# On *λ*-statistical convergence of order *α* of sequences of function

- Mikail Et
^{1}, - Muhammed Çınar
^{2}Email author and - Murat Karakaş
^{3}

**2013**:204

https://doi.org/10.1186/1029-242X-2013-204

© Et et al.; licensee Springer 2013

**Received: **12 December 2012

**Accepted: **10 April 2013

**Published: **23 April 2013

## Abstract

In this study we introduce the concept of ${S}_{\lambda}^{\alpha}(f)$-statistical convergence of sequences of real valued functions. Also some relations between ${S}_{\lambda}^{\alpha}(f)$-statistical convergence and strong ${w}_{\lambda p}^{\beta}(f)$-summability are given.

**MSC:**40A05, 40C05, 46A45.

### Keywords

statistical convergence sequences of function Cesàro summability## 1 Introduction

The idea of statistical convergence was given by Zygmund [1] in the first edition of his monograph published in Warsaw in 1935. The concept of statistical convergence was introduced by Steinhaus [2] and Fast [3] and then reintroduced by Schoenberg [4] independently. Over the years and under different names, statistical convergence has been discussed in the theory of Fourier analysis, ergodic theory, number theory, measure theory, trigonometric series, turnpike theory and Banach spaces. Later on, it was further investigated from the sequence space point of view and linked with summability theory by Connor [5], Edely *et al.* [6], Et *et al.* [7–10], Fridy [11], Güngör *et al.* [12–14], Kolk [15], Orhan *et al.* [16, 17], Mursaleen [18], Kumar and Mursaleen [19], Rath and Tripathy [20], Salat [21], Savaş [22] and many others. In recent years, generalizations of statistical convergence have appeared in the study of strong integral summability and the structure of ideals of bounded continuous functions on locally compact spaces. Statistical convergence and its generalizations are also connected with subsets of the Stone-Čech compactification of the natural numbers. Moreover, statistical convergence is closely related to the concept of convergence in probability.

In the present paper, we introduce and examine the concepts of pointwise *λ*-statistical convergence of order *α* and pointwise $[V,\lambda ]$-summability of order *α* of sequences of real valued functions. In Section 2, we give a brief overview about statistical convergence and strong *p*-Cesàro summability. In Section 3, we establish some inclusion relations between ${w}_{\lambda p}^{\beta}(f)$ and ${S}_{\lambda}^{\alpha}(f)$ and between ${S}_{\lambda}^{\alpha}(f)$ and ${S}_{\lambda}(f)$.

## 2 Definition and preliminaries

*p*-Cesàro convergence of a sequence of real numbers were introduced in the literature independently of one another and followed different lines of development since their first appearance. It turns out, however, that the two definitions can be simply related to one another in general and are equivalent for bounded sequences. The idea of statistical convergence depends on the density of subsets of the set ℕ of natural numbers. The density of a subset

*E*of ℕ is defined by

where ${\chi}_{E}$ is the characteristic function of *E*. It is clear that any finite subset of ℕ has zero natural density and $\delta ({E}^{c})=1-\delta (E)$.

*α*-

*density*of a subset

*E*of ℕ was defined by Çolak [23]. Let

*α*be a real number such that $0<\alpha \le 1$. The

*α*-

*density*of a subset

*E*of ℕ is defined by

where $|\{k\le n:k\in E\}|$ denotes the number of elements of *E* not exceeding *n*.

It is clear that any finite subset of ℕ has zero *α* density and ${\delta}_{\alpha}({E}^{c})=1-{\delta}_{\alpha}(E)$ does not hold for $0<\alpha <1$ in general, the equality holds only if $\alpha =1$. Note that the *α*-*density* of any set reduces to the natural density of the set in case $\alpha =1$.

The order of statistical convergence of a sequence of numbers was given by Gadjiev and Orhan in [24] and after then statistical convergence of order *α* and strong *p*-Cesàro summability of order *α* studied by Çolak [23, 25] and generalized by Çolak and Asma [26].

Let $\lambda =({\lambda}_{n})$ be a nondecreasing sequence of positive real numbers tending to ∞ such that ${\lambda}_{n+1}\le {\lambda}_{n}+1$, ${\lambda}_{1}=1$. The generalized de la Vallée-Poussin mean is defined by ${t}_{n}(x)=\frac{1}{{\lambda}_{n}}{\sum}_{k\in {I}_{n}}{x}_{k}$, where ${I}_{n}=[n-{\lambda}_{n}+1,n]$ for $n=1,2,\dots $ . A sequence $x=({x}_{k})$ is said to be $(V,\lambda )$-summable to a number *ℓ* if ${t}_{n}(x)\to \ell $ as $n\to \mathrm{\infty}$ [27]. If ${\lambda}_{n}=n$, then $(V,\lambda )$-summability is reduced to Cesàro summability. By Λ we denote the class of all nondecreasing sequence of positive real numbers tending to ∞ such that ${\lambda}_{n+1}\le {\lambda}_{n}+1$, ${\lambda}_{1}=1$.

Throughout the paper, unless stated otherwise, by ‘for all $n\in {\mathbb{N}}_{{n}_{o}}$’ we mean ‘for all $n\in \mathbb{N}$ except finite numbers of positive integers’ where ${\mathbb{N}}_{{n}_{o}}=\{{n}_{o},{n}_{o}+1,{n}_{o}+2,\dots \}$ for some ${n}_{o}\in \mathbb{N}=\{1,2,3,\dots \}$.

Let *A* be any non empty set, by $B(A)$ we denote the class of all bounded real valued functions defined on *A*.

## 3 Main results

In this section we give the main results of this paper. In Theorem 3.3, we give the inclusion relations between the sets of ${S}_{\lambda}^{\alpha}(f)$-statistically convergent sequences for different ${\alpha}^{\mathrm{\prime}}s$ and ${\mu}^{\mathrm{\prime}}s$. In Theorem 3.6, we give the relationship between the strong ${w}_{\lambda p}^{\alpha}(f)$-summability and the strong ${w}_{\mu p}^{\beta}(f)$-summability. In Theorem 3.9, we give the relationship between the strong ${w}_{\mu p}^{\beta}(f)$-summability and ${S}_{\lambda}^{\alpha}(f)$-statistical convergence.

**Definition 3.1**Let the sequence $\lambda =({\lambda}_{n})$ be as above and $\alpha \in (0,1]$ be any real number. A sequence of functions $\{{f}_{k}\}$ is said to be ${S}_{\lambda}^{\alpha}(f)$-statistical convergence (or pointwise

*λ*-statistically convergent of order

*α*) to the function

*f*on a set

*A*if, for every $\epsilon >0$,

*α*th power ${({\lambda}_{n})}^{\alpha}$ of ${\lambda}_{n}$, that is ${\lambda}^{\alpha}=({\lambda}_{n}^{\alpha})=({\lambda}_{1}^{\alpha},{\lambda}_{2}^{\alpha},\dots ,{\lambda}_{n}^{\alpha},\dots )$. In this case, we write ${S}_{\lambda}^{\alpha}-lim{f}_{k}(x)=f(x)$ on

*A*. ${S}_{\lambda}^{\alpha}-lim{f}_{k}(x)=f(x)$ means that for every $\delta >0$ and $0<\alpha \le 1$, there is an integer

*N*such that

for all $n>N$ ($=N(\epsilon ,\delta ,x)$) and for each $\epsilon >0$. The set of all pointwise *λ*-statistically convergent function sequences of order *α* will be denoted by ${S}_{\lambda}^{\alpha}(f)$. In this case, we write ${S}_{\lambda}^{\alpha}-lim{f}_{k}(x)=f(x)$ on *A*. For ${\lambda}_{n}=n$ for all $n\in \mathbb{N}$, we shall write ${S}^{\alpha}(f)$ instead of ${S}_{\lambda}^{\alpha}(f)$ and in the special case $\alpha =1$, we shall write ${S}_{\lambda}(f)$ instead of ${S}_{\lambda}^{\alpha}(f)$.

for $\alpha >1$, and so ${S}_{\lambda}^{\alpha}(f)$-statistically converges both to 2 and 0, *i.e.* ${S}_{\lambda}^{\alpha}-lim{f}_{k}(x)=2$ and ${S}_{\lambda}^{\alpha}-lim{f}_{k}(x)=0$. But this is impossible.

**Definition 3.2**Let the sequence $\lambda =({\lambda}_{n})$ be as above, $\alpha \in (0,1]$ be any real number and let

*p*be a positive real number. A sequence of functions $\{{f}_{k}\}$ is said to be strongly ${w}_{\lambda p}^{\alpha}(f)$-summable (or pointwise $[V,\lambda ]$-summable of order

*α*), if there is a function

*f*such that

In this case, we write ${w}_{\lambda p}^{\alpha}-lim{f}_{k}(x)=f(x)$ on *A*. The set of all strongly ${w}_{\lambda p}^{\beta}(f)$-summable sequences of function will be denoted by ${w}_{\lambda p}^{\alpha}(f)$. For ${\lambda}_{n}=n$ for all $n\in \mathbb{N}$, we shall write ${w}_{p}^{\alpha}(f)$ instead of ${w}_{\lambda p}^{\alpha}(f)$ and in the special case $\alpha =1$, we shall write ${w}_{\lambda p}(f)$ instead of ${w}_{\lambda p}^{\alpha}(f)$.

**Theorem 3.3**

*Let*$\lambda =({\lambda}_{n})$

*and*$\mu =({\mu}_{n})$

*be two sequences in*Λ

*such that*${\lambda}_{n}\le {\mu}_{n}$

*for all*$n\in {\mathbb{N}}_{{n}_{o}}$, $0<\alpha \le \beta \le 1$

*and*$\{{f}_{k}\}$

*be a sequence of real valued functions defined on a set*

*A*.

- (i)
*If*$lim\underset{n\to \mathrm{\infty}}{inf}\frac{{\lambda}_{n}^{\alpha}}{{\mu}_{n}^{\beta}}>0$

*then*${S}_{\mu}^{\beta}(f)\subseteq {S}_{\lambda}^{\alpha}(f)$;

- (ii)
*If*$\underset{n\to \mathrm{\infty}}{lim}\frac{{\mu}_{n}}{{\lambda}_{n}^{\beta}}=1$

*then* ${S}_{\lambda}^{\alpha}(f)\subseteq {S}_{\mu}^{\beta}(f)$.

*Proof*(i) Suppose that ${\lambda}_{n}\le {\mu}_{n}$ for all $n\in {\mathbb{N}}_{{n}_{o}}$ and let (1) be satisfied. Then ${I}_{n}\subset {J}_{n}$ and so that $\epsilon >0$ we may write

- (ii)

for all $n\in {\mathbb{N}}_{{n}_{o}}$. Since ${lim}_{n}\frac{{\mu}_{n}}{{\lambda}_{n}^{\beta}}=1$ by (2) the first term and since ${S}_{\lambda}^{\alpha}-lim{f}_{k}(x)=f(x)$ on *A*, the second term of right-hand side of above inequality tends to 0 as $n\to \mathrm{\infty}$. (Note that $(\frac{{\mu}_{n}}{{\lambda}_{n}^{\beta}}-1)\ge 0$ for all $n\in {\mathbb{N}}_{{n}_{o}}$.) This implies that ${S}_{\lambda}^{\alpha}(f)\subseteq {S}_{\mu}^{\beta}(f)$. □

From Theorem 3.3, we have the following results.

**Corollary 3.4**

*Let*$\lambda =({\lambda}_{n})$

*and*$\mu =({\mu}_{n})$

*be two sequences in*Λ

*such that*${\lambda}_{n}\le {\mu}_{n}$

*for all*$n\in {\mathbb{N}}_{{n}_{o}}$

*and*$\{{f}_{k}\}$

*be a sequence of real valued functions defined on a set*

*A*.

*If*(1)

*holds then*,

- (i)
${S}_{\mu}^{\alpha}(f)\subseteq {S}_{\lambda}^{\alpha}(f)$,

*for each*$\alpha \in (0,1]$*and for all*$x\in A$; - (ii)
${S}_{\mu}(f)\subseteq {S}_{\lambda}^{\alpha}(f)$,

*for each*$\alpha \in (0,1]$*and for all*$x\in A$; - (iii)
${S}_{\mu}(f)\subseteq {S}_{\lambda}(f)$

*for all*$x\in A$.

**Corollary 3.5**

*Let*$\lambda =({\lambda}_{n})$

*and*$\mu =({\mu}_{n})$

*be two sequences in*Λ

*such that*${\lambda}_{n}\le {\mu}_{n}$

*for all*$n\in {\mathbb{N}}_{{n}_{o}}$

*and*$\{{f}_{k}\}$

*be a sequence of real valued functions defined on a set*

*A*.

*If*(2)

*holds then*,

- (i)
${S}_{\lambda}^{\alpha}(f)\subseteq {S}_{\mu}^{\alpha}(f)$

*for each*$\alpha \in (0,1]$*and for all*$x\in A$; - (ii)
${S}_{\lambda}^{\alpha}(f)\subseteq {S}_{\mu}(f)$,

*for each*$\alpha \in (0,1]$*and for all*$x\in A$; - (iii)
${S}_{\lambda}(f)\subseteq {S}_{\mu}(f)$

*for all*$x\in A$.

**Theorem 3.6**

*Given for*$\lambda =({\lambda}_{n})$, $\mu =({\mu}_{n})\in \mathrm{\Lambda}$

*suppose that*${\lambda}_{n}\le {\mu}_{n}$

*for all*$n\in {\mathbb{N}}_{{n}_{o}}$, $0<\alpha \le \beta \le 1$

*and*$\{{f}_{k}\}$

*be a sequence of real valued functions defined on a set*

*A*.

*Then*

- (i)
*If*(1)*holds then*${w}_{\mu p}^{\beta}(f)\subset {w}_{\lambda p}^{\alpha}(f)$*for all*$x\in A$; - (ii)
*If*(2)*holds and let*$f(x)\in B(A)$,*then*$B(A)\cap {w}_{\lambda p}^{\alpha}(f)\subset {w}_{\mu p}^{\beta}(f)$*for all*$x\in A$.

*Proof*(i) Omitted.

- (ii)Let $({f}_{k}(x))\in B(A)\cap {w}_{\lambda p}^{\alpha}(f)$ and suppose that (2) holds. Since $({f}_{k}(x))\in B(A)$, then there exists some $M>0$ such that $|{f}_{k}(x)-f(x)|\le M$ for all $k\in \mathbb{N}$ and for all $x\in A$. Now, since ${\lambda}_{n}\le {\mu}_{n}$ and ${I}_{n}\subset {J}_{n}$ for all $n\in {\mathbb{N}}_{{n}_{o}}$, we may write$\begin{array}{rcl}\frac{1}{{\mu}_{n}^{\beta}}\underset{x\in A}{\sum _{k\in {J}_{n}}}|{f}_{k}(x)-f(x){|}^{p}& =& \frac{1}{{\mu}_{n}^{\beta}}\underset{x\in A}{\sum _{k\in {J}_{n}-{I}_{n}}}|{f}_{k}(x)-f(x){|}^{p}+\frac{1}{{\mu}_{n}^{\beta}}\underset{x\in A}{\sum _{k\in {I}_{n}}}|{f}_{k}(x)-f(x){|}^{p}\\ \le & \left(\frac{{\mu}_{n}-{\lambda}_{n}}{{\mu}_{n}^{\beta}}\right){M}^{p}+\frac{1}{{\mu}_{n}^{\beta}}\underset{x\in A}{\sum _{k\in {I}_{n}}}|{f}_{k}(x)-f(x){|}^{p}\\ \le & \left(\frac{{\mu}_{n}-{\lambda}_{n}^{\beta}}{{\mu}_{n}^{\beta}}\right){M}^{p}+\frac{1}{{\mu}_{n}^{\beta}}\underset{x\in A}{\sum _{k\in {I}_{n}}}|{f}_{k}(x)-f(x){|}^{p}\\ \le & (\frac{{\mu}_{n}}{{\lambda}_{n}^{\beta}}-1){M}^{p}+\frac{1}{{\lambda}_{n}^{\alpha}}\underset{x\in A}{\sum _{k\in {I}_{n}}}|{f}_{k}(x)-f(x){|}^{p}\end{array}$

for every $n\in {\mathbb{N}}_{{n}_{o}}$. Therefore, $B(A)\cap {w}_{\lambda p}^{\alpha}(f)\subset {w}_{\mu p}^{\beta}(f)$. □

From Theorem 3.6, we have the following results.

**Corollary 3.7**

*Let*$\lambda =({\lambda}_{n})$

*and*$\mu =({\mu}_{n})$

*be two sequences in*Λ

*such that*${\lambda}_{n}\le {\mu}_{n}$

*for all*$n\in {\mathbb{N}}_{{n}_{o}}$

*and*$\{{f}_{k}\}$

*be a sequence of real valued functions defined on a set*

*A*.

*If*(1)

*holds then*:

- (i)
${w}_{\mu p}^{\alpha}(f)\subset {w}_{\lambda p}^{\alpha}(f)$,

*for each*$\alpha \in (0,1]$*and for all*$x\in A$; - (ii)
${w}_{\mu p}(f)\subset {w}_{\lambda p}^{\alpha}(f)$,

*for each*$\alpha \in (0,1]$*and for all*$x\in A$; - (iii)
${w}_{\mu p}(f)\subset {w}_{\lambda p}(f)$

*for all*$x\in A$.

**Corollary 3.8**

*Let*$\lambda =({\lambda}_{n})$

*and*$\mu =({\mu}_{n})$

*be two sequences in*Λ

*such that*${\lambda}_{n}\le {\mu}_{n}$

*for all*$n\in {\mathbb{N}}_{{n}_{o}}$

*and*$\{{f}_{k}\}$

*be a sequence of real valued functions defined on a set*

*A*.

*If*(2)

*holds then*:

- (i)
$B(A)\cap {w}_{\lambda p}^{\alpha}(f)\subset {w}_{\mu p}^{\alpha}(f)$,

*for each*$\alpha \in (0,1]$*and for all*$x\in A$; - (ii)
$B(A)\cap {w}_{\lambda p}^{\alpha}(f)\subset {w}_{\mu p}(f)$,

*for each*$\alpha \in (0,1]$*and for all*$x\in A$; - (iii)
$B(A)\cap {w}_{\lambda p}(f)\subset {w}_{\mu p}(f)$

*for all*$x\in A$.

**Theorem 3.9**

*Let*

*α*

*and*

*β*

*be fixed real numbers such that*$0<\alpha \le \beta \le 1$, $0<p<\mathrm{\infty}$, ${\lambda}_{n}\le {\mu}_{n}$

*for all*$n\in {\mathbb{N}}_{{n}_{o}}$

*and*$\{{f}_{k}\}$

*be a sequence of real valued functions defined on a set*

*A*.

*Then*:

- (i)
*Let*(1)*holds*,*if a sequence of real valued functions defined on a set**A**is strongly*${w}_{\mu p}^{\beta}(f)$-*summable to**f*,*then it is*${S}_{\lambda}^{\alpha}(f)$-*statistically convergent to**f*; - (ii)
*Let*(2)*holds*, $f(x)\in B(A)$*and*$\{{f}_{k}\}$*be a sequence of bounded real valued functions defined on a set**A*,*if a sequence is*${S}_{\lambda}^{\alpha}(f)$-*statistically convergent to**f**then it is strongly*${w}_{\mu p}^{\beta}(f)$-*summable to**f*.

*Proof*(i) For any function sequence $({f}_{k}(x))$ and $\epsilon >0$, we have

*f*, then it is ${S}_{\lambda}^{\alpha}(f)$-statistically convergent to

*f*.

- (ii)Suppose that ${S}_{\lambda}^{\alpha}-lim{f}_{k}(x)=f(x)$ and $({f}_{k}(x))\in B(A)$. Then there exists some $M>0$ such that $|{f}_{k}(x)-f(x)|\le M$ for all
*k*, then for every $\epsilon >0$ we may write$\begin{array}{rcl}\frac{1}{{\mu}_{n}^{\beta}}\underset{x\in A}{\sum _{k\in {J}_{n}}}|{f}_{k}(x)-f(x){|}^{p}& =& \frac{1}{{\mu}_{n}^{\beta}}\underset{x\in A}{\sum _{k\in {J}_{n}-{I}_{n}}}|{f}_{k}(x)-f(x){|}^{p}+\frac{1}{{\mu}_{n}^{\beta}}\underset{x\in A}{\sum _{k\in {I}_{n}}}|{f}_{k}(x)-f(x){|}^{p}\\ \le & \left(\frac{{\mu}_{n}-{\lambda}_{n}}{{\mu}_{n}^{\beta}}\right){M}^{p}+\frac{1}{{\mu}_{n}^{\beta}}\underset{x\in A}{\sum _{k\in {I}_{n}}}|{f}_{k}(x)-f(x){|}^{p}\\ \le & \left(\frac{{\mu}_{n}-{\lambda}_{n}^{\beta}}{{\mu}_{n}^{\beta}}\right){M}^{p}+\frac{1}{{\mu}_{n}^{\beta}}\underset{x\in A}{\sum _{k\in {I}_{n}}}|{f}_{k}(x)-f(x){|}^{p}\\ =& (\frac{{\mu}_{n}}{{\lambda}_{n}^{\beta}}-\frac{{\lambda}_{n}^{\beta}}{{\lambda}_{n}^{\beta}}){M}^{p}+\frac{1}{{\mu}_{n}^{\beta}}\underset{|{f}_{k}(x)-f(x)|\ge \epsilon}{\sum _{k\in {I}_{n},x\in A}}|{f}_{k}(x)-f(x){|}^{p}\\ +\frac{1}{{\mu}_{n}^{\beta}}\underset{|{f}_{k}(x)-f(x)|<\epsilon}{\sum _{k\in {I}_{n},x\in A}}|{f}_{k}(x)-f(x){|}^{p}\\ \le & (\frac{{\mu}_{n}}{{\lambda}_{n}^{\beta}}-1){M}^{p}\\ +\frac{{M}^{p}}{{\lambda}_{n}^{\alpha}}\left|\{k\in {I}_{n}:|{f}_{k}(x)-f(x)|\ge \epsilon \text{for every}x\in A\}\right|+\epsilon \end{array}$

for all $n\in {\mathbb{N}}_{{n}_{o}}$. Using (2), we obtain that ${w}_{\mu p}^{\beta}-lim{f}_{k}(x)=f(x)$, whenever ${S}_{\lambda}^{\alpha}-lim{f}_{k}(x)=f(x)$. □

From Theorem 3.9, we have the following results.

**Corollary 3.10**

*Let*$\lambda =({\lambda}_{n})$

*and*$\mu =({\mu}_{n})$

*be two sequences in*Λ

*such that*${\lambda}_{n}\le {\mu}_{n}$

*for all*$n\in {\mathbb{N}}_{{n}_{o}}$

*and*$\alpha \in (0,1]$

*be any real number*.

*If*(1)

*holds*,

*then*:

- (i)
*If a sequence of real valued functions defined on a set**A**is strongly*${w}_{\mu p}^{\alpha}(f)$-*summable to**f*,*then it is*${S}_{\lambda}^{\alpha}(f)$-*statistically convergent to**f*; - (ii)
*If a sequence of real valued functions defined on a set**A**is strongly*${w}_{\mu p}(f)$-*summable to**f*,*then it is*${S}_{\lambda}^{\alpha}(f)$-*statistically convergent to**f*; - (iii)
*If a sequence of real valued functions defined on a set**A**is strongly*${w}_{\mu p}(f)$-*summable to**f*,*then it is*${S}_{\lambda}(f)$-*statistically convergent to**f*.

**Corollary 3.11**

*Let*$\lambda =({\lambda}_{n})$

*and*$\mu =({\mu}_{n})$

*be two sequences in*Λ

*such that*${\lambda}_{n}\le {\mu}_{n}$

*for all*$n\in {\mathbb{N}}_{{n}_{o}}$, $\alpha \in (0,1]$

*be any real number*.

*If*(2)

*holds*,

*then*:

- (i)
*If a sequence of bounded real valued functions defined on a set**A**is*${S}_{\lambda}^{\alpha}(f)$-*statistically convergent to**f*,*then it is strongly*${w}_{\mu p}^{\alpha}(f)$-*summable to**f*; - (ii)
*If a sequence of bounded real valued functions defined on a set is*${S}_{\lambda}^{\alpha}(f)$-*statistically convergent to**f*,*then it is strongly*${w}_{\mu p}(f)$-*summable to**f*; - (iii)
*If a sequence of bounded real valued functions defined on a set**A**is*${S}_{\lambda}(f)$-*statistically convergent to**f*,*then it is strongly*${w}_{\mu p}(f)$-*summable to**f*.

## Declarations

### Acknowledgements

Dedicated to Professor Hari M Srivastava.

The authors would like to thank the Management Union of the Scientific Research Projects of Frat University for its financial support under a grant with number *FUBAP* $\mathit{FF}.12.03$. Also, the authors wish to thank the referees for their careful reading of the manuscript and valuable of the manuscript and valuable suggestions.

## Authors’ Affiliations

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