Research Article | Open Access

Abdulcabbar Sönmez, Feyzi Başar, "Generalized Difference Spaces of Non-Absolute Type of Convergent and Null Sequences", *Abstract and Applied Analysis*, vol. 2012, Article ID 435076, 20 pages, 2012. https://doi.org/10.1155/2012/435076

# Generalized Difference Spaces of Non-Absolute Type of Convergent and Null Sequences

**Academic Editor:**Stevo Stevic

#### Abstract

The aim of the present paper is to introduce the spaces and of generalized difference sequences which generalize the paper due to Mursaleen and Noman (2010). These spaces are the *BK*-spaces of non-absolute type and norm isomorphic to the spaces and , respectively. Furthermore, we derive some inclusion relations determine the , , and duals of those spaces, and construct their Schauder bases. Finally, we characterize some matrix classes from the spaces , and to the spaces , , and *c*.

#### 1. Introduction

By , we denote the space of all complex valued sequences. Any vector subspace of is called a *sequence space*. A sequence space with a linear topology is called a -space provided each of the maps defined by is continuous for all , where denotes the complex field and . A *K*-space is called an -space provided is a complete linear metric space. An -space whose topology is normable is called a -space (see [1, pages 272-273]) which contains , the set of all finitely nonzero sequences. We write , and for the classical sequence spaces of all bounded, convergent, and null sequences, respectively, which are -spaces with the usual sup-norm defined by , where, here and in the sequel, the supremum is taken over all . Also by and , we denote the spaces of all absolutely and -absolutely convergent series, respectively, which are -spaces with the usual norm defined by , where . For simplicity in notation, here and in what follows, the summation without limits runs from to . Also by and , we denote the spaces of all bounded and convergent series, respectively.

Let and be two sequence spaces, and let be an infinite matrix of complex numbers , where . Then, we say that defines a matrix mapping from into and we denote it by writing , if for every sequence the sequence , -transform of , exists and is in , where By , we denote the class of all infinite matrices such that . Thus if and only if the series on the right side of (1.1) converges for each and every , and for all . A sequence is said to be -summable to if converges to , which is called the -limit of .

The *domain * of an infinite matrix in a sequence space is defined by
We denote the collection of all finite subsets of by . Also, we write for the sequence whose only nonzero term is a in the th place for each .

The approach of constructing a new sequence space by means of the matrix domain of a particular limitation method has recently been employed by several authors, for example, [2–14]. They introduced the sequence spaces ( and in [14], ( and ( in [10], in [9], (, and in [8], ( in [2], and in [3], and in [4], ( and in [5], and in [6], and in [7] and and in [11], in [12], and in [13]; where, , , , and denote the Nörlund, Cesáro, Riesz, and Euler means, respectively, , , and are, respectively, defined in [5, 9, 12], , and . Also and denote the sequence spaces generated from the Maddox’s spaces and by Başarir [15]. In the present paper, following [2–14], we introduce the difference sequence spaces and of non-absolute type and derive some related results. We also establish some inclusion relations. Furthermore, we determine the -, -, and -duals of those spaces and construct their bases. Finally, we characterize some classes of infinite matrices concerning the spaces and .

The rest of this paper is organized, as follows.

In Section 2, the -spaces and of generalized difference sequences are introduced. Section 3 is devoted to inclusion relations concerning with the spaces and . In Sections 4 and 5, the Schauder bases of the spaces and are given and the -, -, and -duals of the generalized difference sequence spaces and of non-absolute type are determined, respectively. In Section 6, the classes , , , , , and of matrix transformations are characterized, where . Also, by means of a given basic lemma, the characterizations of some other classes involving the Euler, difference, Riesz, and Cesàro sequence spaces are derived. In the final section of the paper, we note the significance of the present results in the literature related with difference sequence spaces and record some further suggestions.

#### 2. The Difference Sequence Spaces and of Non-Absolute Type

The difference sequence spaces have been studied by several authors in different ways (see e.g. [12, 16–21]). In the present section, we introduce the spaces , and , and show that these spaces are -spaces of non-absolute type which are norm isomorphic to the spaces and , respectively.

We assume throughout that is a strictly increasing sequence of positive reals tending to , that is,

Recently, Mursaleen and Noman [22] studied the sequence spaces and of non-absolute type, and later they introduced the difference sequence spaces and in [21] of non-absolute type as follows: Here and after, we use the convention that any term with a negative subscript is equal to zero, for example, and . With the notation of (1.2) we can redefine the spaces and by where denotes the band matrix representing the difference operator, that is, for .

Let and be nonzero real numbers and define the generalized difference matrix by for all . The -transform of a sequence is We note that the matrix can be reduced to the difference matrices in case and . So, the results related to the matrix domain of the matrix are more general and more comprehensive than the consequences of the matrices domain of and include them.

Now, following Başar and Altay [18] and Aydın and Başar [17], we proceed slightly differently to Kızmaz [19] and the other authors following him and employ a technique of obtaining a new sequence space by means of the matrix domain of a triangle limitation method.

We thus introduce the difference sequence spaces and , which are the generalization of the spaces and introduced by Mursaleen and Noman [21], as follows: With the notation of (1.2), we can redefine the spaces and as where denotes the generalized difference matrix defined by (2.4).

It is immediate by (2.7) that the sets and are linear spaces with coordinatewise addition and scalar multiplication, that is, and are the sequence spaces of generalized differences.

On the other hand, we define the triangle matrix by for all . With a direct calculation we derive the equality and every which leads us together with (1.2) to the fact that

Further, for any sequence we define the sequence which will be frequently used as the -transform of , that is, and so we have Where, here and in what follows, the summation running from to is equal to zero when .

Moreover, it is clear by (2.9) that the relation (2.11) can be written as follows: We assume throughout that the sequences and are connected by the relation (2.11).

Now, we may begin with the following theorem which is essential in the text.

Theorem 2.1. *The difference sequence spaces and are BK-spaces with the norm , that is,
*

*Proof. *Since (2.10) holds and and are -spaces with respect to their natural norms (see [23, pages 217-218]) and the matrix is a triangle, Theorem of Wilansky [24, page 63] gives the fact that and are -spaces with the given norms. This completes the proof.

*Remark 2.2. *One can easily check that the absolute property does not hold on the spaces and , that is, and for at least one sequence in the spaces and , and this shows that and are the sequence spaces of non-absolute type, where .

Now, we give the final theorem of this section.

Theorem 2.3. *The sequence spaces and of non-absolute type are norm isomorphic to the spaces and , respectively, that is, and . *

*Proof. *To prove this, we should show the existence of a linear bijection between the spaces and . Consider the transformation defined, with the notation of (2.11), from to by . Then, for every and the linearity of is clear. Further, it is trivial that whenever and hence is injective.

Furthermore, let and define the sequence by
Then, we obtain
Hence, for every , we get by (2.9)
This shows that and since , we conclude that . Thus, we deduce that and . Hence is surjective.

Moreover, one can easily see for every that
which means that is norm preserving. Consequently is a linear bijection which show that the spaces and are linearly isomorphic.

It is clear that if the spaces and are replaced by the spaces and , respectively, then we obtain the fact that . This completes the proof.

#### 3. The Inclusion Relations

In the present section, we establish some inclusion relations concerning with the spaces and . We may begin with the following theorem.

Theorem 3.1. *The inclusion strictly holds. *

*Proof. *It is obvious that the inclusion holds. Further to show that this inclusion is strict, consider the sequence defined by for all . Then, we obtain by (2.9) that
which shows that and hence , where . Thus, the sequence is in but not in . Hence, the inclusion is strict and this completes the proof.

Theorem 3.2. *If , then the inclusion strictly holds. *

*Proof. *Suppose that and . Then and hence , since the inclusion . This shows that . Consequently, the inclusion holds. Further consider the sequence defined by for all . Then, it is trivial that . On the other hand, it can easily seen that . Hence, which means that . Thus, the sequence is in but not in . We therefore deduce that the inclusion is strict. This completes the proof.

On the other hand, we recall that if and , then , namely, is stronger than the ordinary convergence, hence we have the following

Corollary 3.3. *The inclusions and strictly hold. *

Further, it is obvious that the sequence , defined in the proof of Theorem 3.2, is in but not in . This leads us to the following result.

Corollary 3.4. *Although the spaces and overlap, the space does not include the space . *

Now, to prove the next theorem, we need the following lemma [24, page 4].

Lemma 3.5. * if and only if .*

Theorem 3.6. *The inclusion strictly holds if and only if , where the sequence is defined by
*

*Proof. *Suppose that the inclusion holds. Then we obtain that for every and hence the matrix is in the class . Thus it follows by Lemma 3.5 that
Now, by taking into account the definition of the matrix given by (2.8), we have for every that
Thus, the condition (3.3) implies both
Now we have for every that
and since by (3.5), we obtain by (3.6) that
which shows that .

Conversely, suppose that . Then we have (3.8). Further, for every , we derive that
Then, (3.9) and (3.8) together imply that (3.6) holds. On the other hand, we have for every that
Therefore, it follows by (3.6) that . Particularly, if we take and , then we have which shows that (3.5) holds. Thus, we deduce by the relation (3.4) that (3.3) holds. This leads us with Lemma 3.5 to the consequence that . Hence, the inclusion holds and is strict by Corollary 3.4. This completes the proof.

#### 4. The Bases for the Spaces and

In the present section, we give two sequences of the points of the spaces and which form the bases for those spaces.

If a normed sequence space contains a sequence with the property that for every there is a unique sequence of scalars such that
then is called a *Schauder basis* (or briefly *basis*) for . The series which has the sum is then called the expansion of with respect to and is written as .

Now, since the transformation defined from to in the proof of Theorem 2.3 is an isomorphism, the inverse image of the basis of the space is the basis for the new space . Therefore, we have the following.

Theorem 4.1. *Let for all and . Define the sequence for every fixed by
**
Then, the following statements hold. *(a)*The sequence is a basis for the space and any has a unique representation of the form . *(b)*The sequence is a basis for the space and any has a unique representation of the form , where . *

Finally, it easily follows from Theorem 2.1 that and are the Banach spaces with their natural norms. Then by Theorem 4.1 we obtain the following.

Corollary 4.2. *The difference sequence spaces and are seprable. *

#### 5. The -, -, and -Duals of the Spaces and

In this section, we state and prove the theorems determining the -, -, and -duals of the generalized difference sequence spaces and of non-absolute type.

For arbitrary sequence spaces and , the set defined by
is called the *multiplier space* of and . One can easily observe for a sequence space with and that the inclusions and hold, respectively.

With the notation of (5.1), the -, -, and -duals of a sequence space , which are respectively, denoted by , , and , are defined by It is clear that . Also it can be obviously seen that the inclusions , , and hold whenever .

Now, we may begin with quoting the following lemmas (see [25]) which are needed to prove Theorems 5.5 to 5.8.

Lemma 5.1. * if and only if
*

Lemma 5.2. * if and only if
*

Lemma 5.3. * if and only if (5.4) and (5.5) hold, and
*

Lemma 5.4. * if and only if (5.5) holds. *

Now, we prove the following result.

Theorem 5.5. *The -dual of the spaces and is the set
**
where the matrix is defined via the sequence by
**
for all . *

*Proof. *Let . Then, by bearing in mind the relations (2.11) and (2.14), it is immediate that the equality
holds for all . Thus, we observe by (5.9) that whenever or if and only if whenever or . This means that the sequence or if and only if . We therefore obtain by Lemma 5.4 with instead of that if and only if
which leads us to the consequence that . This completes the proof.

Theorem 5.6. *Define the sets , , , and , as follows:
**
Then and .*

*Proof. *Consider the equality
where the matrix is defined by
for all . Then, we deduce by (5.12) that whenever if and only if whenever . This means that if and only if . Therefore, by using Lemma 5.2, we derive from (5.4) and (5.5) that
Therefore, we conclude that .

Similarly, we deduce from Lemma 5.3 with (5.12) that if and only if . Therefore, we derive from (5.4) and (5.5) that (5.14), (5.15) hold.

Further, with a simple calculation one can easily see that the equality
holds for all . Consequently, from (5.6) we obtain that
Hence, we deduce that . This completes the proof.

*Remark 5.7. *We may note by combining (5.17) with the conditions (5.15) that for every sequence .

Finally, we close this section with the following theorem which determines the -dual of the spaces and :

Theorem 5.8. *The -duals of the spaces and are the set .*

*Proof. *The proof of this result follows the same lines that in the proof of Theorem 5.6 using Lemma 5.4 instead of Lemma 5.2.

#### 6. Certain Matrix Mappings Related to the Spaces and

In this final section, we characterize the matrix classes , , , , , and , where . Also, by means of a given basic lemma, we derive the characterizations of some other classes involving the Euler, difference, Riesz, and Cesàro sequence spaces.

For an infinite matrix , we write for brevity that for all provided the convergence of the series.

The following lemmas will be needed in proving our main results.

Lemma 6.1 (see [24, page 57]). *The matrix mappings between the BK-spaces are continuous. *

Lemma 6.2 (see [25, pages 7-8]). * if and only if
*

Lemma 6.3 (see [25, page 5]). * if and only if (5.5) holds and
*

Lemma 6.4 (see [25, page 5]). * if and only if (5.5) and (6.3) hold. *

Now, we give the following results on the matrix transformations.

Theorem 6.5. *Let be an infinite matrix over the complex field. Then, the following statements hold. * *(i) Let . Then, if and only if
* *(ii) if and only if (6.7) and (6.8) hold, and
*

*Proof. *Suppose that the conditions (6.4)–(6.9) hold and take any . Then, we have by Theorem 5.6 that for all and this implies that the -transform of exists. Also, it is clear that the associated sequence is in the space and hence as for some suitable . Further, it follows by combining Lemma 6.2 with (6.5) that the matrix is in the class , where .

Let us now consider the following equality derived by using the relation (2.11) from the th partial sum of the series :
Then, since and , the series converges for every . Furthermore, it follows by (6.4) that the series converges for all and hence as . Therefore, if we pass to limit in (6.12) as then we obtain by (6.8) that
which can be written as follows:
This yields by taking -norm that
which leads us to the consequence that . Hence, .

Conversely, suppose that , where . Then for all which implies with Theorem 5.6 that the conditions (6.6) and (6.7) are necessary.

On the other hand, since and are -spaces, we have by Lemma 6.1 that there is a constant such that
holds for all . Now, . Then, the sequence is in , where the sequence is defined by (4.2) for every fixed .

Since for each fixed , we have

Furthermore, for every , we obtain by (4.2) that
Hence, since the inequality (6.16) is satisfied for the sequence , we have for any that
which shows the necessity of (6.5). Thus, it follows by Lemma 6.2 that .

Now, let and consider the sequence defined by (2.14) for every . Then, such that , that is, the sequences and are connected by the relation (2.11). Therefore, and exist. This leads us to the convergence of the series and for every . We thus deduce that
Consequently, we obtain from (6.12) as that
and since , we conclude that
which shows the necessity of (6.8). Then relation (6.14) holds.

Finally, since and , the necessity of (6.9) is immediate by (6.14). This completes the proof of Part (i) of the theorem.

Since Part (ii) can be proved by using the similar way that used in the proof of Part (i) with Lemma 5.4 instead of Lemma 6.2, we leave the details to the reader.

*Remark 6.6. *It is clear by (6.10) that the limit
exists for each . This just tells us that condition (6.10) implies condition (6.6).

Now, we may note that for , (see [25, pages 7-8]). Thus, by means of Theorem 5.6 and Lemmas 6.2 and 5.4, we immediately conclude the following theorem.

Theorem 6.7. *Let be an infinite matrix over the complex field. Then, the following statements hold. * *(i) Let . Then, *