 Research
 Open Access
 Published:
On the construction of metrics from fuzzy metrics and its application to the fixed point theory of multivalued mappings
Fixed Point Theory and Applications volume 2015, Article number: 226 (2015)
Abstract
We present a procedure to construct a compatible metric from a given fuzzy metric space. We use this approach to obtain a characterization of a large class of complete fuzzy metric spaces by means of a fuzzy version of Caristi’s fixed point theorem, obtaining, in this way, partial solutions to a recent question posed in the literature. Some illustrative examples are also given.
Introduction and preliminaries
Throughout this paper the symbols \(\mathbb{R}\), \(\mathbb{R}^{+}\), and \(\mathbb{N}\) will denote the set of all real numbers, the set of all nonnegative real numbers and the set of all positive integers, respectively. Our basic reference for general topology is [1].
We start by recalling the notion of a continuous tnorm as well as some types of continuous tnorm which will be crucial throughout this paper.
According to [2], a binary operation \(*: [ 0,1 ] \times [ 0,1 ] \rightarrow [ 0,1 ] \) is a continuous tnorm if ∗ satisfies the following conditions: (i) ∗ is associative and commutative; (ii) ∗ is continuous; (iii) \(a*1=a\) for every \(a\in[0,1]\); (iv) \(a*b\leq c*d\) whenever \(a\leq c\) and \(b\leq d\), with \(a,b,c,d\in[0,1]\).
Distinguished examples of continuous tnorm are ∧, Prod, and \(*_{L}\) (the Łukasiewicz tnorm), which are defined as follows: \(a\wedge b=\min \{a,b\}\), \(a \operatorname{Prod} b=ab\), and \(a*_{L}b=\max\{a+b1,0\}\) for all \(a,b\in [0,1]\).
The following wellknown relations hold: \(\wedge\geq \operatorname{Prod} \geq *_{L}\). In fact, ∧ ≥ ∗ for any continuous tnorm ∗.
Two important classes of continuous are the socalled Yager continuous tnorms and Hamacher continuous tnorms, which are constructed as follows.

(A)
Given \(p>0\), define for each \(a,b\in[0,1]\):
$$a*_{Y_{p}}b=1\min\bigl\{ 1,\bigl[(1a)^{p}+(1b)^{p} \bigr]^{1/p}\bigr\} . $$Then \(*_{Y_{p}}\) is a continuous tnorm referred to in the literature as the Yager continuous tnorm (see e.g. [3]).
It is easy to see that \(a*_{Y_{p_{1}}}b\geq a*_{Y_{p_{2}}}b\) whenever \(p_{1}\geq p_{2}\), with \(a,b\in[0,1]\). In particular we see that \(*_{Y_{1}}\) coincides with the Łukasiewicz tnorm.

(B)
Hamacher constructed in [4] a family of continuous tnorms \((*_{H_{\lambda}})_{\lambda\in\mathbb{R}^{+}}\) defined as follows. Given \(\lambda\in\mathbb{R}\), for each \(a,b\in[0,1]\), \(a*_{H_{\lambda}}b=0\) if \(a=b=\lambda=0\), and
$$a*_{H_{\lambda}}b=\frac{ab}{\lambda+(1\lambda)(a+bab)}, $$otherwise. Furthermore, we have \(a*_{H_{\lambda_{1}}}b\leq a*_{H_{\lambda_{2}}}b\) whenever \(\lambda_{1}\geq\lambda_{2}\), with \(a,b\in[0,1]\). In particular we see that \(*_{H_{1}}\) coincides with the product tnorm.
In this paper we shall work with fuzzy metric spaces in the sense of Kramosil and Michalek [5] (see Definition 1 below). At this point it seems suitable to remark that George and Veeramani introduced in [6] an interesting modification of Kramosil and Michalek’s notion. However, from the wellknown fact that every fuzzy metric space \((X,M,*)\) in the sense of George and Veeramani can be considered as a fuzzy metric space in the sense of Kramosil and Michalek, simply putting \(M(x,y,0)=0\) for all \(x,y\in X\), we deduce that the obtained results in this paper remain valid for fuzzy metric spaces in George and Veeramani’s sense.
Definition 1
(Kramosil and Michalek [5])
A fuzzy metric on a set X is a pair \((M,*)\) such that ∗ is a continuous tnorm and M is a function from \(X\times X\times\mathbb{R}^{+}\) to \([0,1]\) such that for all \(x,y,z\in X\):

(FM1)
\(M(x,y,0)=0\);

(FM2)
\(x=y\) if and only if \(M(x,y,t)=1\) for all \(t>0\);

(FM3)
\(M(x,y,t)=M(y,x,t)\);

(FM4)
\(M(x,z,t+s)\geq M(x,y,t)*M(y,z,s)\) for all \(t,s\geq0\);

(FM5)
\(M(x,y,\_):\mathbb{R}^{+}\rightarrow[0,1]\) is left continuous.
By a fuzzy metric space we mean a triple \((X,M,*)\) such that X is a set and \((M,*)\) is a fuzzy metric on X.
It is well known that for each \(x,y\in X\), \(M(x,y,\_)\) is a nondecreasing function on \(\mathbb{R}^{+}\).
Each fuzzy metric \((M,*)\) on a set X induces a topology \(\tau_{M}\) on X which has as a base the family of open sets \(\{B_{M}(x,\varepsilon ,t):\varepsilon\in(0,1),t>0\}\), where \(B_{M}(x,\varepsilon,t)=\{y\in X:M(x,y,t)>1\varepsilon\}\) for all \(\varepsilon\in(0,1)\), \(t>0\).
A Cauchy sequence in a fuzzy metric space \((X,M,*)\) is a sequence \((x_{n})_{n\in\mathbb{N}}\) in X such that for each \(\varepsilon\in(0,1)\) and \(t>0\) there exists an \(n_{0}\in\mathbb{N}\) satisfying \(M(x_{n},x_{m},t)>1\varepsilon\) whenever \(n,m\geq n_{0}\).
A fuzzy metric space \((X,M,*)\) is said to be complete if every Cauchy sequence \((x_{n})_{n\in\mathbb{N}}\) converges with respect to the topology \(\tau_{M}\), i.e., if there exists \(y\in X\) such that for each \(t>0\), \(\lim_{n}M(y,x_{n},t)=1\) (see e.g. [6]).
It is well known (see e.g. [7]) that every fuzzy metric space is metrizable, i.e., given a fuzzy metric space \((X,M,*)\) there exists a metric on X whose induced topology coincides with the topology \(\tau_{M}\). A short and easy proof of this result consists in showing that the countable family \(\{U_{n}:n\in\mathbb{N}\}\), where \(U_{n}=\{(x,y)\in X\times X:M(x,y,1/n)>11/n\}\) for all \(n\in\mathbb{N}\), is a base for a uniformity on X whose induced topology coincides with the topology \(\tau_{M}\), and then to apply the famous Kelley metrization lemma [1], p.185.
However, the important problem of obtaining a general procedure to construct a visual and manageable compatible metric for any fuzzy metric space, in such a way that the fixed point theory for fuzzy metric spaces could be deduced from the classical fixed point theory for metric spaces, remains unsolved.
Radu obtained some partial but interesting solutions to this problem. Thus, he proved in [8] the following theorem.
Theorem 1
[8]
Let \((X,M,*)\) be a fuzzy metric space such that \(* \geq *_{L}\). For each \(x,y\in X\) put
Then \(d_{R}\) is a metric on X such that
for all \(\varepsilon\in(0,1)\). Therefore, the uniformities, and hence the topologies, induced by \((M,*)\) and \(d_{R}\) coincide on X. In particular, \((X,M,*)\) is complete if and only if \((X,d_{R})\) is complete.
Theorem 1 was successfully applied (see e.g. [9–15]) to deduce several fixed point theorems for complete fuzzy metric spaces from the corresponding results for complete metric spaces. See also [16–18] and the references therein, for some recent contributions to the fixed point theory in fuzzy metric spaces and related structures.
Hicks [19] generalized Theorem 1, by replacing the condition \(* \geq *_{L}\) with the following more general condition:
Later on, Radu [20], Theorem 2.1.7, obtained a substantial improvement of Hicks’ result, which is established below in a slightly different form.
Theorem 2
[20]
Let \((X,M,*)\) be a fuzzy metric space. Suppose that there exists a function \(\mu:\mathbb{R}^{+}\rightarrow\mathbb{R}^{+}\) satisfying the following conditions:

(R1)
μ is continuous on \(\mathbb{R}^{+}\);

(R2)
\(\mu(t)=0\Longleftrightarrow t=0\);

(R3)
\(\mu(t+s)\geq\mu(t)+\mu(s)\) for all \(t,s\geq0\);

(R4)
\(M(x,y,t)>1\mu(t)\), \(M(y,z,s)>1\mu(s)\Longrightarrow M(x,z,t+s)>1\mu(t+s)\).
Then the function \(d_{R\mu}:X\times X\rightarrow\mathbb{R}^{+}\) defined as
is a metric on X such that
for all \(\varepsilon\in(0,1)\). Thus the uniformities, and hence the topologies, induced by \((M,*)\) and \(d_{R\mu}\) coincide on X. Moreover, \((X,M,*)\) is complete if and only if \((X,d_{R\mu})\) is complete.
Here, we shall present a modification of Theorem 2 which can be applied to certain cases and instances where Radu’s theorem does not work. From our approach we deduce a fixed point theorem of Caristi type for multivalued mappings which is valid for any complete fuzzy metric space \((X,M,*)\), as well as a characterization of those complete fuzzy metric spaces \((X,M,*)\) with \(* \geq *_{Y_{p}}\), \(p>0\), in terms of Caristi’s fixed point theorem. These results provide partial solutions to a question posed in [9]. Several illustrative examples are also given.
Constructing metrics from fuzzy metrics
We start this section by establishing our promised modification of Theorem 2 above (a background in this direction may be found in [21], Lemma 1.7).
Theorem 3
Let \((X,M,*)\) be a fuzzy metric space. Suppose that there exists a function \(\alpha:\mathbb{R}^{+}\rightarrow \mathbb{R}^{+}\) satisfying the following conditions:

(c1)
α is strictly increasing on \([0,1]\);

(c2)
\(0<\alpha(t)\leq t\) for all \(t\in(0,1)\) and \(\alpha (t)>1\) for all \(t>1\);

(c3)
\((1\alpha(t))*(1\alpha(s))\geq1\alpha(t+s)\) for all \(t,s\in[0,1]\).
Then the function \(d_{\alpha}:X\times X\rightarrow\mathbb{R}^{+}\) defined as
is a metric on X such that \(d_{\alpha}(x,y)\leq1\) for all \(x,y\in X\).
If, in addition, the function α is left continuous on \((0,1]\), then
for all \(\varepsilon\in(0,1)\). Thus the uniformities, and hence the topologies, induced by \((M,*)\) and \(d_{\alpha}\) coincide on X. Moreover, \((X,M,*)\) is complete if and only if \((X,d_{\alpha})\) is complete.
Proof
We first note that \(\alpha(0)=0\) because, by (c1) and (c2), \(\alpha(0)<\alpha(t)\leq t\) for all \(t\in(0,1)\).
Now we prove that \(d_{\alpha}\) is a metric on X such that \(d_{\alpha }(x,y)\leq1\) for all \(x,y\in X\).
Let \(x,y\in X\). Since \(M(x,y,0)=0<1\alpha(0)\) we deduce that \(d_{\alpha}(x,y)\geq0\). Moreover, \(d_{\alpha}(x,y)\leq1\) because, by (c2), \(\alpha (t)>1\) for all \(t>1\).
We also have \(d_{\alpha}(x,x)=0\) because for each \(t>0\), \(M(x,x,t)=1>1\alpha(t)\).
Next we show that \(x=y\) whenever \(d_{\alpha}(x,y)=0\). Indeed, suppose that \(d_{\alpha}(x,y)=0\). Then \(M(x,y,t)>1\alpha(t)\) for all \(t>0\). Choose an arbitrary \(s>0\). Then, for every \(t\in(0,1)\) with \(t< s\), we obtain
so that \(M(x,y,s)=1\). Since s is arbitrary, we deduce that \(x=y\).
Furthermore, we have \(d_{\alpha}(x,y)=d_{\alpha}(y,x)\) because \(M(x,y,t)=M(y,x,t)\) for all \(t>0\).
It remains to prove that \(d_{\alpha}\) satisfies the triangle inequality. To this end, let \(x,y,z\in X\). If \(d_{\alpha}(x,z)+d_{\alpha}(z,y)\geq 1\) we immediately obtain \(d_{\alpha}(x,y)\leq d_{\alpha}(x,z)+d_{\alpha }(z,y)\), because \(d_{\alpha}(x,y)\leq1\).
Hence, we assume, without loss of generality, that \(d_{\alpha }(x,z)+d_{\alpha}(z,y)<1\). In this case, we shall use the following relation for any \(a\in(0,1)\):
(To show (2) suppose that \(d_{\alpha}(x,y)>a\). Then there exists \(t\in(a,1]\) such that \(M(x,y,t)\leq1\alpha(t)\). Since \(t>a\), we deduce that \(\alpha(t)>\alpha(a)\), and thus, \(1\alpha(a)\leq M(x,y,a)\leq M(x,y,t)\leq1\alpha(t)\), so \(\alpha(t)\leq\alpha(a)\), a contradiction.)
Choose an arbitrary \(\varepsilon>0\) such that \(d_{\alpha }(x,z)+d_{\alpha }(z,y)+2\varepsilon<1\). Then, from the definition of \(d_{\alpha}\) and condition (c3), we deduce
It follows from (2) that \(d_{\alpha}(x,y)\leq d_{\alpha }(x,z)+d_{\alpha}(z,y)+2\varepsilon\). Since ε is arbitrary, we conclude that \(d_{\alpha}(x,y)\leq d_{\alpha}(x,z)+d_{\alpha}(z,y)\).
We have proved that \(d_{\alpha}\) is a metric on X.
Now suppose that α is left continuous on \((0,1]\). If \(d_{\alpha }(x,y)<\varepsilon\), with \(\varepsilon\in(0,1)\), then \(M(x,y,t)>1\alpha (\varepsilon)\) by the definition of \(d_{\alpha}\). Conversely, if \(M(x,y,\varepsilon)>1\alpha(\varepsilon)\), then \(d_{\alpha}(x,y)\leq \varepsilon\) by the definition of \(d_{\alpha}\). In that case, if \(d_{\alpha}(x,y)=\varepsilon\), left continuity of \(M(x,y,\_)\) and of α at ε, provide a contradiction. So \(d_{\alpha }(x,y)<\varepsilon\), and thus we have shown the equivalence (1).
From this equivalence it immediately follows that the uniformities, and hence, the topologies induced by \((M,*)\) and \(d_{\alpha}\) coincide. In particular, a sequence in X is a Cauchy sequence in \((X,M,*)\) if and only if it is a Cauchy sequence in \((X,d_{\alpha})\). Consequently \((X,M,*)\) is complete if and only if \((X,d_{\alpha})\) is complete. This concludes the proof. □
Example 1
Let \((X,M,*)\) be a fuzzy metric space such that \(*\geq *_{Y_{p}}\) for some \(p\geq1\). Let \(\alpha:\mathbb{R}^{+}\rightarrow\mathbb {R}^{+}\) be defined as \(\alpha(t)=t^{p}\) for all \(t\in\mathbb{R}^{+}\). Clearly, α is continuous and strictly increasing on \(\mathbb{R}^{+}\), and satisfies conditions (c2) and (c3) of Theorem 3. Hence, the function \(d:X\times X\rightarrow\mathbb{R}^{+}\) defined as
is a metric on X such that
for all \(\varepsilon\in(0,1)\). Thus the uniformities, and hence the topologies, induced by \((M,*)\) and d coincide on X. Furthermore \((X,M,*)\) is complete if and only if \((X,d)\) is complete.
Example 2
Let \((X,M,*)\) be a fuzzy metric space such that \(*\geq *_{Y_{p}}\) for some \(p\in(0,1)\). Let \(\alpha:\mathbb{R}^{+}\rightarrow \mathbb{R}^{+}\) be defined as \(\alpha(t)=t^{1/p}\) for all \(t\in\mathbb{R}^{+}\). Clearly, α is continuous and strictly increasing on \(\mathbb{R}^{+}\), and satisfies conditions (c2) and (c3) of Theorem 3. Hence, the function \(d:X\times X\rightarrow\mathbb{R}^{+}\), defined as
is a metric on X such that
for all \(\varepsilon\in(0,1)\). Thus the uniformities, and hence the topologies, induced by \((M,*)\) and d coincide on X. Furthermore \((X,M,*)\) is complete if and only if \((X,d)\) is complete.
Although Theorem 2 can we also applied to Examples 1 and 2 above, the following provides an instance where Theorem 3 works but not Theorem 2.
Example 3
Let \((X,M,*)\) be a fuzzy metric space such that \(*\geq *_{H_{\lambda}}\) for some \(\lambda\in(0,2]\). Let \(\alpha:\mathbb{R}^{+}\rightarrow\mathbb{R}^{+}\) defined as \(\alpha(t)=\lambda t/2\) if \(t\in [0,1]\), and \(\alpha(t)=(2t+1)/3\) if \(t>1\). It is clear that α is left continuous and strictly increasing on \(\mathbb{R}^{+}\), and it satisfies condition (c2) of Theorem 3.
Next we show that it also satisfies condition (c3). Indeed, for \(\lambda\in (0,2]\) fixed, and \(t,s\in[0,1]\), we have
If \(t+s>1\), we obtain \(\alpha(t+s)=(2(t+s)+1)/3>1\), so, in this case, the inequality
is trivially satisfied.
If \(t+s\leq1\), we have \(\alpha(t+s)=\lambda t/2\), and an easy computation shows that condition
is equivalent to condition \(\lambda(\lambda1)(t+s)\geq2\lambda4\), which is clearly satisfied because \(\lambda\in(0,2]\).
Therefore, we can apply Theorem 3. However, the function α is not continuous at \(t=1\), so it does not satisfy condition (R1) of Theorem 2. Moreover, it does not satisfy condition (R3) of Theorem 2 because for any \(\lambda\in(5/3,2]\) we have
Application to the fixed point theory of multivalued mappings on fuzzy metric spaces
Given a nonempty set X we shall denote by \(\mathcal{P}_{0}(X)\) the collection of all nonempty subsets of X.
Let \((X,d)\) be a metric space. A multivalued mapping \(T:X\rightarrow \mathcal{P}_{0}(X)\) is said to be a Caristi multivalued mapping (on \((X,d)\)) if there is a lower semicontinuous function \(\varphi:X\rightarrow \mathbb{R}^{+}\) such that for each \(x\in X\) there is \(y_{x}\in Tx\) satisfying \(d(x,y_{x})\leq\varphi(x)\varphi(y_{x})\).
In particular, a selfmapping T of a metric space \((X,d)\) is said to be a Caristi mapping if there is a lower semicontinuous function \(\varphi :X\rightarrow\mathbb{R}^{+}\) such that \(d(x,Tx)\leq\varphi(x)\varphi(Tx)\), for all \(x\in X\).
Caristi proved in [22] his celebrated theorem that every Caristi mapping on a complete metric space has a fixed point.
Kirk proved in [23] that Caristi’s fixed point theorem actually characterizes the metric completeness.
On the other hand, it is well known that Caristi’s fixed point theorem admits an easy a natural multivalued generalization (see e.g. [24]).
These results are usually combined as follows.
Theorem 4
For a metric space \((X,d)\) the following are equivalent.

(1)
\((X,d)\) is complete.

(2)
Every Caristi multivalued mapping \(T:X\rightarrow\mathcal {P}_{0}(X)\) has a fixed point, i.e., there is \(z\in X\) such that \(z\in Tz\).

(3)
Every Caristi mapping \(T:X\rightarrow X\) has a fixed point.
In a recent paper [9] it was obtained a fuzzy version of Theorem 4. To this end, the authors of [9] introduced the following notion.
Let \((X,M,*)\) be a fuzzy metric space. A multivalued mapping \(\dot{T}:X\rightarrow\mathcal{P}_{0}(X)\) is called a fuzzy Caristi multivalued mapping if there exists a lower semicontinuous function \(\varphi :X\rightarrow\mathbb{R}^{+}\) such that for each \(x\in X\) there exists \(y_{x}\in Tx\) satisfying the following condition:
The notion of a fuzzy Caristi mapping for a selfmapping \(T:X\rightarrow X\) is defined in the obvious manner (see [9], Definition 2).
Then in Theorem 3 of [9] the following was proved.
Theorem 5
[9]
Let \((X,M,*)\) be a fuzzy metric space such that \(* \geq *_{L}\). The following are equivalent.

(1)
\((X,M,*)\) is complete.

(2)
Every fuzzy Caristi multivalued mapping \(T:X\rightarrow \mathcal{P}_{0}(X)\) has a fixed point.

(3)
Every fuzzy Caristi’s mapping \(T:X\rightarrow X\) has a fixed point.
Remark 1
Actually Theorem 5 was proved in [9] for multivalued mappings from X to the set \(\mathcal{C}_{0}(X)\) of all nonempty closed subsets of \((X,M,*)\). However, the proof remains valid, without changes, for the case that T take values in \(\mathcal{P}_{0}(X)\).
We are going to improve Theorem 5 in two directions, which will provide partial solutions to a question posed in [9], p.1220.
Definition 2
Let \((X,M,*)\) be a fuzzy metric space and \(\dot{T}:X\rightarrow\mathcal{P}_{0}(X)\) a multivalued mapping. We say that T is an αfuzzy Caristi multivalued mapping if there exist a lower semicontinuous function \(\varphi:X\rightarrow\mathbb{R}^{+}\), and a function \(\alpha:\mathbb{R}^{+}\rightarrow\mathbb{R}^{+}\) which is left continuous on \((0,1]\) and satisfies conditions (c1)(c3) of Theorem 3, and such that for each \(x\in X\) there exists \(y_{x}\in Tx\) for which the following holds:
The notion of an αfuzzy Caristi mapping for a selfmap \(T:X\rightarrow X\) is defined in the obvious manner.
Then we obtain the following.
Theorem 6
Let \((X,M,*)\) be a complete fuzzy metric space. Then every αfuzzy Caristi multivalued mapping has a fixed point.
Proof
Let \(T:X\rightarrow\mathcal{P}_{0}(X)\) be an αfuzzy Caristi multivalued mapping. Then there exist a lower semicontinuous function \(\varphi:X\rightarrow\mathbb{R}^{+}\), and a left continuous function \(\alpha:\mathbb{R}^{+}\rightarrow\mathbb{R}^{+}\) satisfying the conditions of Definition 2. By Theorem 3, the function \(d_{\alpha }:X\times X\rightarrow\mathbb{R}^{+}\) defined as
is a metric on X such that
for all \(\varepsilon\in(0,1)\). Thus the topologies, induced by \((M,*)\) and \(d_{\alpha}\) coincide on X, and \((X,M,*)\) is complete if and only if \((X,d_{\alpha})\) is complete.
We now show that T is a Caristi multivalued mapping on \((X,d_{\alpha})\). Indeed, by Definition 2, for each \(x\in X\) there exists \(y_{x}\in Tx\) for which condition (3) is satisfied. Hence, for each \(t>0\) such that \(M(x,y,t)\leq1\alpha(t)\), we deduce that \(\varphi(x)\varphi(y_{x})\geq t\). Consequently, \(d_{\alpha}(x,y)\leq \varphi(x)\varphi(y_{x})\).
Finally, T has a fixed point by Theorem 4, (1) ⟹ (2). □
Theorem 7
Let \((X,M,*)\) be a fuzzy metric space such that \(* \geq *_{Y_{p}}\) for some \(p>0\). The following are equivalent.

(1)
\((X,M,*)\) is complete.

(2)
Every αfuzzy Caristi multivalued mapping has a fixed point.

(3)
Every αfuzzy Caristi mapping has a fixed point.
Proof
(1) ⟹ (2) follows from Theorem 6, and (2) ⟹ (3) is obvious.
(3) ⟹ (1) Construct the metric d of Example 1 if \(p\geq1\), or the metric d of Example 2 if \(p\in(0,1)\). In any case, we shall prove that every Caristi mapping on \((X,d)\) has a fixed point. Indeed, let \(T:X\rightarrow X\) such that there exists a lower semicontinuous function \(\varphi:X\rightarrow\mathbb{R}^{+}\) satisfying
for all \(x\in X\). We claim that T is an αfuzzy Caristi mapping for α taken as in Example 1 or Example 2, respectively. In fact, if for some \(x\in X\) and \(t>0\), we have \(\varphi(x)\varphi(Tx)< t\) but \(M(x,Tx,t)\leq1\alpha(t)\), it follows that \(d(x,Tx)\geq t\), and thus \(\varphi(x)\varphi(Tx)\geq t\), a contradiction. We conclude that T is an αfuzzy Caristi mapping, and by hypothesis, it has a fixed point. Therefore, by Theorem 4, \((X,d)\) is complete, and thus \((X,M,*)\) is complete by Theorem 3 (or by Examples 1 and 2). □
We conclude the paper with an example that illustrates our results in this section.
Example 4
Let \(X=[0,1]\) and let ∗ be a continuous tnorm such that \(* \geq *_{Y_{p}}\), \(p\in(0,1]\). Define \(M:X\times X\times\mathbb{R}^{+}\rightarrow[0,1]\) as \(M(x,y,0)=0\), \(M(x,y,t)=x*y\) if \(x\neq y\) and \(t>0\), and \(M(x,x,t)=1\) for all \(t>0\). It is well known, and easy to check, that \((X,M,*)\) is a complete fuzzy metric space. For each \(x\in X\backslash \{1\}\), fix a subset \(A_{x}\) of X such that \(x\notin A_{x}\) and \(1\in A_{x} \). Define \(T:X\rightarrow\mathcal{P}_{0}(X)\) as \(T1=\{1\}\) and \(Tx=A_{x}\) for all \(x\in X\backslash\{1\}\). We show that T is an αfuzzy Caristi multivalued mapping on X for \(\varphi:X\rightarrow\mathbb{R}^{+}\) given by \(\varphi(1)=0\) and \(\varphi(x)=1\) for all \(x\in X\backslash \{1\}\) and \(\alpha:\mathbb{R}^{+}\rightarrow\mathbb{R}^{+}\) given by \(\alpha(t)=t^{1/p} \) for all \(t\geq0\). Indeed, let \(x\in X\backslash\{1\}\). Take \(y_{x}=1\in T_{x}\). If \(\varphi(x)\varphi(y_{x})< t\), then \(1< t\), and thus, \(1< t^{1/p}\). Hence \(M(x,y_{x},t)\geq0>1t^{1/p}=1\alpha(t)\). Therefore T is an αfuzzy Caristi multivalued mapping, and all conditions of Theorem 6 are satisfied (note that we can also apply Theorem 7, (1) ⟹ (2)).
References
 1.
Kelley, JL: General Topology. Springer, New York (1955)
 2.
Schweizer, B, Sklar, A: Statistical metric spaces. Pac. J. Math. 10, 314334 (1960)
 3.
Klement, E, Mesiar, R, Pap, E: Triangular Norms. Kluwer Academic, Dordrecht (2000)
 4.
Hamacher, H: Über logische Verknüpfungen unscharfer Aussagen und deren zugehörige Bewertungsfunktionen. In: Progress in Cybernetics and Systems Research, pp. 276287. Hemisphere, New York (1975)
 5.
Kramosil, I, Michalek, J: Fuzzy metrics and statistical metric spaces. Kybernetika 11, 326334 (1975)
 6.
George, A, Veeramani, P: On some results in fuzzy metric spaces. Fuzzy Sets Syst. 64, 395399 (1994)
 7.
Gregori, V, Romaguera, S: Some properties of fuzzy metric spaces. Fuzzy Sets Syst. 115, 485489 (2000)
 8.
Radu, V: On the triangle inequality in PMspaces. STPA, West University of Timişoara 39 (1978)
 9.
Abbas, M, Ali, B, Romaguera, S: Multivalued Caristi’s type mappings in fuzzy metric spaces and a characterization of fuzzy metric completeness. Filomat 29(6), 12171222 (2015)
 10.
Cho, YJ, Grabiec, M, Radu, V: On Nonsymmetric Topological and Probabilistic Structures. Nova Science Publishers, New York (2006)
 11.
Hadžić, O, Pap, E: Fixed Point Theory in Probabilistic Metric Spaces. Kluwer Academic, Dordrecht (2001)
 12.
Mihet, D: A note on Hicks type contractions on generalized Menger spaces. STPA, West University of Timişoara 133 (2002)
 13.
Mihet, D: A Banach contraction theorem in fuzzy metric spaces. Fuzzy Sets Syst. 144, 431439 (2004)
 14.
Radu, V: Some fixed point theorems in PM spaces. In: Stability Problems for Stochastic Models. Lecture Notes in Mathematics, vol. 1233, pp. 125133. Springer, Berlin (1985)
 15.
Radu, V: Some remarks on the probabilistic contractions on fuzzy Menger spaces (The Eighth Intern. Conf. on Applied Mathematics and Computer Science, ClujNapoca, 2001). Autom. Comput. Appl. Math. 11(1), 125131 (2002)
 16.
Chauhan, S, Shatanawi, W, Kumar, S, Radenović, S: Existence and uniqueness of fixed points in modified intuitionistic fuzzy metric spaces. J. Nonlinear Sci. Appl. 7, 2841 (2014)
 17.
Hussain, N, Salimi, P, Parvaneh, V: Fixed point results for various contractions in parametric and fuzzy bmetric spaces. J. Nonlinear Sci. Appl. 8, 719739 (2015)
 18.
Mihet, D: Common coupled fixed point theorems for contractive mappings in fuzzy metric spaces. J. Nonlinear Sci. Appl. 6, 3540 (2013)
 19.
Hicks, TL: Fixed point theory in probabilistic metric spaces. Zb. Rad. Prir.Mat. Fak. (Novi Sad) 13, 6372 (1983)
 20.
Radu, V: Some suitable metrics on fuzzy metric spaces. Fixed Point Theory 5, 323347 (2004)
 21.
O’Regan, D, Saadati, R: Nonlinear contraction theorems in probabilistic spaces. Appl. Math. Comput. 195, 8693 (2008)
 22.
Caristi, J: Fixed point theorems for mappings satisfying inwardness conditions. Trans. Am. Math. Soc. 215, 241251 (1976)
 23.
Kirk, WA: Caristi’s fixedpoint theorem and metric convexity. Colloq. Math. 36, 8186 (1976)
 24.
Ansari, QH: Metric Spaces: Including Fixed Point Theory and SetValued Maps. Alpha Science, Oxford (2010)
Acknowledgements
The authors thank the referees for several useful suggestions. Salvador Romaguera and Pedro Tirado acknowledge the support of the Ministry of Economy and Competitiveness of Spain, grant MTM201237894C0201.
Author information
Additional information
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
All authors contributed equally and significantly in writing this paper. All authors read and approved the final version.
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
About this article
Received
Accepted
Published
DOI
MSC
 47H10
 54A40
 54E50
 54E70
 54H25
Keywords
 metric
 fuzzy metric space
 fuzzy Caristi’s multivalued mapping