In this paper, we introduce the convergence of a fuzzy soft filter with the help of the Q-neighborhoods and study the relations between fuzzy soft nets and fuzzy soft filters. In addition, we use fuzzy soft filters to characterize some basic concepts of a fuzzy soft topological space, such as open sets, closure, T2 separation and continuity.
Citation: Rui Gao, Jianrong Wu. Filter with its applications in fuzzy soft topological spaces[J]. AIMS Mathematics, 2021, 6(3): 2359-2368. doi: 10.3934/math.2021143
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In this paper, we introduce the convergence of a fuzzy soft filter with the help of the Q-neighborhoods and study the relations between fuzzy soft nets and fuzzy soft filters. In addition, we use fuzzy soft filters to characterize some basic concepts of a fuzzy soft topological space, such as open sets, closure, T2 separation and continuity.
In 2001, Maji et al. [14] combined fuzzy sets [26] with soft sets [15] and proposed the concept of fuzzy soft sets. After that, the fuzzy soft set was applied to group theory, decision making, medical diagnosis and other fields (see [1,3,7,8,12,17,23,24,25,27]). Meanwhile, the theory of fuzzy soft set has been developed rapidly. Especially, the research on fuzzy soft topology has made a lot of achievements (see [2,6,9,10,11,13,16,18,19,20,21]).
Noting the contribution of the point approach in fuzzy topology, Roy and Samanta [18] defined a fuzzy soft point in a fuzzy soft topological space. In 2018, Ibedou and Abbas [10] redefined this concept. Recently, Gao and Wu [8] studied the properties of fuzzy soft point introduced in [10] deeply, and pointed that the fuzzy soft point given in [10] was more effective than that given in [18]. They also gave the definitions of a fuzzy soft net consisting of fuzzy soft points and its convergence. On these bases, they characterized the continuity of fuzzy soft mappings by the net approach. In 2014, Cetkin and Aygun [4] proposed the concept of fuzzy soft filters. Besides, Izzettin Demir et al. [5] investigated the convergence theory of fuzzy soft filters by using the technique of neighborhoods. Moreover, they used the fuzzy soft filter convergence to characterize closure, continuity, product space and T2 separation. Finally, they defined the notion of a fuzzy soft filter base and a fuzzy soft ultrafilter and obtained a few results analogous to the ones that held for fuzzy ultrafilters.
It is well known that Q-neighborhoods method has more merits than neighborhoods method. In Section 3, this paper redefines the concept of fuzzy soft filters convergence with the help of the Q-neighborhoods. In Section 4, the fuzzy soft filters are used to characterize some basic concepts of a fuzzy soft topological space, such as open sets, closure, T2 separation and continuity. Finally, a brief conclusion is given in Section 5.
Throughout this paper, U refers to an initial universe, and E is the set of all parameters for U. In this case, U is also denoted by (U, E). IU is the set of all fuzzy subsets over U, where I = [0, 1]. The elements −0,−1∈IU respectively refer to the functions −0(x)=0 and −1(x)=1 for all x∈U. For an element A∈IU, if there exists an x∈U such that A(x)=λ>0 and A(y)=0, ∀y∈U∖{x}, then A is called a fuzzy point over U and is denoted by xλ, meanwhile, x and λ are called the support and height of xλ, respectively. The set of all fuzzy points over U is denoted by FP(U).
The definitions in this section are sourced from the existing literature [8,10,18,19].
Definition 2.1. Let A⊆E. A mapping FA:E→IU, is called a fuzzy soft set over (U,E), where FA(e)=ˉ0 if e∈E∖A and FA(e)≠ˉ0 if e∈A.
The set of all fuzzy soft sets over (U, E) is denoted by FS(U, E).
The fuzzy soft set Fφ∈FS(U,E) is called the null fuzzy soft set and is denoted by ˜Φ. Here, Fϕ(e)=ˉ0 for every e∈E.
For FE∈FS(U,E), if FE(e)=ˉ1 for all e∈E, then FE is called the absolute fuzzy soft set and is denoted by ˜E.
Let FA,FB∈FS(U,E). If FA(e)⊆FB(e) for all e∈E, then FA is said to be a fuzzy soft subset of FB and is denoted by FA˜⊆FB or FB˜⊇FA. If FA˜⊆FB and FB˜⊆FA, then FA and FB are said to be equivalent, denoted by FA=FB.
Remark 2.1. If FA˜⊆FB, then A⊆B.
Definition 2.2. Let FA,FB∈FS(U,E).
(1) The complement of FA, denoted by FcA, is then defined as
FcA(e)={ˉ1−FA(e)fore∈A,ˉ1,otherwise. |
(2) The union of FA and FB is also a fuzzy soft set FC defined by FC(e)=FA(e)∪FB(e) for all e∈E, where C=A∪B, and is denoted by FC=FA˜∪FB.
(3) The intersection of FA and FB is also a fuzzy soft set FC defined by FC(e)=FA(e)∩FB(e) for all e∈E, where C=A∩B, and is denoted by FC=FA˜∩FB.
Similarly, the union (intersection) of a family of fuzzy soft sets may be defined as {FCα:α∈Λ} and denoted by ˜∪α∈ΛFCα(˜∩α∈ΛFCα), where Λ is an arbitrary index set.
Remark 2.2. It is therefore clear that:
(1) ˜Φc=˜E,˜Ec=˜Φ;
(2) (˜∪α∈ΛFAα)c=˜∩α∈ΛFcAα, (˜∩α∈ΛFAα)c=˜∪α∈ΛFcAα.
Definition 2.3. A fuzzy soft topology τ over (U,E) is a family of fuzzy soft sets over (U,E) satisfying the following properties;
(1) ˜Φ,˜E∈τ;
(2) if FA,FB∈τ, then FA˜∩FB∈τ;
(3) if FAα∈τ for all α∈Λ (an index set), then ˜∪α∈ΛFAα∈τ.
If τ is a fuzzy soft topology over (U,E), the triple (U,E,τ) is said to be a fuzzy soft topological space. Each element of τ is called an open set. If FcA is an open set, then FA is called a closed set.
Definition 2.4. Let (U,E,τ) be a fuzzy soft topological space, FA∈FS(U,E).
(1) The intersection of all closed sets FB˜⊇FA is called the closure of FA and is denoted by ¯FA.
(2) The union of all open subsets of FA over (U,E,τ) is called the interior of FA and is denoted by intFA.
Definition 2.5. A mapping ξ:E→IU is called a fuzzy soft point over (U,E) if there is an e∈E such that ξ(e)=xλ∈FP(U), and ξ(a)=ˉ0 when a∈E∖{e}.
In this case, ξ is also denoted by Pxλe, and e is called its parameter support. The set of all fuzzy soft points over (U,E) is denoted by FSP(U,E).
A fuzzy soft point is called a point if no confusion arises.
For Pxλe, Pyβf∈FSP(U,E), it is said that Pxλe is equal to Pyβf, denoted by Pxλe = Pyβf, if and only if x = y, λ = β and e = f.
Definition 2.6. A point Pxλe is said to be quasi-coincident with FA∈FS(U,E), which is denoted by Pxλe˜∈FA, if λ+FA(e)(x)>1 for some x∈U.
Definition 2.7. A fuzzy soft set FA is said to be quasi-coincident with FB, which is denoted by FAqFB, if FA(e)(x)+FB(e)(x)>1 for some x∈U and e∈A∩B.
On the contrary, a fuzzy soft set FA is said to be not quasi-coincident with FB, which is denoted by FAˉqFB, if FA(e)(x)+FB(e)(x)⩽1 for all x∈U and e∈A∩B.
Definition 2.8. Let ξ∈FSP(U,E) and FA,FB∈FS(U,E) on a fuzzy soft topological space (U,E,τ).
(1) FA is said to be a neighborhood of ξ if there exists FB∈τ such that ξ∈FB˜⊆FA.
(2) FA is called a Q-neighborhood of ξ if there exists FB∈τ such that ξ˜∈FB˜⊆FA.
The set of all Q-neighborhoods of ξ is denoted by A(ξ).
Remark 2.3. It is clear that A(ξ) is a directed set with the partial order "˜⊆".
Definition 2.9. Let Δ be a directed set with the partial order "≺". If S(δ)∈FSP(U,E) for any δ∈Δ, then {S(δ),δ∈Δ} is said to be a fuzzy soft net over (U,E), and is denoted by S for simplicity.
In particular, if {S(δ),δ∈Δ} is a fuzzy soft net over (U,E), and there exists an FA∈FS(U,E) such that S(δ)∈FA for any δ∈Δ, then {S(δ),δ∈Δ} is said to be a fuzzy soft net in FA.
A fuzzy soft net is called a net for simplicity if no confusion arises.
Definition 2.10. Let FA∈FS(U,E) and S={S(δ),δ∈Δ} be a net over (U,E). If there exists δ0∈Δ such that S(δ)˜∈FA whenever δ0≺δ, then S is said to be eventually quasi-coincident with FA. If for each δ∈Δ there exists δ0∈Δ with δ≺δ0 such that S(δ0)˜∈FA, then S is said to be frequently quasi-coincident with FA.
Definition 2.11. A net {S(δ),δ∈Δ} over (U,E,τ) is said to be convergent to a point ξ if S is eventually quasi-coincident with each Q-neighborhood of ξ. In this case, ξ is called the limit of S and is denoted by limS(δ).
In this section, we introduce the convergence of a fuzzy soft filter by using the Q-neighborhoods and study the relations between fuzzy soft nets and fuzzy soft filters.
Definition 3.1 [4]. A fuzzy soft filter Φ on (U,E,τ) is a nonempty collection of subsets of FS(U,E) with the following properties:
(FSF1) ˜Φ∉F,
(FSF2) If FA,FB∈F, then FA˜∩FB∈F,
(FSF3) If FA∈F and FA˜⊆FB, then FB∈F.
If F1 and F2 are two fuzzy soft filters on (U,E,τ), we say that F1 is finer than F2 (or F2 is coarser than F1) if and only if F1⊇F2.
The set of all fuzzy soft filters over (U,E) is denoted by FSF(U,E).
Example 3.1. Let ξ∈FSP(U,E), F={FA∈FS(U,E)|ξ˜∈FA} be a fuzzy soft filter. A(ξ) is also a fuzzy soft filter and called the Q-neighborhood filter of ξ.
Definition 3.2. Let F be a fuzzy soft filter in a fuzzy soft topological space (U,E,τ). F is said to be convergent to the fuzzy soft point ξ, denoted by , if A(ξ)⊆F.
Now, we investigate the relations between fuzzy soft nets and fuzzy soft filters.
First, we show that a fuzzy soft filter may generate a fuzzy soft net. In fact, let F={FAi,i∈I}∈FSF(U,E) and
S(i)˜∈FAi for all i∈I. | (3.1) |
The index set I forms a directed set under the relation "≺", where i≺i′ if and only if FAi˜⊇FAi′ for any i,i′∈I. Then SF={S(i),i∈I} is a fuzzy soft net.
Next, we show that a fuzzy soft net may generate a fuzzy soft filter. Suppose thatS={S(δ),δ∈Δ} is a fuzzy soft net in a fuzzy soft topological space (U,E,τ). Let
Fs={FA∈FS(U,E),S is eventually quasi-coincident with FA}. | (3.2) |
It is easy to see that FS∈FSF(U,E).
Definition 3.3. (1) Let F={FAδ,δ∈Δ}∈FSF(U,E), and let S(i) be defined as (3.1). Then SF={S(i),i∈I} is said to be the fuzzy soft net generated by F.
(2) Let S={S(δ),δ∈Δ} be a fuzzy soft net in a fuzzy soft topological space (U,E,τ), and FS be defined as (3.2). Then FS is said to be the fuzzy soft filter generated by S.
In the rest of this paper, SF and FS represent the fuzzy soft net generated by F and the fuzzy soft filter generated by S respectively.
Theorem 3.1. Let (U,E,τ) be a fuzzy soft topological space and ξ∈FSP(U,E). F={FAi,i∈I}∈FSF(U,E), S={S(δ),δ∈Δ} is a fuzzy soft net on (U,E). Then:
(1) F converges to ξ if and only if SF converges to ξ.
(2) S converges to ξ if and only if FS converges to ξ.
Proof. (1) (Necessity) Since F converges to ξ, then A(ξ)⊆F. For each FA∈A(ξ), there exists i∈I such that FAi=FA. Therefore, FAi˜⊇FAi′ when i≺i′∈I. So, S(i′)˜∈FA when i≺i′∈I. Thus, SF converges to ξ.
(Sufficiency) Suppose F does not converge to ξ. Then, there exists an FA∈A(ξ) such that FA∉F. Therefore, FAi⧸˜⊆FA for each i∈I. That is, there exist xi∈U and ei∈E such that FAi(ei)(xi)>FA(ei)(xi) for each i∈I. Take λi∈(0,1) such that FAi(ei)(xi)>1−λi>FA(ei)(xi). Let S(i)=P(xi)λiei, then S(i)˜∈FAi and S(i)˜∉FA. Therefore, the fuzzy soft net SF={S(i),i∈I} does not converge to ξ, which is a contradiction. Thus, F converges to ξ.
(2) (Necessity) Since S converges to ξ, then for each FA∈A(ξ), S is eventually quasi-coincident with FA. That is FA∈FS. Therefore, FS converges to ξ.
(Sufficiency) If FS converges to ξ, then A(ξ)⊆F. That is, for each FA∈A(ξ), S is eventually quasi-coincident with FA. Therefore, S converges to ξ.
In this section, we use fuzzy soft filters to describe open sets, closure, T2 separation and continuity in fuzzy soft topological spaces. First, we give a lemma. Its proof is easy.
Lemma 4.1. Let (U,E,τ) be a fuzzy soft topological space. If for any ξ˜∈FA∈FS(U,E), there exists an open set FB∈A(ξ) such that FB˜⊆FA, FC=˜∪{FB|ξ˜∈FA}, then FC=FA.
Theorem 4.1. Let (U,E,τ) be a fuzzy soft topological space. FA∈FS(U,E) is open if and only if FA∈F for any fuzzy soft point ξ˜∈FA and F∈FSF(U,E) with .
Proof. (Necessity) Since FA is open and ξ˜∈FA, then FA∈A(ξ). So FA∈F follows from .
(Sufficiency) Let ξ˜∈FA arbitrarily. Since limA(ξ)=ξ, then FA∈A(ξ). That is, there exists an open set FB∈A(ξ), such that FB˜⊆FA. From Lemma 4.1, FA = ˜∪{FB|ξ˜∈FA} is open.
Remark 4.1. In [8], it is proved that Pxλe˜∈FA if and only if Pxλe∉FcA. Additionally, if Pxλe˜∈FA, then there exists 0<μ<λ such that Px1−μe∈FA.
Theorem 4.2. Let (U,E,τ) be a fuzzy soft topological space. A fuzzy soft point ξ∈¯FA if and only if there exists F∈FSF(U,E) such that and FBqFA for any FB∈F.
Proof. (Necessity) Let ξ∈¯FA, F=A(ξ), then . For any FB∈A(ξ), we suppose that FB is open without loss of generality. To complete the proof, it is sufficient to show that FBqFA. In fact, if FBˉqFA, then FA(d)(y)+FB(d)(y)⩽1 for any y∈U and d∈E. Hence, FA˜⊆FcB. Noting that FcB is closed, one gets ¯FA˜⊆FcB. Therefore, ξ∈FcB. It follows from Remark 4.1 that ξ⧸˜∈FB, which conflicts with FB∈A(ξ). Thus, FBqFA.
(Sufficiency) Suppose that ξ∉¯FA, then ξ˜∈¯FAc from Remark 4.1. Therefore, ¯FAc∈A(ξ). Since , then A(ξ)⊆F. Therefore, ¯FAc∈F, and hence ¯FAcqFA. That is, there exist y∈U and d∈E such that ¯FAc(d)(y)+FA(d)(y)>1. Equivalently, ¯FA(d)(y)<FA(d)(y), which is a contradiction. Thus, ξ∈¯FA.
Definition 4.1. Let (U,E,τ) be a fuzzy soft topological space. If for any two different points ξ and ζ, there exist FA∈A(ξ) and FB∈A(ζ) such that FA˜∩FB=˜Φ, then (U,E,τ) is said to be T2 separated.
Definition 4.2 [5]. A collection B of subsets of FS(U,E) is called a base for a fuzzy soft filter on (U,E,τ) if the following two conditions are satisfied:
(B1) B≠Φ and ˜Φ∉B,
(B2) If FA, FB∈B, then there is a FC∈B such that FC˜⊆FA˜∩FB.
One readily sees that if B is a base for a fuzzy soft filter on (U,E,τ), the collection
FB={FA∈FS(U,E): there exists a FC∈B such that FC˜⊆FA} |
is a fuzzy soft filter on (U,E,τ). We say that the fuzzy soft filter FB is generated by B.
Theorem 4.3. A fuzzy soft topological space (U,E,τ) is T2 separated if and only if for any F∈FSF(U,E) does not converge to two different points at the same time.
Proof. (Necessity) Let F∈FSF(U,E) with =ξ∈FSP(U,E). For any fuzzy soft point ζ≠ξ, since (U,E,τ) be T2 separated, there exist FA∈A(ξ) and FB∈A(ζ) such that FA˜∩FB=˜Φ. From FA∈F, one knows that FB∉F. Therefore, F does not converge to ζ.
(Sufficiency) Suppose (U,E,τ) is not T2 separated. Then there exist two different points ξ and ζ such that FA˜∩FB≠˜Φ for any FA∈A(ξ) and FB∈A(ζ). It is easy to see that B = {FA˜∩FB|FA∈A(ξ), FB∈A(ζ)} is a fuzzy soft filter base, and the fuzzy soft filter generated by B converges to ξ and ζ at the same time, which contradicts with the condition. Thus (U,E,τ) is T2 separated.
The following definition originates from [3].
Definition 4.3. Let φ:U1→U2 and ψ:E1→E2 be two functions. Then, the pair (φ,ψ) is called a fuzzy soft mapping from (U1,E1) to (U2,E2).
(1) Let FA∈FS(U1,E1). Then, the image of FA under (φ,ψ) is the fuzzy soft set over (U2,E2) defined by (φ,ψ)(FA), where
(φ,ψ)(FA)(k)(y)={∨φ(x)=y∨ψ(e)=kFA(e)(x),ifφ−1(y)≠ϕˉ0,otherwise,∀k∈ψ(E1),∀y∈U2. |
(2) Let FB∈FS(U2,E2). Then, the pre-image of FB under (φ,ψ) is the fuzzy soft set over (U1,E1) defined by (φ,ψ)−1(FB), where
(φ,ψ)−1(FB)(e)(x)=FB(ψ(e))(φ(x)), ∀e∈ψ−1(E2), ∀x∈U1 |
If both φ and ψ are injective (surjective), then the fuzzy soft mapping (φ,ψ) is said to be injective (surjective).
The composition of two fuzzy soft mappings (φ,ψ) from (U1,E1) to (U2,E2) and (φ′,ψ′) from (U2,E2) to (U3,E3) is defined as (φ′∘φ,ψ′∘ψ) from (U1,E1) to (U3,E3).
Definition 4.4. Let (U1,E1,τ1) and (U2,E2,τ2) be two fuzzy soft topological spaces, ξ∈FSP(U1,E1). A fuzzy soft mapping (φ,ψ): (U1,E1,τ1)→(U2,E2,τ2) is said to be fuzzy soft continuous at ξ if for any FB∈A((φ,ψ)(ξ)), there exists FA∈A(ξ), such that (φ,ψ)(FA)˜⊆FB.
A fuzzy soft mapping (φ,ψ): (U1,E1,τ1)→(U2,E2,τ2) is said to be fuzzy soft continuous if (φ,ψ) is fuzzy soft continuous at each fuzzy soft point of (U1,E1,τ1).
Remark 4.2. Theorem 6 in [8] implies that the fuzzy soft continuity of a fuzzy soft mapping in this paper is equivalent to that in [22].
Lemma 4.2 [22]. Let (U1,E1,τ1) and (U2,E2,τ2) be two fuzzy soft topological spaces and (φ,ψ): (U1,E1,τ1)→(U2,E2,τ2) is a fuzzy soft mapping. Then (φ,ψ) is fuzzy soft continuous if and only if (φ,ψ)(¯FA)˜⊆¯(φ,ψ)(FA), ∀FA∈FS(U1,E1).
Lemma 4.3. Let (U1,E1,τ1) and (U2,E2,τ2) be two fuzzy soft topological spaces and (φ,ψ): (U1,E1,τ1)→(U2,E2,τ2) is surjective, F∈FSF(U1,E1), then (φ,ψ)(F)∈FSF(U2,E2).
Proof. (FSF1) and (FSF2) are obvious.
(FSF3) Let FA∈F and (φ,ψ)(FA)˜⊆FB∈FS(U2,E2). Since FA˜⊆(φ,ψ)−1(φ,ψ)(FA)˜⊆(φ,ψ)−1(FB), then FA1=(φ,ψ)−1(FB)∈F. Noting that (φ,ψ) is surjective, one gets that FB=(φ,ψ)(φ,ψ)−1(FB)=(φ,ψ)(FA1)∈(φ,ψ)(F).
Therefore, (φ,ψ)(F)∈FSF(U2,E2).
Theorem 4.4. Let (U1,E1,τ1) and (U2,E2,τ2) be two fuzzy soft topological spaces, and (φ,ψ): (U1,E1,τ1)→(U2,E2,τ2) be surjective. Then, (φ,ψ) is fuzzy soft continuous if and only if lim(φ,ψ)(F)(φ,ψ)(ξ) for any F∈FSF(U1,E1) with ∈FSP(U1,E1).
Proof. (Necessity) Since (φ,ψ) is fuzzy soft continuous, by Definition 4.4, for any FB∈A((φ,ψ)(ξ)), there exists FA∈A(ξ), such that (φ,ψ)(FA)˜⊆FB. Let F∈FSF(U1,E1) with . Then FA∈F, and (φ,ψ)(FA)∈(φ,ψ)(F). Therefore, FB∈(φ,ψ)(F). So
.
(Sufficiency) To complete the proof, we shall show that (φ,ψ)(¯FA)˜⊆¯(φ,ψ)(FA) for any FA∈FS(U1,E1). Take ζ∈(φ,ψ)(¯FA) arbitrarily. Then there is ξ∈¯FA such that ζ=(φ,ψ)(ξ). By Theorem 4.2, there exists F∈FSF(U1,E1) such that and FBqFA for any FB∈F. From Lemma 4.3, (φ,ψ)(F)∈FSF(U2,E2), and
.
For any FC∈(φ,ψ)(F), there is FB∈F such that FC=(φ,ψ)(FB). Owing to FBqFA and Theorem 5 in [8], we have ((φ,ψ)(FB))q((φ,ψ)(FA)). That is, FCq((φ,ψ)(FA)). It follows from Theorem 4.2 that ζ∈¯(φ,ψ)(FA). Recalling the arbitrariness of ζ∈(φ,ψ)(¯FA), we have (φ,ψ)(¯FA)˜⊆¯(φ,ψ)(FA).
In this paper, the convergence of a fuzzy soft filter is redefined by the Q-neighborhoods, some important properties of fuzzy soft topological spaces are characterized by the fuzzy soft filter. The obtained results demonstrate that the methods proposed in this paper are very useful and will provide powerful research tools for further research in this field.
This work is supported by the National Natural Science Foundation of China (11971343) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX19_2015).
The authors declare that there is no conflict of interest in this paper.
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