Advances in Fuzzy Systems

Volume 2018, Article ID 8636121, 9 pages

https://doi.org/10.1155/2018/8636121

## Duals and Matrix Classes Involving Cesàro Type Classes of Sequences of Fuzzy Numbers

Department of Mathematics, Gauhati University, Guwahati 781014, India

Correspondence should be addressed to Hemen Dutta; moc.liamffider@80attud_nemeh

Received 31 May 2018; Accepted 26 July 2018; Published 8 August 2018

Academic Editor: Ping-Feng Pai

Copyright © 2018 Hemen Dutta and Jyotishmaan Gogoi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We first define Cesàro type classes of sequences of fuzzy numbers and equip the set with a complete metric. Then we compute the Köthe-Toeplitz dual and characterize some related matrix classes involving such classes of sequences of fuzzy numbers.

#### 1. Introduction

In 1965, Zadeh [1] introduced the concept of fuzzy sets and fuzzy set operations as an extension of the classical notion of the set theory. Later on several authors have discussed different aspects of the theory of fuzzy sets and applied it in various areas of science and engineering such as fuzzy topological spaces, similarity relations and fuzzy orderings, fuzzy possibility theory, fuzzy measures of fuzzy events, and fuzzy mathematical programming. Nowadays, fuzzy set theory is used as a powerful mathematical tool in solving complex real life problems which yields a notion of uncertainty and vagueness. Matloka [2] introduced bounded and convergent sequences of fuzzy numbers and studied their properties. In [3], Nanda studied sequences of fuzzy numbers and proved that every Cauchy sequence of fuzzy numbers is convergent. Since then, different classes of sequences of fuzzy numbers were introduced and studied by various authors. For the works on convergence of fuzzy sequences and series, we refer to Nuray and Savaş [4], Diamond and Kloeden [5], Matloka [2], Esi [6], Kaleva [7], Nanda [8],[3], Dubois and Prade [9], Altınok, Çolak, and Altın [10], Stojaković and Stojaković [11],[12], and Mursaleen, Srivastava, and Sharma [13]. In [14], Subrahmanyam defined the Cesàro summability of sequences of fuzzy numbers and proved some related Tauberian theorems. Some interesting results related to Cesàro summability method of sequences of fuzzy numbers and the Tauberian conditions which guarantee the convergence of summable sequences of fuzzy numbers can be found in Subrahmanyam [14], Talo and Çakan [15], Altın, Mursaleen, and Altınok [16], and Yavuz [17],[18].

*Definition 1 (Goetschel and Voxman [19]). *A fuzzy number is a fuzzy set on the real axis, i.e., a mapping , which satisfies the following four conditions:(i) is normal; i.e., there exists an such that (ii) is fuzzy convex; i.e., for all (iii) is upper semicontinuous.(iv)The set is compact, where denotes the closure of the set in the usual topology of .We denote the set of all fuzzy numbers on by and called it the space of fuzzy numbers. -level set of is defined by

The set is a closed, bounded, and nonempty interval for each which is defined by can be embedded in , since each can be regarded as a fuzzy number defined as

*Definition 2 (Talo and Başar [20]). *Let and . Then the operations addition, scalar multiplication, and product are defined on byand where it is immediate thatandfor all

*Definition 3 (Talo and Başar [20]). *Let be the set of all closed bounded intervals of real numbers such that . Define the relation on as follows:where . Then is a complete metric space (see Diamond and Kloeden [5], Nanda [8]). Talo and Başar [20] defined the metric on by means of Hausdorff metric asThe partial ordering relation on is defined as follows:

*Definition 4 (Talo and Başar [20]). * is a nonnegative fuzzy number if and only if for all It is immediate that if is a nonnegative fuzzy number.

One can see that

Lemma 5 (Bede and Gal [21]). * and. Then *(i)* is a complete metric space.*(ii)*(iii)**(iv)**(v)*

*Lemma 6 (Talo and Başar [20]). The following statements hold:(i)(ii)If *

*By we denote the set of all single sequences of fuzzy numbers on *

*Matloka [2] introduced bounded and convergent sequences of fuzzy numbers and studied their properties. We now quote the following definitions given by Talo and Başar [20] which we will use in a later part of this paper.*

*Definition 7. *A sequence of fuzzy numbers is said to be bounded if the set of fuzzy numbers consisting of the terms of the sequence is a bounded set. That is to say that a sequence is said to be bounded if and only if there exist two fuzzy numbers and such that for all This means that and for all

The fact that the boundedness of the sequence is equivalent to the uniform boundedness of the functions and on Therefore, one can say that the boundedness of the sequence is equivalent to the fact that

*Definition 8. *Consider the sequence of fuzzy numbers . If for every there exists and for all , then we say that the sequence is said to be convergent to the limit and writeand we have the sets consisting of the bounded, convergent, and convergent to sequences of fuzzy numbers (Talo and Başar [20]) as follows: Throughout the text, the summations without limits run from to ; for example,means that

*Definition 9 (Talo and Başar [20]). *Let Then the expressionis called a series corresponding to the sequence of fuzzy number. We denote If the sequence () converges to a fuzzy number , then we say that the series converges to and write which implies as thatuniformly in Conversely, if the fuzzy numbers , and converge uniformly in , then defines a fuzzy number such that

Otherwise, we say the series of fuzzy numbers diverges. Additionally, if the sequence is bounded then we say that the series of fuzzy numbers is bounded.

*Definition 10 (Talo and Başar [20]). *Let be a space of convergent sequences of fuzzy numbers. The sum of a serieswith respect to this rule is defined by

*Definition 11. *Following Khan and Rahman [22], we define the Cesàro sequence space as follows:

If is a positive sequence of real numbers, then, for ,where and denotes summation over the range If for all , then reduces to defined byFollowing Maddox [23], throughout the paper we use the following inequality.

For any and we havewhere and

The classical analogy of was introduced and studied by Lim [24].

The classical Cesàro sequence space and its algebraic dual and related matrix transformations were introduced and studied by various authors like Shiue [25], Leibowitz [26], Lim [24], Khan and Khan [27],[28], Khan and Rahman [22], Johnson and Mohapatra [29], Rahman and Karim [30], etc.

The main purpose of this paper is to define and study the Cesàro sequence space and determine the Köthe-Toeplitz dual and give some related matrix transformations.

*2. Complete Metric Structure*

*2. Complete Metric Structure*

*We equip with a metric and show that this set is complete with respect to the metric defined in the following theorem.*

*Theorem 12. is complete with the metric defined bywhere *

*Proof. *We first show that is a metric space.

It is obvious that and

Now we prove the triangle inequality.

Suppose .Then, using Minkowski’s inequality,Next, to show that is complete under , let us consider that is a Cauchy sequence in Then, for given there exists such thatwhich implies that ; that is, is a Cauchy sequence in So converges to a limit, say ; i.e.,

Suppose For given , there exists such that, for any ,Letting , we obtainSince is arbitrary, letting , we obtainwhich implies that

Next we show that

We have that is bounded in ; i.e., there exists such that Now, for any ,Now,Letting , we obtainThis implies that

This step completes the proof.

*3. Computation of the Köthe-Toeplitz Dual*

*3. Computation of the Köthe-Toeplitz Dual*

*Definition 13 (Talo and Başar [20]). *The Köthe-Toeplitz dual or the dual of a set , denoted by , is defined as follows:where denotes the absolutely summable sequences of fuzzy numbers defined as follows:We now give the following theorem by which the Köthe-Toeplitz dual of will be determined.

*Theorem 14. If and , then*

*Proof. *Let and

We define We want to show that

Let and Then, using inequality (26) together with Lemma 6, we getwhich implies that Thus

Conversely, suppose that for all but Then, for every ,So, following Khan and Rahman [22], we can define a sequence such that and we haveNow we define a sequence as follows:and , where is such thatandwhere the maximum is taken with respect to in .

Therefore, It follows that is divergent. MoreoverNowThus, So, , which is a contradiction to our assumption. Hence

That is,

Then, combining the two results, we obtain

This step completes the proof.

*4. Characterization of Matrix Classes*

*4. Characterization of Matrix Classes*

*An infinite matrix is one of the most general linear operators between two sequence spaces. The study of theory of matrix transformations has always been of great interest to mathematicians in the study of sequence spaces, which is motivated by special results in summability theory.*

*Definition 15 (Talo and Başar [20]). *Let and be any two-dimensional infinite matrix of fuzzy numbers. Then we say that defines a mapping from into and denote it by if, for every sequence , the transform of , , given byexists for each each and is in .

if and only if the series on the right hand side of (53) converges for each and every and we have . A sequence of fuzzy numbers is said to be to if converges to which is called the limit of . Also by we denote that preserves the limit; that is, limit of is equal to the limit of for all .

We writewhere, for each , the maximum is taken with respect to

*Theorem 16. Let be an infinite matrix of fuzzy numbers and . Then if there exists an integer such that whereand *

*Proof. *Suppose there exists an integer such that . Let ThenUsing inequality (26), we obtainTherefore,

The necessity of the above theorem is still open; i.e., we do not know if then the condition (55) holds or not.

*Theorem 17. Let be an infinite matrix of fuzzy numbers and . Then if (55) holds and, for fixed , we have*

*Proof. *Suppose conditions (55) and (58) hold. ThenFrom (59), using similar argument as in Theorem 14, it is easy to verify that the seriesThis implies that Thus, exists.

Now, for each and , we can choose an integer such thatThen It follows thatas . This shows that which proves the theorem.

*Corollary 18. Let be an infinite matrix of fuzzy numbers and . Then if (55) holds and, for fixed , we have*

*Proof. *The proof is obvious.

*Data Availability*

*Data Availability*

*The paper is theoretical in nature and all necessary references are included in the References with proper citation within the main text.*

*Conflicts of Interest*

*Conflicts of Interest*

*The authors declare that they have no conflicts of interest.*

*Acknowledgments*

*Acknowledgments*

*The authors would like to express their gratitude to the University Grants Commission, New Delhi, India, for offering Fellowship to the author J. Gogoi via Award Letter no F./2015-16/NFO-2015-17-OBC-ASS-36722/(SA-III/Website).*

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