Research Article | Open Access

A. A. Abd El-Latif, H. Aygün, V. Çetkin, "On -Double Fuzzy Filter Spaces", *Advances in Fuzzy Systems*, vol. 2018, Article ID 3871282, 15 pages, 2018. https://doi.org/10.1155/2018/3871282

# On -Double Fuzzy Filter Spaces

**Academic Editor:**Kemal Kilic

#### Abstract

We give in this paper the definitions of -double fuzzy filter base and -double fuzzy filter structures where and are strictly two-sided commutative quantales, and we also investigate the relations between them. Moreover, we propose second-order image and preimage operators of -double fuzzy filter base and study some of its fundamental properties. Finally, we handle the given structures in the categorical aspect. For instance, we show that the category -**DFIL** of -double fuzzy filter spaces and filter maps between these spaces is a topological category over the category** SET**.

#### 1. Introduction

Kubiak [1] and Šostak [2] introduced the notion of -fuzzy topological space as a generalization of -topological spaces introduced by Chang [3]. At the bottom of it lies the degree of openness of an -fuzzy set. A general approach to the study of topological-type structures on fuzzy powersets was developed in [4–14].

On the other hand, Atanassov [15] introduced the idea of intuitionistic (double graded) fuzzy set. Çoker and his coworker(s) [16, 17] introduced the idea of topology of intuitionistic fuzzy sets. Recently, Mondal and Samanta [18] introduced the notion of intuitionistic gradation of openness which is a generalization of both fuzzy topological spaces [2] and the topology of intuitionistic fuzzy sets [16].

Working under the name “intuitionistic” did not continue because doubts were thrown about the suitability of this term, especially when working in the case of complete lattice These doubts were quickly ended in 2005 by Gutiérrez García and Rodabaugh [19]. They argued that this term is unsuitable in mathematics and applications. They concluded that they work under the name “double.”

The notion of -filter was introduced by Höhle and Šostak [7] as an expansion of fuzzy filter [20–25]. In recent years, -filters were used to introduce many kinds of lattice-valued convergence spaces [26–28]. -filter is an important tool to study -fuzzy topology [29, 30] and -fuzzy uniform space [26]. The structure of this paper is as follows. In Section 2, we recall some fundamental definitions related to quantale lattice by giving illustrative examples and also recall some definitions necessary for the main sections. In Section 3, we define -double fuzzy filter and -double fuzzy filter base and then study relations between them. In the next two sections, we consider two types of second-order Zadeh image and preimage operators of -double fuzzy filter base and examine their characteristics by giving examples.

#### 2. Preliminaries

Throughout this paper, let be a nonempty set. Let be a complete lattice with the least element and the greatest element . For , for all . The second lattice belonging to the context of our work is denoted by and and

A complete lattice is called completely distributive, if for any family in the following identity holds:

*Definition 1 (see [24, 31–33]). *A triple is called a strictly two-sided commutative quantale (stsc-quantale, for short) iff it satisfies the following properties:(L1) is a commutative semigroup.(L2), for all (L3) is distributive over arbitrary joins:

An stsc-quantale is an -distributive quantale (or stsc-biquantale [34]) if is distributive over nonempty meets:

*Remark 2 (see [24, 25, 31–33, 35]). *(1) A complete lattice satisfying the infinite distributive law is an stsc-quantale. In particular, the unit interval is an -distributive quantale.

(2) Every left-continuous t-norm on is an stsc-quantale.

(3) Every continuous t-norm on is an –distributive quantale.

(4) Every GL-monoid is an stsc-quantale.

(5) Let be an stsc-quantale. For each , we define Then, it satisfies Galois correspondence; that is,

*Definition 3 (see [1, 7, 24, 29, 31, 33, 35–38]). *Let be an stsc-quantale. A mapping is called an order-reversing involution if it satisfies the following conditions:

(1) , for each .

(2) If , then , for each .

An stsc-quantale is called a Girard monoid [37] if

Hence, in case is a Girard monoid, residuation induces an order-reversing involution In this paper, we always assume that (resp., ) is a Girard monoid with an order-reversing involution , and the operation is defined by unless otherwise specified, where denote the quantale operations on .

*Remark 4 (see [39]). *When the underlying lattice is the unit interval of the real numbers, the notion of a Girard monoid coincides with the notion of a left-continuous t-norm with strong induced negation .

Lemma 5 (see [34]). *Let be a Girard monoid. For each , one has the following properties:**(1) If , then , , and **(2) **Let be a complete lattice and be a function. The Zadeh image and preimage operators and are defined by *

Lemma 6 (see [40]). *Let be an stsc-quantale and be a function. For each and , one has the following properties:*(1)* with equality if is injective.*(2)*.*

*Definition 7 (see [40]). *Basic scheme for second-order image operators: let be a function.

*Case 1. *ConsiderThis is the Zadeh image operator of the Zadeh image operator. We denote it by ; that is, for all and ,

*Case 2. *ConsiderThis is the Zadeh preimage operator of the Zadeh preimage operator. We denote it by ; that is, for all and , Basic scheme for second-order preimage operators: let be a function.

*Case 1. *ConsiderThis is the Zadeh image operator of the Zadeh preimage operator. We denote it by ; that is, for all and ,

*Case 2. *Consider This is the Zadeh preimage operator of the Zadeh image operator. We denote it by ; that is, for all and , In this paper, we consider additional operators as follows.

Define the operator as , for all and .

Define the operator as , for all and .

All algebraic operations on can be extended pointwise to the sets and as follows: for all , and ;(1) iff (2)(3) iff

*Definition 8 (see [41]). *The pair of maps is called an -double fuzzy topology on if it satisfies the following conditions:

(LO1) , for each ,

(LO2) , ,

(LO3) and , for each ,

(LO4) and , for each .

The triplet is called an -double fuzzy topological space (-dfts, for short). and may be interpreted as gradation of openness and gradation of nonopenness, respectively.

Let and be -double fuzzy topologies on . We say that is finer than ( is coarser than ) if and for all .

Let and be -dfts’s. A function is called LF-continuous iff and , for all .

Thus, we have the category **-DFTOP** where the objects are -dfts’s and the morphisms are -continuous maps between these spaces.

*Example 9. *Let be a set, and Then, is a left-continuous t-norm (Lukasiewicz t-norm) with strong induced negation . Let be defined as follows: Define as follows:Then, the pair is a -dft on .

*Remark 10. *(1) If with an order-reversing involution -dfts is the concept of Mondal and Samanta [18].

(2) If and are frames with 0 and 1, -dfts is the concept of Gutiérrez García and Rodabaugh [19].

(3) If , -dfts is the concept of Abd El-latif [42].

*Definition 11 (see [29, 30]). *A map is called an -filter if it fulfills the following conditions:

(LF1) and .

(LF2) , for each .

(LF3) If , then .

The pair, , is called an -filter space.

#### 3. -Double Fuzzy Filters and -Double Fuzzy Filter Bases

*Definition 12. *The pair of maps is called an -double fuzzy filter (briefly, -dff) on if it fulfills the following axioms:

(DFF1) , for each .

(DFF2) , and , .

(DFF3) and , for each .

(DFF4) If , then and .

The triplet is called an -double fuzzy filter space (briefly, -dffs).

If and are two -dffs on , we say that is finer than (or is coarser than ), denoted by if and only if and , for each .

*Definition 13. *Let and be two -dffs’s. Then, a map is said to be

(i) a filter map if and only if and ;

(ii) a filter preserving map if and only if and .

Normally, the composition of filter maps (resp., filter preserving maps) is a filter map (resp., filter preserving map).

Hence, we get to the category -**DFIL** with objects of all the -dffs’s and the morphisms are filter maps between these spaces.

*Remark 14. *(i) Let be an -filter on and defined by . Then, the pair is an -dff on . Therefore, -dff is a generalization of -filter due to Höhle and Šostak [7, 29].

(ii) If , the definition of -dff coincides with the definition of a proper -intuitionistic fuzzy filter due to Abd El-latif [42].

Theorem 15. *Each -dff produces an -dfts .*

*Proof. *When contrasting the axioms of -dff and -dft, we find (DFF4) implying (DFT4).

Let . Then, for all ; due to (DFF4), we have that and for all . So,Then, we can get an -dft defined by

Theorem 16. *Let and be -dffs’s. If is a filter map, then is an LF-continuous map.*

*Proof. *Let . If or , then the proof is easy. Let and . Then, from the definition of double filter map and Theorem 15, we have

Corollary 17. *The function defined by and is a functor.*

*Notation 18. *Let be two maps and . Then, and are defined as follows:

*Definition 19. *The pair of maps is called an -double fuzzy filter base (briefly, -dffb) on if it fulfills the following axioms:

(DFFB1) , for each .

(DFFB2) , and , .

(DFFB3) and , for each .

If and are two -dffb’s on , we say is finer than (or is coarser than ) denoted by if and only if and , for each .

*Remark 20. *(i) An -dffb is a generalization of -filter base due to Kim and Ko [40].

(ii) If is an -dff, then is an -dffb with and .

(iii) If is an -dffb, then, by (DFFB3), implies and .

Theorem 21. *If is an -dffb, then is the coarsest -dff which satisfies and .*

*Proof. *(DFF1) For each , (DFF2) and (DFF4) are easily checked.

(DFF3) Suppose that there exist such that By the definition of and (), there exist with and such thatSince is an -dffb, Since , we have It is a contradiction. Thus, , for each . Similarly, , for each .

Let be another -dff which is finer than , that is, and . Then, we have

Theorem 22. * are maps fulfilling the following conditions:**(C1) , for each ,**(C2) and and for each finite index set , if , then and .**We define the maps as where and are taken for every finite index set such that , respectively. Then, the following properties are satisfied:**(i) is an -dffb on .**(ii) If , and is an -dffb on , then and .*

*Proof. *(i) (DFFB1) For each , the following is valid:(DFFB2) It is clear by condition (C2).

(DFFB3) For each and for any two finite index sets with and , since , by the definition of and , we get If supremum and infimum are taken over finite index set , respectively, then by (2) and (3), Thus, is an -dffb on .

(ii) For any finite family , the following are true: Then, and .

Theorem 23. *Let and be two -dffb’s on and , respectively, and be a function. Then, one has the following properties:**(i) is a filter map if and only if and .**(ii) is a filter preserving map if and only if and .**(iii) If and , then is a filter map.**(iv) If and , then is a filter preserving map.*

*Proof. *Proving condition (i) is enough since the other conditions are similarly proved.

(i) (:) Since and , for each , it is trivial.

(:) Let and , for each . We will show that is a filter map. For arbitrary , we have Thus, is a filter map.

*Example 24. *Let be a set, be the stsc-quantale with Lukasiewicz t-norm, and be defined by . Define the maps as follows:

It can be seen by easy computation that

(1) and are not -double fuzzy filters but they are -double fuzzy filter bases, so they generate -double fuzzy filters and .

(2) is not an -double fuzzy filter base and it does not satisfy condition (C2) of Theorem 22.

(3) Since , is a filter map and is a filter preserving map though and

We also note that if is considered as a frame, then is an -double fuzzy filter base.

#### 4. The Types , of Preimages and Images of -Double Fuzzy Filter Bases

Theorem 25. *Let be a function and be an -dffb on . Then, the following properties are satisfied.**(i) If implies and , then is an -dffb on and is the coarsest -dff on for which is a filter map.**(ii) If is surjective, then is an -dffb.**(iii) If implies and , is injective, and is an -dff on , then is an -dff on .*

*Proof. *(i) (DFFB1) For each , we have(DFFB2) Since , then and . By assumption, and