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# On statistical convergence and strong Cesàro convergence by moduli

*Journal of Inequalities and Applications*
**volume 2019**, Article number: 298 (2019)

## Abstract

In this paper we will establish a result by Connor, Khan and Orhan (Analysis 8:47–63, 1988; Publ. Math. (Debr.) 76:77–88, 2010) in the framework of the statistical convergence and the strong Cesàro convergence defined by a modulus function *f*. Namely, for every modulus function *f*, we will prove that a *f*-strongly Cesàro convergent sequence is always *f*-statistically convergent and uniformly integrable. The converse of this result is not true even for bounded sequences. We will characterize analytically the modulus functions *f* for which the converse is true. We will prove that these modulus functions are those for which the statistically convergent sequences are *f*-statistically convergent, that is, we show that Connor–Khan–Orhan’s result is sharp in this sense.

## Introduction

A sequence \((x_{k})\) on a normed space \((X,\|\cdot \|) \) is said to be strongly Cesàro convergent to *L* if

The strong Cesàro convergence for real numbers was introduced by Hardy–Littlewood [14] and Fekete [12] in connection with the convergence of Fourier series (see [35], for historical notes, and the most recent monograph [5]).

A sequence \((x_{n})\) is statistically convergent to *L* if for any \(\varepsilon >0\) the subset \(\{k: |x_{k}-L|<\varepsilon \}\) has density 1 on the natural numbers. The term statistical convergence was first presented by Fast [11] and Steinhaus [34] independently in the same year 1951. Actually, a root of the notion of statistical convergence can be detected in [36], where he used the term almost convergence which turned out to be equivalent to the concept of statistical convergence.

Both concepts were developed independently and surprisingly enough, both are related thanks to a result by Connor ([6]) which was sharpened by Khan and Orhan ([15]). Among other results, Khan and Orhan show that a sequence is strongly Cesàro convergent if and only if it is statistically convergent and uniformly integrable. In this circle of ideas, a significant number of deep and beautiful results have been obtained by Connor, Fridy, Khan, Mursaleen, Orhan... and many others (see [2, 9, 13, 26, 27, 31,32,33]). Moreover, the convergence methods are an active research area with important applications (see the recent monograph by Mursaleen [24]). For instance, there are applications in many fields, such as approximation theory [3, 10, 22, 23, 25, 28]. On the other hand, from the point of view of infinite dimensional spaces, there are interesting results that characterize properties of normed spaces by means of some convergence types (see for instance [8, 16,17,18]).

Let us recall that \(f: \mathbb{R}^{+}\to \mathbb{R}^{+}\) is said to be a modulus function if it satisfies:

- (1)
\(f(x)=0 \) if and only if \(x=0\).

- (2)
\(f(x+y)\leq f(x)+f(y)\) for every \(x,y\in \mathbb{R}^{+}\).

- (3)
*f*is increasing. - (4)
*f*is continuous from the right at 0.

In [19, 29, 30] the authors extended the notion of strong Cesàro convergence with respect to a modulus function, and in [1] it was introduced the concept of *f*-statistical convergence in which underlies a new concept of *f*-density of subsets of natural numbers (where *f* is a modulus function). A modulus function *f* was used by Maddox ([20]) to obtain a representation of statistical convergence in terms of strong summability. Later, it was used by Connor ([7]) to study the concepts of strong matrix summability with respect to a modulus. In this paper we will consider only unbounded modulus functions, since the bounded case is reduced only to trivial examples.

In [4] the authors studied the relationship between the *f*-statistical convergence and other Cesàro convergence types defined with respect to a modulus *f*. It has been observed that there is not enough structure to establish Connor–Khan–Orhan’s result in any direction. The aim of this paper is to establish, for the *f*-statistical convergence and a suitable *f*-strong Cesàro convergence that will be introduced in Sect. 2, equivalences analogous to those obtained in Connor–Khan–Orhan’s result.

The notion of *f*-strong Cesàro convergence that we introduce is very handy to use, and it fits as a glove to the *f*-statistical convergence. In fact, we will prove that if a sequence \((x_{n})\) is *f*-strongly Cesàro convergent to *L* then \((x_{n})\) is *f*-statistically convergent to *L* and it is uniformly integrable.

However, the converse of the above result, is not always true, even for bounded sequences. Thus, the following questions arises:

For which modulus functions

fit is possible to obtain the converse of Connor–Khan–Orhan’s result. That is, for which modulus functions do we find that all uniformly integrable andf-statistically convergent sequences aref-strongly Cesàro convergent?

We answer the above question by characterizing analytically such modulus functions, which will be called *compatible modulus functions*. And surprisingly, we can show that such compatible modulus functions are those for which all statistically convergent sequences are *f*-statistically convergent, that is, in some sense Connor–Khan–Orhan’s result is quite sharp. The paper concludes with a brief section devoted to related issues, references and open questions.

## Preliminary results

Let *X* be a normed space. A sequence \((x_{n})\subset X\) is said to be uniformly integrable if

If a sequence \((x_{n})\) is uniformly integrable then \((x_{n}-L)\) is also uniformly integrable for every \(L\in X\). If we consider \(L_{\mu }^{1}[0,1]\) where *μ* is the Lebesgue measure, a sequence \((x_{n})\) is uniformly integrable if and only if the set of simple functions

is uniformly integrable in \(L^{1}_{\mu }[0,1]\) (here \(\chi _{A}(\cdot )\) denotes the characteristic function of *A*). This measure theoretic approach was used by Khan and Orhan in [15], providing an answer to a problem posed by Connor [7] in the *A*-statistical-convergence setting and to another open question posed by Miller ([21]).

Next we define *f*-strong Cesàro convergence:

### Definition 2.1

Let *f* be a modulus function. A sequence \((x_{n})\) is said to be *f*-strongly Cesàro convergent to *L* if

Let us observe that if *f* is bounded, then the constant sequence \(x_{n}=L\) is the only sequence which is *f*-strongly Cesàro convergent to *L*. Indeed, if for some *k*, \(\|x_{k}-L\|=c> 0\) then

which gives the desired result.

In [1], by means of a new concept of density of a subset of \(\mathbb{N}\), it is defined the following non-matrix concept of convergence.

### Definition 2.2

A sequence \((x_{n})\) is said to be *f*-statistically convergent to *L* if for every \(\varepsilon >0\).

Analogously, if *f* is bounded, the only sequences \((x_{n})\) that converge *f*-statistically are the constant sequences. Thus, in what follows we will suppose that *f* is an unbounded modulus function.

Let us observe that if *f* is a modulus function, for all \(x\in \mathbb{R}^{+}\) and \(m\in \mathbb{N}\) we have \(f (\frac{x}{m} ) \geq \frac{1}{m} f(x)\). Indeed, \(f(x)=f(\frac{1}{m}mx)\leq mf( \frac{x}{m})\). As a consequence, it was pointed out in [1] that if \(x_{n}\) is *f*-statistically convergent to *L*, then \((x_{n})\) is statistically convergent to *L*. A similar result remains true for the *f*-strong Cesàro convergence.

### Proposition 2.3

*Let**f**be a modulus function*. *If*
\((x_{n})\)*is**f*-*strongly Cesàro convergent to**L*, *then*
\((x_{n})\)*is strongly Cesàro convergent to**L*.

### Proof

Indeed, for all \(p\geq 1\), there exists \(n_{0}\), such that

for all \(n\geq n_{0}\). That is,

and since *f* is increasing, we have

for all \(n\geq n_{0}\), that is, \((x_{n})\) is strongly Cesàro convergent to *L*. □

However, the converse of the above statement is not true, as it is shown in the following example.

### Example 2.4

Let us consider the modulus function \(f(x)=\log (x+1)\) and the sequence \((x_{k})\) defined as:

Then \((x_{k})\) is strongly Cesàro convergent to 0, indeed

However, \((x_{n})\) is not *f*-strongly Cesàro convergent to 0:

### Definition 2.5

A modulus function *f* is said to be compatible if for any \(\varepsilon >0\) there exist \(\varepsilon '>0\) and \(n_{0}(\varepsilon )\in \mathbb{N}\) such that \(\frac{f(n\varepsilon ')}{f(n)}<\varepsilon \) for all \(n\geq n _{0}\).

### Example 2.6

The functions \(f(x)=x^{p}+x^{q}\), \(0< p,q\leq 1\), \(f(x)=x^{p}+\log (x+1)\), \(f(x)=x+\frac{x}{x+1}\) are modulus functions which are compatible. And \(f(x)=\log (x+1)\), \(f(x)=W(x)\) where *W* is the *W*-Lambert function restricted to \(\mathbb{R}^{+}\) (that is, the inverse of \(xe^{x}\)) are modulus functions which are not compatible. Indeed, let us show that \(f(x)=x+\log (x+1)\) is compatible.

On the other hand if \(f(x)=\log (x+1)\), since

we find that \(f(x)=\log (x+1)\) is not compatible.

### Proposition 2.7

*Let**f**be a compatible modulus function*. *If*
\((x_{n})\)*is statistically convergent to**L**then*
\((x_{n})\)*is**f*-*statistically convergent to**L*.

### Proof

Let us fix \(\varepsilon >0\) arbitrarily small. Since *f* is compatible, there exist \(\varepsilon ' >0\) and \(n_{0}(\varepsilon )\) such that if \(n\geq n_{0}\) then

Let \(c>0\) and let us fix \(\varepsilon '>0\). Since \((x_{n})\) is statistically convergent to *L* then there exists \(m_{0}(\varepsilon )\) (since \(\varepsilon '\) depends on *ε* we find that \(m_{0}\) depends actually on *ε*) such that if \(n\geq m_{0}\)

Since *f* is increasing we have

for all \(n\geq \max \{m_{0},n_{0}\}\), which gives the desired result. □

For *f*-strong Cesàro convergence, we obtain a similar result.

### Proposition 2.8

*Let**f**be a compatible modulus function*. *Then*, *if*
\((x_{n})\)*is strongly Cesàro convergent to**L**then*
\((x_{n})\)*is**f*-*strongly Cesàro convergent to**L*.

### Proof

Let us suppose that \((x_{n})\) is strongly Cesàro convergent to *L*. Then for any \(\varepsilon '>0\) there exists \(n\geq n_{0}\) such that

and since *f* is increasing, then

thus

then by applying the same argument as above we get the desired result. □

### Proposition 2.9

*Let**f**be a modulus function*.

- (1)
*If all statistically convergent sequences are**f*-*statistically convergent*,*then**f**must be compatible*. - (2)
*If all strongly Cesàro convergent sequences are**f*-*strongly Cesàro convergent*,*then**f**must be compatible*.

### Proof

Let \((x_{n})\) be a sequence which is statistically convergent to *L* but not *f*-statistically convergent to *L*. There exist \(\varepsilon _{0}>0\), a constant \(c>0\) and a subsequence \((n_{k})\) such that

On the other hand, since \(x_{n}\to L\) statistically, for all \(\varepsilon >0\) there exists \(n_{0}(\varepsilon )\) such that

for all \(n\geq n_{0}(\varepsilon )\). And since *f* is increasing

In particular for all \(n_{k}\geq n_{0}(\varepsilon )\)

that is, the modulus *f* is not compatible. Hence, we have proved (1).

The proof of (2) follows by a similar argument and is left to the reader. □

## Main results

Let us recall Connor–Khan–Orhan’s result.

### Theorem 3.1

*A sequence is strongly Cesàro convergent to**L**if and only if it is statistically convergent to**L**and uniformly integrable*.

This result connects two concepts that were introduced historically in different times and by different authors. Sometimes, it is easier to verify that a sequence is strongly convergent than to verify that it is statistically convergent. Conversely, if our sequence is uniformly integrable, we do not know if it has limit and we want to check that the sequence is strongly Cesàro convergent, it is usually simpler to check that the sequence is statistically Cauchy. This great advantage so useful pushes us to know what happens in the *f*-statistical-convergence setting.

### Theorem 3.2

*Let*
\((x_{n})\)*be a sequence*, *then if*
\((x_{n})\)*is**f*-*strongly Cesàro convergent to**L**then*
\((x_{n})\)*is**f*-*statistically convergent to**L**and*
\((x_{n})\)*is uniformly integrable*.

### Proof

In order to prove that \((x_{n})\) is *f*-statistically convergent to *L*, it is sufficient to show that, for all \(m\in \mathbb{N}\),

Indeed, let \(\varepsilon >0\) and let us consider *m* such that \(\frac{1}{m+1}\leq \varepsilon \leq \frac{1}{m}\). Then we get

therefore, since *f* is increasing

thus taking limits on *n* we obtain what we desired.

Therefore, let \(m\in \mathbb{N}\), and let us show Eq. (1). We have

Since \((x_{n})\) is *f*-strongly Cesàro convergent to *L*, we have

therefore dividing Eq. (2) by \(f(n)\), and taking the limit as \(n\to \infty \) we obtain for each \(m\in \mathbb{N}\)

which implies that \((x_{n})\) is *f*-statistically convergent to *L*.

On the other hand, since \((x_{n})\) is *f*-strongly Cesàro convergent to *L* then by applying Proposition 2.3 and Connor–Khan–Orhan’s result, we find that \((x_{n})\) is uniformly integrable as we desired. □

Let us denote by \(c_{0}(X)\) the sequences on *X* which are convergent to 0, and \(\ell _{1}(X)\) the sequences \((x_{n})\subseteq X\) such that \(\sum_{n} \|x_{n}\|<\infty \). From the theorem above we deduce that if \((x_{n}) \subseteq X\), and

- (1)
for every

*f*modulus \((x_{n})\) is*f*-strongly Cesáro convergent to*L*

then

- (2)
for every

*f*modulus \((x_{n})\) is*f*-statistically convergent to*L*.

Moreover in [1] it was proved that the statement (2) is equivalent to \((x_{n} - L) \in c_{0} (X)\), i.e., the sequence converges to zero. Analogously we have the following.

### Theorem 3.3

*A sequence*
\((x_{n}) \subseteq X\)*satisfies* (1) *if and only if the sequence*
\((x_{n} - L)\)*belongs to*
\(\ell _{1} (X)\).

### Proof

It is trivial to see that if \(\sum_{n\in \mathbb{N}} \|x_{n} - L\| < +\infty \) then for every *f* modulus \((x_{n})\) is *f*-strongly Cesáro convergent to *L*.

Conversely, let us suppose that for every *f* modulus the sequence \((x_{n})\) is *f*-strongly Cesáro convergent to *L* but \((x_{n} - L) \notin \ell _{1}(X)\).

We consider the set of natural numbers *A* defined by

(where \(\lfloor x \rfloor \) means the greatest integer smaller than *x*) it is clear that *A* is infinite, so using Lemma 3.4 in [1] there exists *g* an unbounded modulus such that

but this is a contradiction. □

### Theorem 3.4

*Let us suppose that**f**is a compatible modulus function and*
\((x_{n})\)*is a uniformly integrable sequence*. *Then if*
\((x_{n})\)*is**f*-*statistically convergent to**L**then*
\((x_{n})\)*is**f*-*strongly Cesàro convergent to**L*. *Moreover*, *if**f**is a modulus such that all uniformly integrable and**f*-*statistically convergent sequences*
\((x_{n})\)*are**f*-*strongly Cesàro convergent*, *then the modulus**f**must be compatible*.

### Proof

Let \((x_{n}) \) be a bounded sequence such that \((x_{n}) \) is *f*-statistically convergent to *L* and uniformly integrable.

Let us consider \(\varepsilon > 0\). Since *f* is compatible there exists \(\varepsilon '>0\) such that

for all \(n\geq n_{0}(\varepsilon )\).

Now, since \((x_{n})\) is uniformly integrable, there exists a natural number \(M>0\) large enough satisfying \(\frac{1}{M}<\varepsilon '\) and for all \(n\in \mathbb{N}\)

And since \((x_{n})\) is *f*-statistically convergent to *L*, there exists a natural number, which we abusively denote by \(n_{0}(\varepsilon )\), such that for all \(n\geq n_{0}(\varepsilon )\)

Therefore

Since *f* is increasing, according to (7) we find that for all \(n\geq n_{0}(\varepsilon )\) the first term of (8) is

On the other hand, let us estimate the second summand of the inequality (8). Using that *f* is increasing and by applying firstly the inequality (6) and later inequality (5) we have for \(n\geq n_{0}(\varepsilon )\)

Finally, for the third summand in (8) by applying inequality (5) we obtain if \(n\geq n_{0}(\varepsilon )\)

Thus, by using inequalities (9), (10), and (11) into inequality (8) we obtain if \(n\geq n_{0}(\varepsilon )\)

that is, \((x_{n})\) is *f*-strongly Cesàro convergent to *L* as we desired.

Now, let us suppose that there exists a uniformly integrable sequence \((x_{n})\) which is *f*-statistically convergent to *L* and \((x_{n})\) is not *f*-strongly Cesàro convergent to *L*. We want to show that in such a case *f* is not compatible.

Let \(\varepsilon >0\), we wish to prove that there exist \(c'>0\) and an increasing subsequence \(\{n_{k}\}\) such that \(c'\leq \frac{f(n_{k} \varepsilon )}{f(n_{k})}\), for all *k*.

Since \((x_{n})\) is uniformly integrable, given \(\varepsilon '>0\) there exist integers \(M>0\) and \(n_{0}(\varepsilon ')\) large enough such that

for all \(n\geq n_{0}(\varepsilon ')\). Therefore

for all \(n\geq n_{0}(\varepsilon ')\). On the other hand, since \((x_{n})\) is *f*-statistically convergent to *L*, there exists an integer which we denote abusively by \(n_{0}(\varepsilon ')\) such that

for all \(n\geq n_{0}(\varepsilon )\).

Therefore, by using inequalities (12) and (13) we have shown that, for every \(\varepsilon >0\) and \(\varepsilon '>0\), there exist integers \(M>0\) and \(n_{0}(\varepsilon ')\) such that for all \(n\geq n_{0}\) we have

for all \(n\geq n_{0}(\varepsilon ')\).

Now, let us observe that if \((x_{n})\) does not converge *f*-strongly Cesàro to *L*, there exist a subsequence \(n_{k}\) and \(c>0\) such that \(c\leq h_{n_{k}}\) for all *k*. Thus, for all \(\varepsilon >0\) and \(\varepsilon '>0\) there exists \(n_{1}(\varepsilon ')>0\) such that if \(n_{k}\geq n_{1}(\varepsilon ')\) we have

Since the first term, \(\varepsilon '>0\), of the above inequality is arbitrarily small, there exists a subsequence of \(n_{k}\) such that \(0<\frac{c}{2}\leq \frac{f(n_{k}\varepsilon )}{f(n_{k})}\) as we desired. □

### Remark 3.5

Let us observe that uniform integrability in Theorem 3.4 is necessary. Set \(n_{j}=j^{2}\) and let us define

The sequence \((x_{k})\) is not uniformly integrable, it is statistically convergent to 0, and it is not strongly Cesàro summable.

### Remark 3.6

Let us observe that the first part of Theorem 3.4 can be obtained directly by using several results. Namely, the converse of Proposition 2.7, Connor–Khan–Orhan’s result and Proposition 2.8. Indeed, if \((x_{n})\) is *f*-statistically convergent, then \((x_{n})\) is statistically convergent. By Connor–Khan–Orhan’s result, since \((x_{n})\) is uniformly bounded, we find that \((x_{n})\) is strongly Cesàro convergent. And finally, since *f* is a compatible modulus, we find that \((x_{n})\) is *f*-strongly Cesàro convergent.

## Concluding remarks and open questions

Given a modulus function *f*, if \(A\subset \mathbb{N}\), the *f*-density of *A*; \(d_{f}(A)\) is defined by

whenever this limit exists. When \(f(x)=x\) then we have the usual density of natural numbers. It is well known that if \(d_{f}(A)=0\) then \(d(A)=0\); the converse in general is not true. Using Proposition 2.7 we can get the following result.

### Corollary 4.1

*If**f**is an unbounded modulus*, *the following conditions are equivalent*:

- (1)
*For every*\((x_{n}) \subset X\)*and*\(x \in X\),*if*\((x_{n})\)*is statistically convergent to**x**then it is also**f*-*statistically convergent to the same**x*. - (2)
*For every*\(A \subset \mathbb{N}\),*if*\(d(A) = 0\)*then*\(d_{f}(A) = 0\). - (3)
*f**is compatible*.

While the usual density works well with complements, that is, \(d(\mathbb{N}\setminus A)=1-d(A)\), however, this property fails for *f*-density. The importance of this property is that it allows us to define statistically convergence by looking at the complement. For this reason, it will be interesting to characterize the modulus function *f* for which the following property is satisfied: For any \(A\subset N\) if \(d_{f}(\mathbb{N}\setminus A)=1\) then \(d_{f}(A)=0\). As was pointed out in [1] if \(f(x)=\log (1+x)\) then the last property is not satisfied. Is it possible to find a counterexample for a compatible module function?

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## Acknowledgements

The authors are supported by Ministerio de Ciencia, Innovación y Universidades under PGC2018-101514-B-100, by Junta de Andalucía FQM-257 and Plan Propio de la Universidad de Cádiz.

## Funding

All authors are supported by Junta de Andalucía FQM-257 and Plan Propio de la Universidad de Cádiz. F. León-Saavedra, M.C. Listán-García and F.J. Pérez Fernández are supported by FEDER/Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación PGC2018-101514-B-100.

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León-Saavedra, F., Listán-García, M.d.C., Pérez Fernández, F.J. *et al.* On statistical convergence and strong Cesàro convergence by moduli.
*J Inequal Appl* **2019, **298 (2019). https://doi.org/10.1186/s13660-019-2252-y

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### Keywords

- Statistical convergence
- Strong Cesàro convergence
- Modulus function
- Uniformly bounded sequence