Abstract
In the present paper, Stancu type generalizations of the q-analog of Lupaş Bernstein operators with shifted knots are introduced. Some approximation results and rate of convergence for these operators are investigated. A Voronovskaja type theorem and local approximation results for the mentioned operators are studied. The extra parameters γ, δ, q, a and b provide more flexibility for approximation.
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1 Introduction
Approximation theory basically deals with approximation of functions by simpler functions or more easily calculated functions. Broadly it is divided into theoretical and constructive approximation. In 1912 Bernstein [5] was the first to construct a sequence of positive linear operators as follows:
where \(u\in[0,1]\), f is bounded on \([0,1]\).
A constructive proof of the well-known Weierstrass approximation theorem using a probabilistic approach was provided. Here \(C[0,1]\) denotes the set of all continuous functions on \([0,1]\) which is equipped with the sup-norm \(\Vert\cdot \Vert\). He showed that if \(f\in C[0,1]\), then \(B_{m}(f;u)\) converges to \(f(u)\) uniformly on \([0,1]\). One can find a detailed monograph about the Bernstein polynomials in [12, 13].
Before proceeding, let us recall some basic definitions and notations of quantum calculus [9]. For any fixed real number \(q>0\) satisfying the conditions \(0< q\leq1\), the q-integer \([k]_{q}\), for \(k\in\mathbb{N}\) is defined as
and the q-factorial by
The q-binomial expansion is
and the q-binomial coefficients are as follows:
From the above
and
Gauss-formula is defined as
After development of q-calculus, Lupaş [14] in 1987 introduced the q-Lupaş operator (rational) as follows:
and studied its approximation properties.
Similarly, Phillips [23] in 1996 constructed another q-analog of Bernstein operators (polynomials) as follows:
where \(B_{m,q}\): \(C[0,1]\rightarrow C[0,1]\) defined for any \(m\in\mathbb{N}\) and any function \(f\in C[0,1]\).
Bases of these operators have been used in computer aided geometric design (CAGD) to study curves and surfaces. From then onward it became an active area of research in approximation theory as well as CAGD. In the recent past, q-analogs of various operators were investigated by several researchers (see [6, 15, 19, 22, 24]). Also see [1, 2, 4, 10, 11, 16–18, 21, 25] for other modifications.
In 1968 Stancu [26] showed that the Bernstein–Stancu polynomials
converge to continuous function \(f(u)\) uniformly in \([0,1]\) for each real γ, δ such that \(0\leq\gamma\leq\delta\).
In 2010, a new construction of Bernstein–Stancu type polynomials with shifted knots was introduced by Gadjiev and Gorhanalizadeh [8]:
where \(\frac{\gamma_{2}}{n+\delta_{2}}\leq u\leq\frac{n+\gamma_{2}}{n+ \delta_{2}}\) and \(\gamma_{k}\), \(\delta_{k}\) (\(k=1,2\)) are positive real numbers provided \(0\leq\gamma_{1}\leq\gamma_{2}\leq\delta_{1} \leq\delta_{2}\). It is clear that, for \(\gamma_{2}=\delta_{2}=0\), the polynomials (1.6) turn into the Bernstein–Stancu polynomials (1.5) and if \(\gamma_{1}=\gamma_{2}=\delta_{1}=\delta _{2}=0\) then these polynomials turn into the classical Bernstein polynomials.
Khalid et al. studied Bezier curves and surfaces using basis of shifted Bernstein polynomial in [11]. Recently, Mursaleen et al. [20] introduced and studied Lupaş Bernstein shifted operators based on q-integers as follows:
or
where \(\frac{\gamma}{[m]_{q}+\delta}\leq u\leq\frac{[m]_{q}+\gamma }{[m]_{q}+\delta}\) and γ, δ are positive real numbers provided \(0\leq\gamma\leq\delta\). In case \(\gamma=\delta=0\), the above operators (1.7) reduce to Lupaş q-Bernstein operators [14].
Motivated by the above work, in the next section we present a Stancu type modification of Lupaş q-Bernstein shifted operators and will study its approximation properties.
2 Construction of Lupaş q-Bernstein–Stancu shifted operators
Let \(\gamma,\delta\in\mathbb{N}_{0}\) (the set of all non-negative integers) be such that \(0\leq\gamma\leq\delta\) and \(0\leq a \leq b\), then we have
or
where \(\frac{\gamma}{[m]_{q}+\delta}\leq u\leq\frac{[m]_{q}+\gamma }{[m]_{q}+\delta}\) and γ, δ are positive real numbers provided \(0\leq\gamma\leq\delta\). In the case \(a= b=0\), the above operators (2.1) reduces to (1.7).
3 Definitions and auxiliary results
Lemma 3.1
Let \(S^{(\gamma,\delta,a,b)}_{m,q}(f;u)\)be given by (2.1). Then the following properties hold:
- (1)
\(S^{(\gamma,\delta,a,b)}_{m,q}(1;u)=1\),
- (2)
\(S^{(\gamma,\delta,a,b)}_{m,q}(t;u)= \frac{[m]_{q}+\delta}{{[m]_{q}}+b} ( u-\frac{\gamma}{[m]_{q}+ \delta} ) +\frac{a}{{[m]_{q}}+b}\),
- (3)
\(S^{(\gamma,\delta,a,b)}_{m,q}(t^{2};u)= (\frac{q^{2}[m-1]_{q}}{ {[m]_{q}}+b} ) ( \frac{[m]_{q}+\delta}{{[m]_{q}}+b} ) \frac{ (u-\frac{\gamma}{[m]_{q}+\delta} )^{2}}{ \lbrace\frac{[m]_{q}+ \gamma}{[m]_{q}+\delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta } ) \rbrace} + ( \frac{[m]_{q}+\delta}{{([m]_{q}}+b)^{2}} ) (u-\frac{ \gamma}{[m]_{q}+\delta} )+ \frac{a}{{([m]_{q}}+b)^{2}}\),
- (4)
\(S^{(\gamma,\delta,a,b)}_{m,q}(t^{3};u)= ( \frac{[m]_{q}+ \delta}{{[m]_{q}}+b} ) \frac{q^{6}[m-1]_{q}[m-2]_{q} (u-\frac{ \gamma}{[m]_{q}+\delta} )^{3}}{{([m]_{q}}+b)^{2} \lbrace\frac{[m]_{q}+ \gamma}{[m]_{q}+\delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta } ) \rbrace \lbrace\frac{[m]_{q}+\gamma}{[m]_{q}+ \delta}-u+q^{2} ( u-\frac{\gamma}{[m]_{q}+\delta} ) \rbrace}+(2+3a+q) (\frac{q^{2}[m-1]_{q}}{{([m]_{q}}+b)^{2}} ) ( \frac{[m]_{q}+ \delta}{[m]_{q}+b} ) \frac{ (u-\frac{\gamma}{[m]_{q}+ \delta} )^{2}}{ \lbrace\frac{[m]_{q}+\gamma}{[m]_{q}+ \delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta} ) \rbrace}+ {(1+3a^{2})} (\frac{[m]_{q}+\delta}{{([m]_{q}}+b)^{3}} ) (u-\frac{\gamma}{[m]_{q}+\delta} )+ \frac{a^{3}}{{([m]_{q}}+b)^{3}}+ \frac{3a^{2}([m]_{q}+\delta)}{{([m]_{q}}+b)^{3}}\),
- (5)
\(S^{(\gamma,\delta,a,b)}_{m,q}(t^{4};u)=\frac {q^{12}[m-1]_{q}[m-2]_{q}[m-3]_{q}}{ {([m]_{q}}+b)^{3}} (\frac{[m]_{q}+\delta}{{[m]_{q}}+b} ) \times\frac{1}{ \lbrace\frac{[m]_{q}+\gamma}{[m]_{q}+\delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta} ) \rbrace} \frac{ (u-\frac{\gamma}{[m]_{q}+\delta} )^{4}}{ \lbrace\frac{[m]_{q}+ \gamma}{[m]_{q}+\delta}-u+q^{2} ( u-\frac{\gamma}{[m]_{q}+ \delta} ) \rbrace \lbrace\frac{[m]_{q}+\gamma}{[m]_{q}+ \delta}-u+q^{3} ( u-\frac{\gamma}{[m]_{q}+\delta} ) \rbrace} +\frac{[m-1]_{q}[m-2]_{q}}{{([m]_{q}}+b)^{3}} (\frac{[m]_{q} + \delta}{[m]_{q}+b} ) \frac{(q^{8}+2q^{7}+3q^{6}+4aq^{2}) (u-\frac{ \gamma}{[m]_{q}+\delta} )^{3}}{ \lbrace\frac{[m]_{q}+ \gamma}{[m]_{q}+\delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta } ) \rbrace \lbrace\frac{[m]_{q}+\gamma}{[m]_{q}+ \delta}-u+q^{2} ( u-\frac{\gamma}{[m]_{q}+\delta} ) \rbrace} +(q^{4}+3q^{3}+3q^{2}+6a^{2}q^{2})\frac{[m-1]_{q}}{{([m]_{q}}+b)^{3}} ( \frac{[m]_{q}+\delta}{[m]_{q}+b} ) \frac{ (u-\frac{ \gamma}{[m]_{q}+\delta} )^{2}}{ \lbrace\frac{[m]_{q}+ \gamma}{[m]_{q}+\delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta } ) \rbrace}+(1+4a^{3}) ( \frac{[m]_{q}+\delta}{ {([m]_{q}}+b)^{4}} ) (u-\frac{\gamma}{[m]_{q}+\delta} )+ \frac{a^{4}}{{([m]_{q}}+b)^{4}}+ \frac{6a^{2}([m]_{q}+\delta)}{{([m]_{q}}+b)^{4}}\).
Proof
(1) By using definition of q-binomial coefficients and Gauss-formula, we have
(2) For \(f(t)=t\), we have
(3) For \(f(t)=t^{2}\), we have
after calculating the values of A, B and C we get
which is the required result.
(4) For \(f(t)=t^{3}\), we have
After calculating the values of D, E, F and G, we get
(5) For \(f(t)=t^{4}\), we have
After calculating the values of H, I, J, K and L, we obtain
□
Lemma 3.2
By using the linearity of operators \(S^{(\gamma,\delta,a,b)}_{m,q}(f;u)\)and by Lemma 3.1, for all \(u\in [\frac{\gamma}{[m]_{q}+\delta},\frac{[m]_{q}+\gamma }{[m]_{q}+\delta} ]\), we can acquire the central moments as
- (1)
\(S^{(\gamma,\delta,a,b)}_{m,q}((t-u);u)=\frac{[m]_{q}+\delta}{ {[m]_{q}}+b} ( u-\frac{\gamma}{[m]_{q}+\delta} ) +\frac{a}{ {[m]_{q}}+b}-u\),
- (2)
\(S^{(\gamma,\delta,a,b)}_{m,q}((t-u)^{2};u)= ( \frac{q^{2}[m-1]_{q}}{[m]_{q}+b} ) ( \frac{[m]_{q}+\delta}{[m]_{q}+b} ) \frac{ (u-\frac{\gamma }{[m]_{q}+\delta} )^{2}}{ \lbrace\frac{[m]_{q}+\gamma}{[m]_{q}+ \delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta} ) \rbrace} + ( \frac{1}{[m]_{q}+b}-2u ) ( \frac{[m]_{q}+\delta }{[m]_{q}+b} ) (u-\frac{\gamma}{[m]_{q}+\delta} )+ ( \frac{a}{[m]_{q}+b} ) ( \frac{1}{[m]_{q}+b}-2u ) +u^{2}\),
- (3)
\(S^{(\gamma,\delta,a,b)}_{m,q}((t-u)^{3};u) = ( \frac{[m]_{q}+ \delta}{{[m]_{q}}+b} ) \frac{q^{6}[m-1]_{q}[m-2]_{q} (u-\frac{ \gamma}{[m]_{q}+\delta} )^{3}}{{([m]_{q}}+b)^{2} \lbrace\frac{[m]_{q}+ \gamma}{[m]_{q}+\delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta } ) \rbrace \lbrace\frac{[m]_{q}+\gamma}{[m]_{q}+ \delta}-u+q^{2} ( u-\frac{\gamma}{[m]_{q}+\delta} ) \rbrace} + (\frac{2+3a+q}{({[m]_{q}}+b)}-3u ) (\frac{q^{2}[m-1]_{q}}{ {([m]_{q}}+b)} ) ( \frac{[m]_{q}+\delta}{[m]_{q}+b} ) \frac{ (u-\frac{\gamma}{[m]_{q}+\delta} )^{2}}{ \lbrace\frac{[m]_{q}+ \gamma}{[m]_{q}+\delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta } ) \rbrace} + (\frac{1+3a^{2}}{{([m]_{q}}+b)^{2}}-\frac{3u}{{[m]_{q}}+b}+3u ^{2} ) (\frac{[m]_{q}+\delta}{{[m]_{q}}+b} ) (u-\frac{ \gamma}{[m]_{q}+\delta} )+\frac{a^{3}}{{([m]_{q}}+b)^{3}} +3a^{2} (\frac{[m]_{q}+\delta}{{([m]_{q}}+b)^{3}} ) -\frac{3u ^{2}a}{{([m]_{q}}+b)^{2}}+3u^{2} (\frac{[m]_{q}+\delta}{{[m]_{q}}+b} ) (u-\frac{\gamma}{[m]_{q}+\delta} )+ \frac{3a^{2}}{{([m]_{q}}+b)}-u^{3}\),
- (4)
\(S^{(\gamma,\delta,a,b)}_{m,q}((t-u)^{4};u)=\frac {q^{12}[m-1]_{q}[m-2]_{q}[m-3]_{q}}{ {([m]_{q}}+b)^{3}} (\frac{[m]_{q} +\delta}{{[m]_{q}}+b} ) \frac{1}{ \lbrace\frac{[m]_{q}+\gamma}{[m]_{q}+\delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta} ) \rbrace} \frac{ (u-\frac{\gamma}{[m]_{q}+\delta} )^{4}}{ \lbrace\frac{[m]_{q}+ \gamma}{[m]_{q}+\delta}-u+q^{2} ( u-\frac{\gamma}{[m]_{q}+ \delta} ) \rbrace \lbrace\frac{[m]_{q}+\gamma}{[m]_{q}+ \delta}-u+q^{3} ( u-\frac{\gamma}{[m]_{q}+\delta} ) \rbrace} +\frac{[m-1]_{q}[m-2]_{q}}{{([m]_{q}}+b)^{2}} (\frac{[m]_{q} + \delta}{{[m]_{q}}+b} ) \frac{ ( \frac{q^{8}+3q^{5}+2q^{6}+4aq ^{2}}{[m]_{q}}-4uq^{6} ) (u-\frac{\gamma}{[m]_{q}+\delta } )^{3}}{ \lbrace\frac{[m]_{q}+\gamma}{[m]_{q}+\delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta} ) \rbrace \lbrace\frac{[m]_{q}+ \gamma}{[m]_{q}+\delta}-u+q^{2} ( u-\frac{\gamma}{[m]_{q}+ \delta} ) \rbrace} + ( \frac{q^{4}+3q^{3}+3q^{2}+6a^{2}q^{2}}{{([m]_{q}}+b)^{2}}-\frac{8uq ^{2}+4uq^{3}+12auq^{2}}{[m]_{q}+b}-2u^{2}q^{2} ) \frac{[m-1]_{q}}{[m]_{q}+b} ( \frac{[m]_{q}+\delta}{[m]_{q}+b} )\frac{ (u-\frac{\gamma}{[m]_{q}+\delta} )^{2}}{ \lbrace\frac{[m]_{q}+ \gamma}{[m]_{q}+\delta}-u+q ( u-\frac{\gamma}{[m]_{q}+\delta } ) \rbrace} + ( \frac{1+4a^{3}}{{([m]_{q}}+b)^{3}}- \frac{4u+12a^{2}u}{{([m]_{q}}+b)^{2}}+\frac{2u^{2}}{{[m]_{q}}+b}-4u ^{3} ) ( \frac{[m]_{q}+\delta}{{[m]_{q}}+b} ) (u-\frac{\gamma}{[m]_{q}+\delta} ) + ( \frac{3a^{2}}{ {[m]_{q}}+b}-12a^{2}u ) ( \frac{[m]_{q}+\delta}{{([m]_{q}}+b)^{3}} )+ \frac{a^{4}}{{([m]_{q}}+b)^{4}}- \frac{4a^{3}u}{{([m]_{q}}+b)^{3}}- \frac{4au ^{2}}{{([m]_{q}}+b)^{2}} \frac{4au^{3}}{{[m]_{q}}+b}+u^{4}\).
We can easily see that \(S^{(\gamma,\delta,a,b)}_{m,q}(f;u)\)are positive linear operators.
4 Main results
Firstly, we prove some theorems on the convergence of \(S^{(\gamma, \delta,a,b)}_{m,q}(f;u)\) to \(f(u)\).
Theorem 4.1
Let \(f \in C[0,1]\)and the sequence \(q_{m}\)satisfying \(0< q_{m}<1\)such that \(q_{m}\to1\)as \(m\to\infty\). Then
Proof
From Lemma 3.1, it follows that
Consider the sequence of operators
Then obviously
and using (4.1) we obtain
Now, by applying the Korovkin theorem [12] (see also [3]) to the sequence of positive linear operators \(S^{*}_{m}\), we obtain
for every continuous function f. Therefore (4.2) gives
and the proof is completed. □
Theorem 4.2
Iffbe a continuous function on \([0,1]\)and taking \(0< q<1\), then
where
Proof
For any \(u,y\in[a,b]\), it is well known that
Therefore, we get
Choosing \(\sigma=\sigma_{m}=\sqrt{S^{(\gamma,\delta,a,b)}_{m,q} ((t-u)^{2};u ) )}\), we have
Thus, we obtain the desired result. □
Theorem 4.3
(Voronovskaja type theorem)
Let \(f^{\prime\prime}\in C[0,1]\)and \({(q_{m})}_{m\in\mathbb{N}}\subseteq(0,1)\)be a sequence such that \(q_{m}\to1\)as \(m\to\infty\)and \(q^{m}_{m}\to0\)as \(m\to\infty\). Then
uniformly on \([\frac{\gamma}{[m]_{q_{m}}+\delta},\frac{[m]_{q _{m}}+\gamma}{[m]_{q_{m}}+\delta} ]\).
Proof
By the Taylor formula we may write
where \(r(t,u)\) is the remainder term and \(\lim_{t\rightarrow u}r(t,u)=0\). Applying \(S^{(\gamma,\delta ,a,b)}_{m,q_{m}}(f;u)\) to (4.3), we obtain
By the Cauchy–Schwartz inequality, we have
Observe that \(r^{2}(u,u)=0\) and \(r^{2}(\cdot ,u)\in C[0,1]\), then it follows from Theorem 4.2 that
uniformly with respect to \(u\in [\frac{\gamma}{[m]_{q_{m}}+ \delta},\frac{[m]_{q_{m}}+\gamma}{[m]_{q_{m}}+\delta} ]\). From (4.4) and (4.5), we get
Now we compute the following:
uniformly in \([\frac{\gamma}{[m]_{q_{m}}+\delta},\frac{[m]_{q _{m}}+\gamma}{[m]_{q_{m}}+\delta} ]\).
Finally using the above two equalities, we have
This completes the proof of the theorem. □
5 Local approximation
If \(\sigma>0\) and \(W^{2}=\{s\in C[0,1]; s^{\prime},s^{\prime\prime }\in C[0,1]\}\), then the K-functional is defined as
By [7], p. 177, Theorem 2.4, there exists an absolute constant \(C>0\) such that
where the second order modulus of smoothness for \(f\in C[0,1]\) is defined as
The usual modulus of continuity for \(f\in C[0,1]\) is defined as
Our next main result is the following local approximation theorem.
Theorem 5.1
Letfbe a continuous function on \([0,1]\)with \(0< q<1\). Then, for every \(u\in [\frac{\gamma}{[m]_{q}+\delta},\frac{[m]_{q}+\gamma}{[m]_{q}+ \delta} ]\), we have
where
Proof
We define
From Lemma 3.1, we find
and
Let \(s\in W^{2}\). By using Taylor’s formula we have
we get
which implies that
From Lemma 3.2(1), we have
On the other hand, we have
Now, for \(f\in C[0,1]\) and \(s\in W^{2}\), by using (5.7) and (5.8), we get
Taking the infimum on the right hand side over all \(s\in W^{2}\), we obtain
Now using (5.3), we have
This completes the proof. □
6 Conclusion
It can be concluded that the parameters γ, δ, q, a and b will provide more modeling flexibility for approximation of functions and bases of these operators can be used to draw curves and surfaces in CAGD. Also the approximation results derived for shifted intervals will be very helpful when it comes to implementation using computers for simulation purposes.
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The authors are extremely thankful to the referees for their valuable comments and suggestion. The second author is also grateful to Council of Scientific and Industrial Research (CSIR), India, for providing the Senior Research Fellowship (09/1172(0001)/2017-EMR-I).
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Mursaleen, M., Qasim, M., Khan, A. et al. Stancu type q-Bernstein operators with shifted knots. J Inequal Appl 2020, 28 (2020). https://doi.org/10.1186/s13660-020-2303-4
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DOI: https://doi.org/10.1186/s13660-020-2303-4