Lemma 1
We have
$$ \lambda_{kn}= \int_{A_{n}}q_{nk} ( t ) \,dt= \biggl( \frac {n}{n+\beta _{2}} \biggr) ^{n+1}\frac{1}{n+1},\quad k=0,1,\ldots,n. $$
(2.1)
Proof
For \(p,q=1,2,\ldots\) , set
$$\begin{aligned} B^{\ast} ( p,q ) :=& \int_{A_{n}} \biggl( x-\frac{\alpha_{2}}{n+\beta_{2}} \biggr) ^{p-1} \biggl( \frac{n+\alpha_{2}}{n+\beta_{2}}-x \biggr) ^{q-1}\,dx \\ = & \int_{0}^{\frac{n}{n+\beta_{2}}}x^{p-1} \biggl( \frac{n}{n+\beta_{2}} -x \biggr) ^{q-1}\,dx. \end{aligned}$$
Then
$$\begin{aligned} B^{\ast} ( p,q ) =&\frac{q-1}{p} \int_{0}^{\frac{n}{n+\beta _{2}}}x^{p} \biggl( \frac{n}{n+\beta_{2}}-x \biggr) ^{q-2}\,dx \\ =&\frac{q-1}{p} \int_{0}^{\frac{n}{n+\beta_{2}}} \biggl( \frac {n}{n+\beta_{2}}x^{p-1}-x^{p-1} \biggl( \frac{n}{n+\beta_{2}}-x \biggr) \biggr) \biggl( \frac{n}{n+\beta_{2}}-x \biggr) ^{q-2}\,dx \\ =&\frac{q-1}{p}\cdot\frac{n}{n+\beta_{2}}B^{\ast} ( p,q-1 ) -\frac{q-1}{p}B^{\ast} ( p,q ) , \end{aligned}$$
which implies that
$$ B^{\ast} ( p,q ) =\frac{q-1}{p+q-1}\cdot\frac{n}{n+\beta _{2}}B^{\ast} ( p,q-1 ) . $$
Therefore,
$$\begin{aligned} \lambda_{kn} =&\binom{n}{k}B^{\ast} ( k+1,n-k+1 ) \\ =& \biggl( \frac{n}{n+\beta_{2}} \biggr) ^{n-k}\binom{n}{k} \frac{ ( n-k ) ( n-k-1 ) \cdots2\cdot1}{ ( n+1 ) n\cdots ( k+2 ) }B^{\ast} ( k+1,1 ) \\ =& \biggl( \frac{n}{n+\beta_{2}} \biggr) ^{n-k}\frac{k+1}{ ( n+1 ) }\int_{0}^{\frac{n}{n+\beta_{2}}}x^{k}\,dx \\ =& \biggl( \frac{n}{n+\beta_{2}} \biggr) ^{n+1}\frac{1}{n+1}. \end{aligned}$$
 □
Lemma 2
For any
\(x\in A_{n}\), we have
$$ \widetilde{S}_{n,\alpha,\beta} \bigl( ( t-x ) ^{2},x \bigr) \leq \frac{C}{n}\delta_{n}^{2} ( x ) . $$
(2.2)
Proof
Write
$$ \widetilde{D}_{n,\alpha,\beta} ( f,x ) := \biggl( \frac {n+\beta_{2}}{n} \biggr) ^{2n+1}\sum_{k=0}^{n}q_{nk} ( x ) ( n+1 ) \int_{A_{n}}q_{nk} ( t ) f ( t ) \,dt. $$
Then [7]
$$\begin{aligned}& \widetilde{D}_{n,\alpha,\beta} ( 1,x ) =1, \widetilde {D}_{n,\alpha,\beta} ( t,x ) =\frac{n}{n+2}x+\frac{n+2\alpha _{2}}{ ( n+2 ) ( n+\beta_{2} ) }, \\& \widetilde{D}_{n,\alpha,\beta} \bigl( t^{2},x \bigr) = \biggl( x- \frac{ \alpha_{2}}{n+\beta_{2}} \biggr) ^{2}\frac{n ( n-1 ) }{ ( n+2 ) ( n+3 ) } \\& \hphantom{\widetilde{D}_{n,\alpha,\beta} \bigl( t^{2},x \bigr) ={}}{}+\frac{n}{n+\beta_{2}} \biggl( x-\frac{\alpha_{2}}{n+\beta_{2}} \biggr) \frac{4n}{ ( n+2 ) ( n+3 ) } \\& \hphantom{\widetilde{D}_{n,\alpha,\beta} \bigl( t^{2},x \bigr) ={}}{}+ \biggl( \frac{n}{n+\beta_{2}} \biggr) ^{2} \frac{2}{ ( n+2 ) ( n+3 ) }+\frac{2n\alpha_{2}}{ ( n+2 ) ( n+\beta _{2} ) } \biggl( x-\frac{\alpha_{2}}{n+\beta_{2}} \biggr) \\& \hphantom{\widetilde{D}_{n,\alpha,\beta} \bigl( t^{2},x \bigr) ={}}{}+\frac{2n\alpha_{2}}{ ( n+2 ) ( n+\beta_{2} ) ^{2}}+ \biggl( \frac{\alpha_{2}}{n+\beta_{2}} \biggr) ^{2}, \end{aligned}$$
(2.3)
and
$$ \widetilde{D}_{n,\alpha,\beta} \bigl( ( t-x ) ^{2},x \bigr) \leq \frac{C}{n}\delta_{n}^{2} ( x ) . $$
By the facts that
$$\begin{aligned}& \widetilde{S}_{n,\alpha,\beta} ( 1,x ) =\widetilde {D}_{n,\alpha ,\beta} ( 1,x ) =1, \\& \widetilde{S}_{n,\alpha,\beta} ( t,x ) =\frac{n}{n+\beta _{1}}\widetilde{D}_{n,\alpha,\beta} ( t,x ) +\frac{\alpha_{1}}{n+\beta_{1}}, \end{aligned}$$
(2.4)
and
$$ \widetilde{S}_{n,\alpha,\beta} \bigl( t^{2},x \bigr) =\frac {n^{2}}{ ( n+\beta_{1} ) ^{2}} \widetilde{D}_{n,\alpha,\beta} \bigl( t^{2},x \bigr) + \frac{2n\alpha_{1}}{ ( n+\beta_{1} ) ^{2}}\widetilde{D}_{n,\alpha,\beta} ( t,x ) + \frac{\alpha _{1}^{2}}{ ( n+\beta_{1} ) ^{2}}, $$
we get
$$\begin{aligned} \widetilde{S}_{n,\alpha,\beta} \bigl( ( t-x ) ^{2},x \bigr) =&\frac{n^{2}}{ ( n+\beta_{1} ) ^{2}}\widetilde{D}_{n,\alpha ,\beta } \bigl( ( t-x ) ^{2},x \bigr) \\ &{}+ \biggl( \frac{2n^{2}x}{ ( n+\beta_{1} ) ^{2}}+\frac {2n\alpha_{1}}{ ( n+\beta_{1} ) ^{2}}-\frac{2nx}{n+\beta_{1}} \biggr) \widetilde{D}_{n,\alpha,\beta} ( t,x ) \\ &{}+\frac{\alpha_{1}^{2}}{ ( n+\beta_{1} ) ^{2}}-\frac{2\alpha _{1}x}{n+\beta_{1}}+x^{2}-\frac{n^{2}}{ ( n+\beta_{1} ) ^{2}}x^{2} \\ =&\frac{n^{2}}{ ( n+\beta_{1} ) ^{2}}\widetilde{D}_{n,\alpha ,\beta} \bigl( ( t-x ) ^{2},x \bigr) +\frac{ ( \beta _{1}^{2}+4\beta_{1} ) n+2\beta_{1}^{2}}{ ( n+\beta_{1} ) ^{2} ( n+2 ) }x^{2} \\ &{}+\frac{2\alpha_{1} ( \beta_{1}+\beta_{2}+2 ) n^{2}+2n\alpha _{1} ( \beta_{1}\beta_{2}+2\beta_{1}+2\beta_{2} ) +4\alpha _{1}\beta_{1}\beta_{2}}{ ( n+\beta_{1} ) ^{2} ( n+2 ) ( n+\beta_{2} ) }x \\ &{}+\frac{\alpha_{1}^{2}}{ ( n+\beta_{1} ) ^{2}} \\ \leq&\widetilde{D}_{n,\alpha,\beta} \bigl( ( t-x ) ^{2},x \bigr) + \frac{C}{n^{2}} \\ \leq&\frac{C}{n}\delta_{n}^{2} ( x ) . \end{aligned}$$
 □
Lemma 3
For any given
\(\gamma\geq0\), we have
$$ \sum_{k=0}^{n}\biggl\vert \frac{k+\alpha_{2}}{n+\beta_{2}}-x\biggr\vert ^{\gamma}\bigl\vert q_{nk} ( x ) \bigr\vert \leq C\frac {\delta _{n}^{\gamma} ( x ) }{n^{\gamma/2}}, \quad x\in [ 0,1 ] . $$
(2.5)
Proof
It was showed in [3] that
$$ \sum_{k=0}^{n}\biggl\vert \frac{k+\alpha_{1}}{n+\beta_{1}}-x\biggr\vert ^{\gamma}\bigl\vert q_{nk} ( x ) \bigr\vert \leq C\frac{ ( \delta_{n}^{\ast} ( x ) ) ^{\gamma}}{n^{\gamma /2}}, \quad x\in [ 0,1 ] , $$
(2.6)
where \(\delta_{n}^{\ast} ( x ) :=\psi ( x ) +\frac {1}{\sqrt{n}}\) and \(\psi ( x ) =\sqrt{x ( 1-x ) }\). We verify that
$$ \delta_{n}^{\ast} ( x ) \sim\delta_{n} ( x ) ,\quad x\in [ 0,1 ] . $$
(2.7)
In fact, when \(x\in [ \frac{2\alpha_{2}+1}{n+\beta_{2}},\frac{ n-\beta_{2}+2\alpha_{2}}{n+\beta_{2}} ] \), we have
$$\begin{aligned}& \frac{1}{2}x\leq x-\frac{\alpha_{2}}{n+\beta_{2}}\leq x, \\& \frac{1}{2} ( 1-x ) \leq\frac{n+\alpha_{2}}{n+\beta _{2}}-x\leq 1-x. \end{aligned}$$
Thus,
$$ \psi ( x ) \sim\varphi ( x ) , $$
which implies (2.7) for \(x\in [ \frac{2\alpha_{2}+1}{n+\beta _{2}},\frac{n-\beta_{2}+2\alpha_{2}}{n+\beta_{2}} ] \). When \(x\in [ 0,\frac{2\alpha_{2}+1}{n+\beta_{2}} ) \cup ( \frac{n-\beta _{2}+2\alpha_{2}}{n+\beta_{2}},1 ] \), we have
$$ \delta_{n}^{\ast} ( x ) \sim\delta_{n} ( x ) \sim \frac{1}{\sqrt{n}}, $$
(2.8)
and thus (2.7) also holds.
Now, by (2.6) and (2.7), we have
$$\begin{aligned} \sum_{k=0}^{n}\biggl\vert \frac{k+\alpha_{2}}{n+\beta_{2}}-x\biggr\vert ^{\gamma}\bigl\vert q_{nk} ( x ) \bigr\vert \leq &\sum_{k=0}^{n}\biggl\vert \frac{k+\alpha_{2}}{n+\beta_{2}}-\frac {k+\alpha _{1}}{n+\beta_{1}}\biggr\vert ^{\gamma}\bigl\vert q_{nk} ( x ) \bigr\vert +\sum_{k=0}^{n} \biggl\vert \frac{k+\alpha_{1}}{n+\beta_{1}} -x\biggr\vert ^{\gamma}\bigl\vert q_{nk} ( x ) \bigr\vert \\ \leq&\frac{C}{n^{\gamma}}\sum_{k=0}^{n}\bigl\vert q_{nk} ( x ) \bigr\vert +C\frac{\delta_{n}^{\gamma} ( x ) }{n^{\gamma /2}} \\ \leq&C\frac{\delta_{n}^{\gamma} ( x ) }{n^{\gamma/2}}. \end{aligned}$$
 □
Lemma 4
For any
\(x\in A_{n}\), we have
$$ \sum_{k=0}^{n}q_{nk} ( x ) ( n+1 ) \int_{A_{n}}\delta _{n}^{2} ( t ) q_{nk} ( t ) \,dt\leq C\delta _{n}^{2} ( x ) $$
(2.9)
and
$$ \sum_{k=0}^{n-1}q_{n-1,k} ( x ) n \int_{A_{n}}\delta _{n}^{-2} ( t ) q_{n+1,k+1} ( t ) \,dt\leq C\delta_{n}^{-2} ( x ) . $$
(2.10)
Proof
By a similar calculation to that of Lemma 1, we have
$$ \int_{A_{n}}\varphi^{2} ( t ) q_{nk} ( t ) \,dt= \biggl( \frac{n}{n+\beta_{2}} \biggr) ^{n+3}\frac{ ( n-k+1 ) ( k+1 ) }{ ( n+3 ) ( n+2 ) ( n+1 ) }. $$
(2.11)
On the other hand, we have
$$\begin{aligned} \sum_{k=0}^{n} \biggl( \frac{k}{n}- \frac{k^{2}}{n^{2}} \biggr) q_{nk} ( x ) =& \biggl( \frac{n}{n+\beta_{2}} \biggr) ^{n-1} \biggl( x-\frac{\alpha_{2}}{n+\beta_{2}} \biggr) - \biggl( \frac{n}{n+\beta_{2}} \biggr) ^{n-1}\frac{ ( x-\frac{\alpha_{2}}{n+\beta_{2}} ) }{n} \\ &{}-\frac{n-1}{n} \biggl( \frac{n}{n+\beta_{2}} \biggr) ^{n-2} \biggl( x-\frac{\alpha_{2}}{n+\beta_{2}} \biggr) ^{2} \\ =&\frac{n-1}{n} \biggl( \frac{n}{n+\beta_{2}} \biggr) ^{n-2}\varphi ^{2} ( x ) . \end{aligned}$$
Therefore,
$$\begin{aligned} \sum_{k=0}^{n}q_{nk} ( x ) ( n+1 ) \int_{A_{n}}\delta _{n}^{2} ( t ) q_{nk} ( t ) \,dt \leq& 2\sum_{k=0}^{n}q_{nk} ( x ) ( n+1 ) \int_{A_{n}} \biggl( \varphi^{2} ( t ) + \frac{1}{n} \biggr) q_{nk} ( t ) \,dt \\ \leq&2\sum_{k=0}^{n}q_{nk} ( x ) \biggl( \frac{n}{n+\beta _{2}} \biggr) ^{n+3}\frac{ ( n-k+1 ) ( k+1 ) }{ ( n+3 ) ( n+2 ) } \\ &{}+ \frac{C}{n}\sum_{k=0}^{n}q_{nk} ( x ) \\ \leq&C\sum_{k=0}^{n}q_{nk} ( x ) \biggl( \frac{ ( n-k ) k}{n^{2}}+\frac{1}{n} \biggr) +\frac{C}{n} \\ \leq&C\delta_{n}^{2} ( x ) , \end{aligned}$$
which proves (2.9).
By Lemma 1, we have
$$\begin{aligned} n \int_{A_{n}}\delta_{n}^{-2} ( t ) q_{n+1,k+1} ( t ) \,dt \leq&Cn \int_{A_{n}} \bigl( \varphi^{-2} ( t ) +n \bigr) q_{n+1,k+1} ( t ) \,dt \\ \leq&Cn \biggl( \int_{A_{n}}\varphi^{-2} ( t ) q_{n+1,k+1} ( t ) \,dt+1 \biggr) \\ =&Cn \biggl( \frac{ ( n+1 ) n}{ ( k+1 ) ( n-k ) }\int_{A_{n}}q_{n-1,k} ( t ) \,dt+1 \biggr) \\ \leq&Cn \biggl( \frac{ ( n+1 ) }{ ( k+1 ) ( n-k ) }+1 \biggr) \\ \leq&Cn. \end{aligned}$$
Then
$$\begin{aligned} \sum_{k=0}^{n-1}q_{n-1,k} ( x ) n \int_{A_{n}}\delta _{n}^{-2} ( t ) q_{n+1,k+1} ( t ) \,dt \leq &Cn\sum_{k=0}^{n}q_{n-1,k} ( x ) \\ =&Cn\leq C\delta_{n}^{-2} ( x ) . \end{aligned}$$
Hence, (2.10) is proved. □
Lemma 5
If
f
is
r
times differentiable on
\([ 0,1 ] \), then
$$\begin{aligned} \widetilde{S}_{n,\alpha,\beta}^{(r)} ( f,x ) =& \biggl( \frac{ n+\beta_{2}}{n} \biggr) ^{2n+1} \biggl( \frac{n}{n+\beta_{1}} \biggr) ^{r} \frac{ ( n+1 ) !n!}{ ( n-r ) ! ( n+r ) !}\sum_{k=0}^{n-r}q_{n-r,k}(x) \\ &{}\times \int_{\frac{\alpha_{2}}{n+\beta_{2}}}^{\frac{n+\alpha_{2}}{n+\beta_{2}}}q_{n+r,k+r}(t)f^{(r)} \biggl( \frac{nt+\alpha_{1}}{n+\beta _{1}} \biggr) \,dt. \end{aligned}$$
(2.12)
Proof
By using Leibniz’s theorem, we have
$$\begin{aligned} \widetilde{S}_{n,\alpha,\beta}^{(r)} ( f,x ) =& \biggl( \frac{ n+\beta_{2}}{n} \biggr) ^{2n+1}\sum_{i=0}^{r} \sum_{k=i}^{n-r+i}\binom {r}{i}\frac{(-1)^{r-i} ( n+1 ) !}{ ( k-i ) ! ( n-k-r+i ) !} \\ &{}\times \biggl( x-\frac{\alpha_{2}}{n+\beta_{2}} \biggr) ^{k-i} \biggl( \frac{n+\alpha_{2}}{n+\beta_{2}}-x \biggr) ^{n-k-r+i} \int_{\frac{\alpha _{2}}{n+\beta_{2}}}^{\frac{n+\alpha_{2}}{n+\beta_{2}}}q_{nk}(t)f \biggl( \frac{nt+\alpha_{1}}{n+\beta_{1}} \biggr) \,dt \\ =& \biggl( \frac{n+\beta_{2}}{n} \biggr) ^{2n+1}\sum _{k=i}^{n-r+i}\sum_{i=0}^{r} \binom{r}{i}\frac{(-1)^{r-i} ( n+1 ) !}{ ( n-r ) !}q_{n-r,k-i}(x) \\ &{}\times \int_{\frac{\alpha_{2}}{n+\beta_{2}}}^{\frac{n+\alpha_{2}}{n+\beta_{2}}}q_{nk}(t)f \biggl( \frac{nt+\alpha_{1}}{n+\beta_{1}} \biggr) \,dt \\ =& \biggl( \frac{n+\beta_{2}}{n} \biggr) ^{2n+1}\frac{ ( n+1 ) !}{ ( n-r ) !}\sum _{k=0}^{n-r}(-1)^{r}q_{n-r,k}(x) \\ &{}\times \int_{\frac{\alpha_{2}}{n+\beta_{2}}}^{\frac{n+\alpha_{2}}{n+\beta_{2}}}\sum_{i=0}^{r} \binom{r}{i}(-1)^{i}q_{n,k+i}(t)f \biggl( \frac{nt+\alpha_{1}}{n+\beta_{1}} \biggr) \,dt. \end{aligned}$$
Since
$$ \frac{d^{r}}{dt^{r}}q_{n+r,k+r}(t)=\sum_{i=0}^{r} \binom {r}{i}(-1)^{i}\frac{ ( n+r ) !}{n!}q_{n,k+i}(t), $$
we have
$$\begin{aligned} \widetilde{S}_{n,\alpha,\beta}^{(r)} ( f,x ) =& \biggl( \frac{n+\beta_{2}}{n} \biggr) ^{2n+1}\frac{ ( n+1 ) !n!}{ ( n-r ) ! ( n+r ) !}\sum_{k=0}^{n-r}q_{n-r,k}(x) \\ &{}\times\int_{\frac{\alpha_{2}}{n+\beta_{2}}}^{\frac{n+\alpha_{2}}{n+\beta_{2}}}(-1)^{r}q_{n+r,k+r}^{(r)}(t)f \biggl( \frac{nt+\alpha_{1}}{n+\beta_{1}} \biggr) \,dt. \end{aligned}$$
We obtain the required result by integrating by parts r times. □
Set
$$\begin{aligned}& \Vert f\Vert _{0}=\sup_{x\in A_{n}} \bigl\{ \bigl\vert \delta _{n}^{\alpha ( \lambda-1 ) }(x)f ( x ) \bigr\vert \bigr\} ; \\& C_{\alpha,\lambda}= \bigl\{ f\in C ( A_{n} ) ,\Vert f\Vert _{0}< +\infty \bigr\} ; \\& \Vert f\Vert _{1}=\sup_{x\in A_{n}} \bigl\{ \bigl\vert \delta _{n}^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)f^{\prime} ( x ) \bigr\vert \bigr\} ; \\& C_{\alpha,\lambda}^{1}= \bigl\{ f\in C_{\alpha,\lambda}, \Vert f\Vert _{1}< +\infty \bigr\} ; \\& \Vert f\Vert _{2}=\sup_{x\in A_{n}} \bigl\{ \bigl\vert \delta _{n}^{2+\alpha ( \lambda-1 ) }(x)f^{\prime\prime} ( x ) \bigr\vert \bigr\} ; \\& C_{\alpha,\lambda}^{2}= \bigl\{ f\in C_{\alpha,\lambda},f^{\prime } \in \mathit{A.C.}_{\mathrm{loc}},\Vert f\Vert _{2}< +\infty \bigr\} ; \\& K_{\alpha,\lambda}^{1} ( f,t ) =\inf_{g\in C_{\alpha,\lambda }^{1}} \bigl\{ \Vert f-g\Vert _{0}+t\Vert g\Vert _{1} \bigr\} ; \\& K_{\alpha,\lambda}^{2} ( f,t ) =\inf_{g\in C_{\alpha,\lambda }^{2}} \bigl\{ \Vert f-g\Vert _{0}+t\Vert g\Vert _{2} \bigr\} . \end{aligned}$$
Lemma 6
If
\(0\leq\lambda\leq1\), \(0<\alpha<2\), then
$$\begin{aligned}& \bigl\Vert \widetilde{S}_{n,\alpha,\beta} ( f ) \bigr\Vert _{1}\leq Cn^{1/ ( 2-\lambda ) }\Vert f\Vert _{0},\quad f\in C_{\alpha,\lambda}, \end{aligned}$$
(2.13)
$$\begin{aligned}& \bigl\Vert \widetilde{S}_{n,\alpha,\beta} ( f ) \bigr\Vert _{1}\leq C\Vert f\Vert _{1},\quad f\in C_{\alpha,\lambda }^{1}. \end{aligned}$$
(2.14)
Proof
Firstly, we prove (2.13) by considering the following two cases.
Case 1. \(x\in B_{n}:= [ \frac{\alpha_{2}+1}{n+\beta_{2}}, \frac{n+\alpha_{2}-1}{n+\beta_{2}} ] \). In this case, we have
$$ \varphi ( x ) \geq\min \biggl( \varphi \biggl( \frac{\alpha _{2}+1}{n+\beta_{2}} \biggr) , \varphi \biggl( \frac{n+\alpha_{2}-1}{n+\beta _{2}} \biggr) \biggr) \geq \frac{C}{\sqrt{n}}, $$
which means that
$$ \delta_{n}(x)\sim\varphi ( x ) \quad \text{for }x\in B_{n}. $$
(2.15)
By simple calculations, we have
$$ q_{nk}^{\prime} ( x ) =n\varphi^{-2} ( x ) \biggl( \frac{k+\alpha_{2}}{n+\beta_{2}}-x \biggr) q_{nk} ( x ) $$
(2.16)
and
$$\begin{aligned} \delta_{n} \biggl( \frac{nt+\alpha_{1}}{n+\beta_{1}} \biggr) =&\sqrt{ \biggl( t-\frac{\alpha_{2}}{n+\beta_{2}}+\frac{\alpha_{1}-\beta _{1}t}{n+\beta_{1}} \biggr) \biggl( \frac{n+\alpha_{2}}{n+\beta_{2}}-t+\frac {\beta _{1}t-\alpha_{1}}{n+\beta_{1}} \biggr) }+\frac{1}{\sqrt{n}} \\ =&\sqrt{\varphi^{2} ( t ) +O \biggl( \frac{1}{n} \biggr) }+ \frac {1}{\sqrt{n}}\sim\varphi ( t ) +\frac{1}{\sqrt{n}}=\delta _{n} ( t ) . \end{aligned}$$
(2.17)
By (2.1), (2.15)-(2.17), and Hölder’s inequality, we have
$$\begin{aligned}& \bigl\vert \delta_{n}^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)\widetilde{S}_{n,\alpha,\beta}^{\prime} ( f,x ) \bigr\vert \\& \quad \leq Cn\varphi^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) -2}(x) \biggl( \frac{n+\beta_{2}}{n} \biggr) ^{2n+1} \\& \qquad {}\times\sum_{k=0}^{n}q_{nk}(x) \biggl\vert \frac{k+\alpha_{2}}{n+\beta _{2}}-x\biggr\vert ( n+1 ) \biggl\vert \int_{A_{n}}f \biggl( \frac{nt+\alpha_{1}}{n+\beta_{1}} \biggr) q_{nk}(t)\,dt\biggr\vert \\& \quad \leq Cn\Vert f\Vert _{0}\varphi^{ ( \frac {2}{2-\lambda}-\alpha ) ( 1-\lambda ) -2}(x)\sum _{k=0}^{n}q_{nk}(x)\biggl\vert \frac{k+\alpha_{2}}{n+\beta _{2}}-x\biggr\vert ( n+1 ) \biggl\vert \int_{A_{n}}\delta _{n}^{\alpha ( 1-\lambda ) } ( t ) q_{nk}(t)\,dt\biggr\vert \\& \quad \leq Cn\Vert f\Vert _{0}\varphi^{ ( \frac {2}{2-\lambda}-\alpha ) ( 1-\lambda ) -2}(x)\sum _{k=0}^{n}q_{nk}(x)\biggl\vert \frac{k+\alpha_{2}}{n+\beta _{2}}-x\biggr\vert \biggl( ( n+1 ) \int_{A_{n}}\delta_{n}^{2} ( t ) q_{nk}(t)\,dt \biggr) ^{\alpha ( 1-\lambda ) /2} \\& \qquad {}\times \biggl( ( n+1 ) \int_{A_{n}}q_{nk}(t)\,dt \biggr) ^{1-\alpha ( 1-\lambda ) /2} \\& \quad \leq Cn\Vert f\Vert _{0}\varphi^{ ( \frac {2}{2-\lambda}-\alpha ) ( 1-\lambda ) -2}(x)\sum _{k=0}^{n}q_{nk}(x)\biggl\vert \frac{k+\alpha_{2}}{n+\beta _{2}}-x\biggr\vert \biggl( ( n+1 ) \int_{A_{n}}\delta_{n}^{2} ( t ) q_{nk}(t)\,dt \biggr) ^{\alpha ( 1-\lambda ) /2}. \end{aligned}$$
By (2.9), (2.15) (2.5), and Hölder’s inequality again, we have
$$\begin{aligned}& \bigl\vert \delta_{n}^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)\widetilde{S}_{n,\alpha,\beta}^{\prime} ( f,x ) \bigr\vert \\& \quad \leq Cn\Vert f\Vert _{0}\varphi^{ ( \frac {2}{2-\lambda}-\alpha ) ( 1-\lambda ) -2}(x) \Biggl( \sum_{k=0}^{n}q_{nk}(x)\biggl\vert \frac{k+\alpha_{2}}{n+\beta_{2}}-x\biggr\vert ^{\frac{1}{1-\alpha ( 1-\lambda ) /2}} \Biggr) ^{1-\alpha ( 1-\lambda ) /2} \\& \qquad {}\times \Biggl( \sum_{k=0}^{n}q_{nk}(x) ( n+1 ) \int _{A_{n}}\delta _{n}^{2} ( t ) q_{nk}(t)\,dt \Biggr) ^{\alpha ( 1-\lambda ) /2} \\& \quad \leq Cn^{1/2}\Vert f\Vert _{0} \varphi^{\frac{2 ( 1-\lambda ) }{2-\lambda}-1}(x)\leq Cn^{1/ ( 2-\lambda ) } \Vert f\Vert _{0}. \end{aligned}$$
(2.18)
Case 2. \(x\in B_{n}^{c}= [ \frac{\alpha_{2}}{n+\beta_{2}},\frac {\alpha _{2}+1}{n+\beta_{2}} ) \cup ( \frac{n+\alpha_{2}-1}{n+\beta _{2}},\frac{n+\alpha_{2}}{n+\beta_{2}} ] \). In this case, we have
$$ \delta_{n} ( x ) \sim\frac{1}{\sqrt{n}},\quad x\in B_{n}^{c}. $$
(2.19)
Noting that
$$ q_{nk}^{\prime} ( x ) =n \bigl( q_{n-1,k-1} ( x ) -q_{n-1,k} ( x ) \bigr) $$
with \(q_{n-1,-1} ( x ) =q_{n-1,n} ( x ) =0\), we get
$$ \widetilde{S}_{n,\alpha,\beta}^{\prime} ( f,x ) =n\sum _{k=0}^{n-1}q_{n-1,k}(x) (x) ( n+1 ) \int_{A_{n}}f \biggl( \frac{nt+\alpha_{1}}{n+\beta_{1}} \biggr) \bigl( q_{n,k+1}(t)-q_{n,k}(t) \bigr) \,dt. $$
Then, by using (2.17) and Hölder’s inequality twice,
$$\begin{aligned}& \bigl\vert \delta_{n}^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)\widetilde{S}_{n,\alpha,\beta}^{\prime} ( f,x ) \bigr\vert \\& \quad \leq Cn\delta_{n}^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)\Vert f\Vert _{0}\Biggl\vert \sum_{k=0}^{n-1}q_{n-1,k}(x) ( n+1 ) \int_{A_{n}}\delta _{n}^{\alpha ( 1-\lambda ) } ( t ) \bigl( q_{n,k+1}(t)+q_{n,k}(t) \bigr) \,dt\Biggr\vert \\& \quad \leq Cn\delta_{n}^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)\Vert f\Vert _{0}\sum_{k=0}^{n-1}q_{n-1,k}(x) \biggl( ( n+1 ) \int _{A_{n}}\delta _{n}^{2}(t) \bigl( q_{n,k+1}(t)+q_{n,k}(t) \bigr) \,dt \biggr) ^{\frac {\alpha ( 1-\lambda ) }{2}} \\& \quad \leq Cn\delta_{n}^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)\Vert f\Vert _{0} \Biggl( \sum_{k=0}^{n-1}q_{n-1,k}(x) ( n+1 ) \int_{A_{n}}\delta _{n}^{2}(t) \bigl( q_{n,k+1}(t)+q_{n,k}(t) \bigr) \,dt \Biggr) ^{\frac {\alpha ( 1-\lambda ) }{2}} \\& \quad \leq Cn\delta_{n}^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)\Vert f\Vert _{0}\delta_{n}^{\alpha ( 1-\lambda ) } \\& \quad \leq Cn^{\frac{1}{2-\lambda}} \Vert f\Vert _{0}, \end{aligned}$$
(2.20)
where in the fourth inequality, we used the following fact, which can be deduced exactly in the same way as (2.10):
$$ \sum_{k=0}^{n-1}q_{n-1,k}(x) ( n+1 ) \int_{A_{n}}\delta _{n}^{2}(t)q_{nk^{\ast}}(t) \,dt\leq C\delta_{n}^{2}(x). $$
We obtain (2.13) by combining (2.18) and (2.20).
Now, we begin to prove (2.14). If \(( \frac{2}{2-\lambda }-\alpha ) ( \lambda-1 ) <0\), by (2.12) and using Hölder’s inequality twice, we get
$$\begin{aligned}& \bigl\vert \delta_{n}^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)\widetilde{S}_{n,\alpha,\beta}^{\prime} ( f,x ) \bigr\vert \\ & \quad \leq C\Vert f\Vert _{1}\Biggl\vert \delta_{n}^{ ( \frac {2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)n \sum_{k=0}^{n-1}q_{n-1,k} ( x ) \int_{A_{n}}q_{n+1,k+1}(t)\delta_{n}^{ ( \frac{2}{2-\lambda }-\alpha ) ( \lambda-1 ) }(t) \,dt\Biggr\vert \\ & \quad \leq C\Vert f\Vert _{1}\delta_{n}^{ ( \frac {2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x) \sum_{k=0}^{n-1}q_{n-1,k} ( x ) \biggl( n \int_{A_{n}}q_{n+1,k+1}(t)\delta_{n}^{-2}(t) \,dt \biggr) ^{\frac{1}{2} ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) } \\ & \qquad {}\times \biggl( n \int_{A_{n}}q_{n+1,k+1}(t)\,dt \biggr) ^{1-\frac {1}{2} ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) } \\ & \quad \leq C\Vert f\Vert _{1}\delta_{n}^{ ( \frac {2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x) \sum_{k=0}^{n-1}q_{n-1,k} ( x ) \biggl( n \int_{A_{n}}q_{n+1,k+1}(t)\delta_{n}^{-2}(t) \,dt \biggr) ^{\frac{1}{2} ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) } \\ & \qquad (\text{by (2.1)}) \\ & \quad \leq C\Vert f\Vert _{1}\delta_{n}^{ ( \frac {2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x) \Biggl( \sum_{k=0}^{n-1}q_{n-1,k} ( x ) n \int _{A_{n}}q_{n+1,k+1}(t)\delta _{n}^{-2}(t) \,dt \Biggr) ^{\frac{1}{2} ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) } \\ & \quad \leq C\Vert f\Vert _{1}, \end{aligned}$$
where, in the last inequality, (2.10) is applied.
If \(( \frac{2}{2-\lambda}-\alpha ) ( \lambda-1 ) >0\), by using (2.9) instead of (2.10), we also can deduce that
$$ \bigl\vert \delta_{n}^{ ( \frac{2}{2-\lambda}-\alpha ) ( 1-\lambda ) }(x)\widetilde{S}_{n,\alpha,\beta}^{\prime} ( f,x ) \bigr\vert \leq C\Vert f\Vert _{1}. $$
 □
Lemma 7
If
\(0\leq\lambda\leq1\), \(0<\alpha<2\), then
$$\begin{aligned}& \bigl\Vert \widetilde{S}_{n,\alpha,\beta} ( f ) \bigr\Vert _{2}\leq Cn\Vert f\Vert _{0}, \quad f\in C_{\alpha ,\lambda}, \end{aligned}$$
(2.21)
$$\begin{aligned}& \bigl\Vert \widetilde{S}_{n,\alpha,\beta} ( f ) \bigr\Vert _{2}\leq C\Vert f\Vert _{2}, \quad f\in C_{\alpha,\lambda }^{2}. \end{aligned}$$
(2.22)
Proof
It can be proved in a way similar to Lemma 6. □
Lemma 8
For
\(0< t<\frac{1}{8}\), \(\frac{t}{2}\leq x\leq1-\frac{t}{2}\), \(x\in [ 0,1 ] \), \(\beta<2\), we have
$$ \int_{-t/2}^{t/2}\delta_{n}^{-\beta} ( x+u ) \, du\leq C(\beta )t\delta_{n}^{-\beta}(x). $$
(2.23)
Lemma 9
For
\(0< t<\frac{1}{4}\), \(t\leq x\leq1-t\), \(x\in [ 0,1 ] \), \(0\leq \beta\leq2\), we have
$$ \int_{-t/2}^{t/2} \int_{-t/2}^{t/2}\delta_{n}^{-\beta} ( x+u+v ) \,du\,dv\leq Ct^{2}\delta_{n}^{-\beta}(x). $$
(2.24)
It has been shown in [9] that Lemma 8 and Lemma 9 are valid when \(\delta_{n} ( t ) \) is replaced by \(\delta_{n}^{\ast} ( t ) \), which combining with (2.8) proves Lemma 8 and Lemma 9.