Function contractive maps in triangular symmetric spaces

Some fixed point results are given for a class of functional contractions acting on (reflexive) triangular symmetric spaces. Technical connections with the corresponding theories over (standard) metric and partial metric spaces are also being established.


Introduction
Let X be a nonempty set. By a symmetric over X we shall mean any map d : X × X → R + := [0, ∞[ with (cf. Hicks and Rhoades [9]) (a01) d(x, y) = d(y, x), ∀x, y ∈ X; the couple (X, d) will be referred to as a symmetric space.
Call the symmetric d, triangular, provided (a02) d(x, z) ≤ d(x, y) + d(y, z), for all x, y, z ∈ X; and reflexive triangular, when it fulfills (the stronger condition) (a03) d(x, z) + d(y, y) ≤ d(x, y) + d(y, z), ∀x, y, z ∈ X. Further, let us say that the symmetric d is sufficient, in case (a04) d(x, y) = 0 =⇒ x = y; hence, x = y =⇒ d(x, y) > 0. The class of sufficient triangular symmetric spaces -also called: dislocated metric spaces (cf. Hitzler [10, Ch 1, Sect 1.4]), or metric-like spaces (cf. Amini-Harandi [5]) -is comparable with the one of (standard) metric spaces. Moreover, the class of (sufficient) reflexive triangular symmetric spaces has multiple connections with the one of partial metric spaces, due to Matthews [16]. As we shall see below, the fixed point theory for functional contractive maps in sufficient (reflexive) triangular symmetric spaces is a common root of both corresponding theories in standard metric spaces and partial metric spaces. This ultimately tells us that, for most of the function contractions taken from the list in Rhoades [18], any such theory over partial metric spaces is nothing but a clone of the corresponding one developed for standard metric spaces. Further aspects will be delineated elsewhere.

Preliminaries
Let (X, d) be a symmetric space; where d(., .) is triangular. Call the subset Y in P 0 (X), d-singleton provided y 1 , y 2 ∈ Y =⇒ d(y 1 , y 2 ) = 0; here, P 0 (X) denotes the class of all nonempty subsets of X.
(A) We introduce a 0d-convergence and 0d-Cauchy structure on X as follows. Given the sequence (x n ) in X and the point x ∈ X, let us say that (x n ), 0d-converges to x (written as: The set of all such points x will be denoted 0d − lim n (x n ); when it is nonempty, then (x n ) is called 0d-convergent; note that, in this case, 0d − lim n (x n ) is a dsingleton, because d is triangular. We stress that the concept (b01) does not match the standard requirements in Kasahara [13]; because, for the constant sequence As d is triangular, any 0d-convergent sequence is 0d-Cauchy too; but, the reciprocal is not in general true. Let us say that (X, d) is 0-complete, if each 0d-Cauchy sequence is 0d-convergent.
(C) Let F (A) stand for the class of all functions from A = ∅ to itself. For any ϕ ∈ F (R + ), the following conditions will be considered: (b05) ϕ is normal: ϕ(0) = 0 and (ϕ(t) < t, ∀t > 0) (b06) ϕ is asymptotic normal: ϕ is normal, and for each sequence (r n ) in For the last condition, we need some conventions. Given the normal function ϕ ∈ F (R + ) and the point s in R 0 By this very definition, we have the representation moreover, from the normality condition, The following limit property holds. Given the sequence (t n ; n ≥ 0) in R + and the point s ∈ R + , define t n ↓ s (as n → ∞), provided [t n ≥ s, ∀n] and t n → s.

Lemma 2.
Let the function ϕ ∈ F (R + ) be normal; and s ∈ R 0 + be arbitrary fixed.
Taking the infimum over ε > 0 in this relation, yields the desired fact.
i) Call the normal function ϕ ∈ F (R + ), right admissible, whenever (b07) holds with Q = ∅; clearly, any such function is nearly right admissible. For example, the normal function ϕ is right admissible, whenever it is right usc on R 0 + ; i.e.: (b08) lim sup t→s+ ϕ(t) ≤ ϕ(s), for each s ∈ R 0 + . Note that (b08) holds whenever ϕ is right continuous on R 0 + . ii) Suppose that the normal function ϕ ∈ F (R + ) is increasing on R + . Then, by a well known result (see, for instance, Natanson, [17, Ch 8, Sect 1]), there exists a denumerable subset Q = Q(ϕ) of R 0 + such that ϕ is (bilaterally) continuous on R 0 + \ Q. This, in particular, tells us that lim sup wherefrom, ϕ is nearly right admissible.

Main result
Let (X, d) be a symmetric space; with, in addition, (c01) d is triangular and (X, d) is 0-complete. Further, let T : X → X be a selfmap of X. Call z ∈ X, d-fixed iff d(z, T z) = 0; the class of all such elements will be denoted as Fix(T ; d). Technically speaking, the points in question are obtained by a limit process as follows. Let us say that Now, concrete circumstances guaranteeing such properties involve (in addition to (c01)) contractive selfmaps T with the d-asymptotic property: (c02) lim n d(T n x, T n+1 x) = 0, ∀x ∈ X. These may be described as follows. Denote, for x, y ∈ X: y)), ∀x, y ∈ X. The main result of this note is Theorem 1. Suppose that the d-asymptotic map T is (d, G; ϕ)-contractive, for some nearly right admissible normal function ϕ ∈ F (R + ). Then, T is a global Picard operator (modulo d).
Proof. Assume that Fix(T ; d) is nonempty. Given z 1 , z 2 ∈ Fix(T ; d), we have In addition (from the triangular property) so that, L(z 1 , z 2 ) ≤ d(z 1 , z 2 ); which tells us that G(z 1 , z 2 ) = d(z 1 , z 2 ). On the other hand, again from the choice of our data, and the triangular property, Combining with the contractive condition yields (for any choice of G) wherefrom (as ϕ is normal), d(z 1 , z 2 ) = 0; so that, Fix(T ; d) is d-singleton. It remains now to establish the Picard property. Fix some x 0 ∈ X; and put (x n = T n x 0 ; n ≥ 0); note that, as T is d-asymptotic, (x n ; n ≥ 0) is 0d-semi-Cauchy.
By the contractive condition, we then have (for G = M 1 ) Passing to limit as n → ∞ in either of these, yields a contradiction. Hence, z is an element of Fix(T ; d); and the proof is complete.

Reflexive triangular case
Now, it remains to determine circumstances under which T is d-asymptotic. Let (X, d) be a symmetric space, with (d01) d is reflexive triangular and (X, d) is 0-complete. Further, let T be a selfmap of X; and fix G ∈ {M 1 , M 2 , M 3 }.
Denote for simplicity Fix(T ) = {z ∈ X; z = T z}; each point of this set is called fixed under T . For both practical and theoretical reasons, it would be useful to determine under which extra conditions upon d, the above result involving Fix(T ; d) may give appropriate information about the points of Fix(T ). Call the symmetric d on X, an almost partial metric provided (d02) d is reflexive triangular and sufficient (see above). The following auxiliary fact will be in effect for us. Let us say that the subset Y ∈ P 0 (X) is a singleton, provided Y = {y}, for some y ∈ X.
The proof is almost immediate; so, we do not give details. Now, assume in the following that (d03) d is an almost partial metric and (X, d) is 0-complete.
Theorem 3. Let the selfmap T be (d, G; ϕ)-contractive, for some nearly right admissible asymptotic normal function ϕ ∈ F (R + ). Then, T n x 0d −→ z as n → ∞, for each x ∈ X. (A) Clearly, each (standard) metric on X is an almost partial metric. Then, Theorem 3 includes the main result in Leader [14]; see also Cirić [7]. In fact, its argument mimics the one in that paper. The only "specific" fact to be underlined is related to the reflexive triangular property of our symmetric d.
(B) According to Matthews [16], call the symmetric d, a partial metric provided it is reflexive triangular and , y), ∀x, y ∈ X (Matthews property). Note that, by the reflexive triangular property, one has (with z = x) d(x, x) + d(y, y) ≤ 2d(x, y), ∀x, y ∈ X; (4.6) and this, along with (d04), yields d=sufficient; i.e.: each partial metric is an almost partial metric. Hence, Theorem 3 is applicable to such objects; its corresponding form is just the main result in Altun et al [4]; see also Romaguera [19]. It is to be stressed here that the Matthews property (d05) was not effectively used in the quoted statement. This forces us to conclude that this property is not effective in most fixed point results based on such contractive conditions. On the other hand, the argument used here is, practically, a clone of that developed for the standard metric setting. Hence -at least for such results -it cannot get us new insights for the considered matter; see also Haghi et al [8]. Clearly, the introduction of an additional (quasi-) order structure on X does not change this conclusion. Hence, the results in the area due to Altun and Erduran [3] are but formal copies of the ones (in standard metric spaces) due to Agarwal et al [2]; see also Turinici [21]. Finally, we may ask whether this reduction scheme comprises as well the class of contraction maps in general complete partial metric spaces taken as in Ilić et al [11]. Formally, such results are not reducible to the above ones. But, from a technical perspective, this is possible; see Turinici [22] for details.

Triangular symmetrics
Let (X, d) be a symmetric space, taken as in (c01); and T be a selfmap of X. Further, take some G ∈ {M 1 , M 2 }.
The argument is based on the evaluation (4.3) being retainable in our larger setting; we do not give details. Now, by simply combining this with Theorem 1, we have (under (c01)) Theorem 4. Suppose that T is (d, G; ϕ)-contractive, for some nearly right admissible asymptotic normal function ϕ ∈ F (R + ). Then, T is a global Picard operator (modulo d).
A basic particular case of this result is to be stated under the lines below. Call the symmetric d(., .) on X, a weak almost partial metric, provided (e01) d is triangular and sufficient (see above). Note that, in such a case, Lemma 4 is still retainable. Assume in the following that (e02) d is a weak almost partial metric and (X, d) is 0-complete.
The proof mimics the one of Theorem 3 (if one takes Theorem 4 as starting point); so, it will be omitted. Now, let us give two important examples of such objects.
(A) Clearly, each (standard) metric on X is a weak almost partial metric. Then, Theorem 5 includes the main result in Boyd and Wong [6]; see also Matkowski [15].
(B) Remember that the symmetric d is called a partial metric provided it is reflexive triangular and (d04)+(d05) hold. As before, (4.6) tells us (via (d04)) that each partial metric is a weak almost partial metric; hence, Theorem 5 is applicable to such objects. In particular, when ϕ is linear (ϕ(t) = λt, t ∈ R + , for some λ ∈ [0, 1[), one recovers the Banach type fixed point result in Aage and Salunke [1]; which, in turn, includes the one in Valero [23]. It is to be stressed here that the Matthews property (d05) was not effectively used in the quoted statement; in addition, the (stronger) reflexive triangular property of d was replaced by the triangular property of the same. As before, the argument used here is, practically, a clone of that developed in the standard metric setting (see above). Further developments of these facts to cyclic fixed point results may be found in Karapinar and Salimi [12].