The uniform convergence of a sequence of operators to a continuous function was introduced by Bohman [
9] and Korovkin [
16]. Through
q-calculus various modifications of Bernstein operators [
7] have been studied so far [
10,
18,
31]. The
\((p,q)\)-integers are the generalization of the
q-integers, which has an important role in the representation theory of quantum calculus in the physics literature. Recently, the approximation by the
\((p,q)\)-analog of a positive linear operator has become an active area of research. For the theory and numerical implementations of the
\((p,q)\)-analog of Bernstein operators introduced by Mursaleen et al. [
22] and other
\((p,q)\)-analogs, the reader may refer to [
1‐
5,
11‐
15,
19‐
21] and [
32]. For most recent work on the
\((p,q)\)-approximation we refer to [
8,
24,
26].
The
\((p,q)\)-integer,
\((p,q)\)-binomial expansion and the
\((p,q)\)-binomial coefficients are defined by
It can easily be verified by induction that
The
\((p,q)\)-analog of Euler’s identity is defined by
Let
\(f:[0,1]\longrightarrow \mathbb{R}\) and
\(q>p>1\). The
\((p,q)\)-Bernstein operators [
22] of
f is defined as
$$ B_{p,q}^{n}(f;x):=\sum_{k=0}^{n}f \biggl( \frac{[k]_{p,q}}{p^{k-n}[n]_{p,q}} \biggr) p_{n,k}(p,q;x),\quad n \in \mathbb{N}, $$
(1.1)
where the polynomial
\(p_{n,k}(p,q;x)\) is given by
(1.2)
For
\(p=1\),
\(B_{p,q}^{n} (f;x)\) turns into the
q-Bernstein operator. We have
$$ B_{p,q}^{n}(f;0)=f(0), \qquad B_{p,q}^{n}(f;1)=f(1), \quad n \in \mathbb{N}. $$
(1.3)
The following
\((p,q)\)-difference form of Bernstein operators [
25] is given by
$$ B_{p,q}^{n}(f;x):=\sum_{r=0}^{n} \lambda _{p,q}^{n} f \biggl[ 0,\frac{p ^{n-1}[1]_{p,q}}{[n]_{p,q}},\ldots, \frac{p^{n-r}[r]_{p,q}}{[n]_{p,q}} \biggr] x^{r}, $$
(1.4)
where
\(f[x_{0},x_{1},\ldots,x_{n}]\) indicates the
nth order divided difference of
f with pairwise distinct node, that is,
$$\begin{aligned}& f[x_{0}]=f(x_{0}), \qquad f[x_{0},x_{1}]= \frac{f(x_{1})-f(x_{0})}{x_{1}-x_{0}}, \\& f[x_{0},x_{1},\ldots,x_{n}]=\frac{f[x_{1},\ldots,x_{n}]-f[x_{0}, \ldots,x_{n-1}]}{[x_{n}-x_{0}]} \end{aligned}$$
and
\(\lambda _{p,q}^{n}\) is given by
(1.5)
and
\(\lambda _{p,q}^{0}=\lambda _{p,q}^{1}=1\),
\(0\leq \lambda _{p,q} ^{n}\leq 1\),
\(r=0,1,\ldots,n\).
Furthermore, as
\(n\rightarrow \infty \),
\(B_{p,q}^{n}(f;x)\rightarrow f(x)\) uniformly on any compact subset of
\((-\alpha,\alpha)\) and
\(B_{p,q}^{n}(f;x)\rightarrow \infty \) for
\(\vert x \vert > \alpha \),
\(x\notin \mathbb{J}_{p,q}\). Therefore, it is left to examine the case
\(\alpha \in \mathbb{J}_{p,q} \setminus \{0\}\) which is exactly the subject of the present paper. Let the function
\(f_{m}:\mathbb{R}\rightarrow \mathbb{R}\) be defined by
$$ f_{m}(x)= \textstyle\begin{cases} \frac{1}{(x-p^{m}q^{-m})^{j}},& x\in \mathbb{R}\setminus \{p^{m}q^{-m}\}, \\ a,& x=p^{m}q^{-m}, \end{cases}\displaystyle m\in \mathbb{N}_{0}, a \in \mathbb{R}. $$
(1.8)