1
Let \(\mathcal{P}_{n,q}^{\ast }( \cdot ; \cdot )\) be the operators defined by (1.7). Then, we have
Proof
We prove this lemma by using the definition of generalized q-gamma function defined by Definition 1.1. More precisely,
If \(u=0\), then \(f(t)=1\), and hence
If \(u=1\), then \(f(t)=t\), and hence
Take \(u=2\), then, for \(f(t)=t^{2}\), we have
From [11] and by (1.6), we obtain
For \(u=3\), \(f(t)=t^{3}\) and for \(u=4\), \(f(t)=t^{4}\), we get
and
A simple calculation leads to
Hence by using the result for \(D_{n,q}(f;x)\) defined by (1.6) with \(f(t)=t^{3}\) and \(f(t)=t^{4}\) (see [11]), we get the required result. □
Lemma 2.2
Let \(\mathcal{P}_{n,q}^{\ast }( \cdot ; \cdot )\) be the operators defined by (1.7). Then, we have
3 Korovkin and weighted Korovkin type approximation
Korovkin’s approximation theory [4] has many applications in classical approximation theory, as well as in other branches of mathematics. In this section we obtained some approximation results via well known Korovkin’s type theorem and weighted Korovkin’s type theorem for the operators defined by (1.7).
Let \(C_{B}(\mathbb{R^{+}})\) be the set of all bounded and continuous functions on \(\mathbb{R^{+}}=[0,\infty )\), which is a linear normed space with
Let
In order to obtain the convergence results for the operators \(\mathcal{P}_{n,q}^{\ast }( \cdot ; \cdot )\) defined by (1.7), we take \(q=q_{n}\) (\(0< q_{n}<1\)) such that
for some constant α (\(0\leqq \alpha <1\)).
Theorem 3.1
Let \(q=q_{n}\), with \(0< q_{n}<1\), satisfy (3.1). Then, for any function \(f\in C[0, \infty )\cap E\),
Proof
The proof is based on the well-known Korovkin’s theorem regarding the convergence of a sequence of linear and positive operators, so it is enough to prove the conditions
uniformly on \([0,1]\).
Clearly, \(\frac{1}{[n]_{q}}\rightarrow 0\), (\(n\rightarrow \infty \)) we have
This completes the proof. □
We recall the weighted spaces of functions on \(\mathbb{R}^{+}\), which are defined as follows:
where \(\sigma (x)=1+x^{2}\) is a weight function and \(M_{f}\) is a constant depending only on f. Note that \(Q_{\sigma }(\mathbb{R}^{+})\) is a normed space with the norm \(\Vert f\Vert _{\sigma }= \sup_{x\geq 0}\frac{\vert f(x)\vert }{\sigma (x)}\).
Theorem 3.2
Let \(q=q_{n}\), with \(0< q_{n}<1\), satisfy (3.1). Then, for any function \(f\in Q_{\sigma }^{k}(\mathbb{R}^{+})\), we have
Proof
Take \(f(t)=t^{\tau }\). Then since \(f(t)\in C_{\sigma }^{k}(\mathbb{R} ^{+}) \), by Korovkin’s theorem, it satisfies \(\mathcal{P}_{n,q_{n}} ^{\ast }(t^{\tau }; x)\rightarrow x^{\tau }\) uniformly, whenever \(n\rightarrow \infty \). Therefore, by applying Lemma 2.1, since \(\mathcal{P}_{n,q_{n}}^{\ast }(1; x)=1\), we have
and
Then, clearly, \(\frac{1}{[n]_{q_{n}}}\rightarrow 0\) as \(n\rightarrow \infty \), which implies that
In similar way,
Thus we have
This completes the proof. □
4 Order of approximation
The modulus of continuity of f denoted by \(\omega (f;\delta )\) gives the maximum oscillation of f in any interval of length not exceeding \(\delta >0 \). For a function \(f\in C_{B}(\mathbb{R}^{+})\), it is given by
and, for any \(\delta >0\), one has
Theorem 4.1
Let \(f\in C_{B}(\mathbb{R}^{+})\) and \(x\in [0,\infty )\). Then we have
where \(q=q_{n}\) are numbers such that \(0< q_{n}<1\) and (3.1) holds, and \(\omega (f;\cdot )\) is the modulus of continuity defined by (4.1).
Proof
We prove it by using (4.1)–(4.2) and Cauchy–Schwarz inequality. Indeed,
where \(\mathcal{P}_{n,q_{n}}^{\ast } ( (q^{k+2\upsilon \theta _{k} }_{n} t-x )^{2};x ) \leq \mathcal{P}_{n,q_{n}}^{ \ast } ((t-x)^{2};x )\). And if we now choose \(\delta = \delta _{n}=\sqrt{\frac{1}{[n]_{q_{n}}}}\), then we get our result. □
Corollary 4.2
For \(\delta _{n}=\mathcal{P}_{n,q_{n}}^{\ast } ( (q^{k+2 \upsilon \theta _{k} }_{n} t-x )^{2};x )\), we have
5 Rate of convergence
Now we give the rate of convergence of the operators \(\mathcal{P}_{n,q} ^{\ast }(f;x)\) in terms of the elements of the usual Lipschitz class \(\operatorname{Lip}_{M}(\nu )\).
Let \(f\in C[0,\infty )\), \(M>0\) and \(0<\nu \leq 1\). The class \(\operatorname{Lip}_{M}(\nu )\) is defined as
Theorem 5.1
Let \(q=q_{n}\) be such that \(q_{n}\in (0,1)\) and (3.1) holds. Then, for each \(f\in \operatorname{Lip}_{M}(\nu )\) with \(M>0\), \(0<\nu \leq 1\), we have
Proof
We prove it by using (5.1) and Hölder’s inequality. Indeed,
Therefore,
This completes the proof. □
Let \(C_{B}[0,\infty )\) denote the space of all bounded and continuous functions defined on \(\mathbb{R}^{+}=[0,\infty )\) and
with the norm
also set
Theorem 5.2
Let \(\mathcal{P}_{n,q}^{\ast }( \cdot ; \cdot )\) be the operators defined by (1.7). Then, for \(q=q_{n}\) such that \(q_{n}\in (0,1) \) and any \(\psi \in C_{B}^{2}(\mathbb{R}^{+})\),
where \(\Delta _{n,q_{n}}=\frac{1}{{q_{n}}[n]_{q_{n}}}+\frac{1}{{2q_{n} ^{2}}[n]_{q_{n}}^{2}} (1+\frac{1}{q_{n}} )\) and \(\varPhi _{n,q_{n}}(x)=\frac{1}{2{q_{n}}^{2}[n]_{q_{n}}} (1+{q_{n}} ^{2}[1+2\upsilon ]_{q_{n}} )x\).
Proof
Let \(\psi \in C_{B}^{2}(\mathbb{R}^{+})\). Then, by using the generalized mean value theorem in the Taylor series expansion, we have
By applying the linearity property of \(\mathcal{P}_{n,{q_{n}}}^{ \ast }\), we have
which implies that
From (5.3) we have \(\Vert \psi ^{\prime } \Vert _{C_{B}(\mathbb{R}^{+})}\leq \Vert \psi \Vert _{C_{B}^{2}(\mathbb{R}^{+})}\) and \(\Vert \psi ^{\prime \prime }\Vert _{C_{B}(\mathbb{R}^{+})}\leq \Vert \psi \Vert _{C_{B}^{2}(\mathbb{R}^{+})}\), as well as
This completes the proof. □
6 Direct theorem
In 1968, J. Peetre [22] introduced a functional known as Peetre’s K-functional, which is defined by
There exits a positive constant \(C>0\) such that \(K_{2}(f,\delta ) \leq C\omega _{2}(f,\delta ^{\frac{1}{2}})\), \(\delta >0\), where the second-order modulus of continuity is given by
Theorem 6.1
For \(f\in C_{B}(\mathbb{R}^{+})\), \(x\in {}[ 0,\infty )\) and \(q=q_{n}\) satisfying (3.1), we have
where \(\mathcal{D}\) is a positive constant.
Proof
We prove this by using Theorem 5.2. Let \(\psi \in C_{B}(\mathbb{R}^{+}) \), then
By taking the infimum over all \(\psi \in C_{B}^{2}(\mathbb{R}^{+})\) and using (6.1), we get
Now, for an absolute constant \(\mathcal{D}>0\) provided in [7], we use the relation
This completes the proof. □
Atakut and Ispir [5] introduced the weighted modulus of continuity defined as
for an arbitrary \(f\in Q_{\sigma }^{k}(\mathbb{R}^{+})\). The two main properties of this modulus of continuity are \(\lim_{\delta \rightarrow 0}\varOmega (f;\delta )=0\) and
where \(t,x\in {}[ 0,\infty )\).
Theorem 6.2
Let \(q=q_{n}\) be numbers such that \(q_{n}\in (0,1)\) as \(n\rightarrow \infty \). Then, for every \(f\in Q_{\sigma }^{k}(\mathbb{R}^{+})\),
where the positive constant \(\mathcal{C}=1+\mathcal{C}_{1}+4 \mathcal{C}_{2}\) and
Proof
We use (6.3)–(6.4) and the Cauchy–Schwarz inequality. Thus we have
and
From Lemma 2.2, we easily see that
where
Now there exits a constant \(\mathcal{C}_{1}> 0\) such that
We easily conclude that
where
Since, \(\lim_{n \to \infty }\frac{1}{[n]_{q_{n}}^{i} }=0\) for all \(i=1,2,3,4\) and \(\lim_{n \to \infty }q_{n} =1\), for a constant \(\mathcal{C}_{1}>0\), we have
In the view of (6.7), we easily see that
Finally, in the light of equation (6.5) by combining (6.6)–(6.10), if we choose \(\delta =\sqrt{ \chi _{\upsilon ,q_{n}}(n)}\) and take the supremum over \(x\in {}[ 0,\chi _{\upsilon ,q_{n}}(n))\), we get the desired result. □
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The second author would like to thank Prince Sultan University for funding this work through research group “Nonlinear Analysis Methods in Applied Mathematics (NAMAM)” group number RG-DES-2017-01-17.
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Nasiruzzaman, M., Mukheimer, A. & Mursaleen, M. Approximation results on Dunkl generalization of Phillips operators via q-calculus. Adv Differ Equ 2019, 244 (2019). https://doi.org/10.1186/s13662-019-2178-1
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DOI: https://doi.org/10.1186/s13662-019-2178-1