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Fixed point theorems for a class of maps in normed Boolean vector spaces
Fixed Point Theory and Applications volumeÂ 2012, ArticleÂ number:Â 47 (2012)
Abstract
In this article, we obtain some fixed and common fixed point theorems for a class of maps on normed Boolean vector spaces satisfying the property (E.A) without using continuity. Our results extend and unify some known results.
2000 Mathematics Subject Classification: Primary 06E30.
1 Introduction and preliminaries
Fixed point theory of Boolean functions has many potential applications to errorcorrecting codes, to switching circuits and to the relationship between the consistency of a Boolean equation, cryptography, convergence of some recursive parallel array processes in Boolean arrays and many others. However, there are only a limited number of results available in literature dealing with fixed point theory for Boolean valued functions (see, for instance, Ghilezan [1] and Rudeanu [2]). In addition, most of these results are in finite dimensional spaces. Recently, Rao and Pant [3] obtained some fixed and common fixed point theorems for asymptotically regular maps on finite dimensional normed Boolean vector spaces (for details of Boolean vector spaces, we refer to Subrahmanyam [4, 5]). The purpose of this article is to obtain some coincidence and common fixed point theorems in infinite dimensional normed Boolean vector spaces for certain classes of maps without using continuity conditions. These maps satisfy the property (E.A) introduced and studied by Aamri and Moutawakil [6] for the first time. It is interesting to note that the above property presents a nice generalization of noncompatible maps. Results obtained herein extend certain results of [6, 7] among others to normed Boolean vector spaces.
Definition 1.1. [6] Let X be a metric space and S, T : X â†’ X. Then the maps S and T are said to satisfy the property (E.A) if there exits a sequence {x_{ n } } in X such that
When T = I, the identity map on X, we obtain the corresponding definition for a single map satisfying the property (E.A) (see [7]).
Example 1.2. [6]. Let X = [0, âˆž) endowed with the usual metric. Define S, T : X â†’ X by Sx=\frac{x}{4} and Tx=\frac{3x}{4} for all x âˆˆ X. Consider a sequence {x}_{n}=\frac{1}{n}. Clearly \underset{n\xe2\u2020\u2019\mathrm{\xe2\u02c6\u017e}}{\text{lim}}S{x}_{n}=\underset{n\xe2\u2020\u2019\mathrm{\xe2\u02c6\u017e}}{\text{lim}}T{x}_{n}=0, and S and T satisfy the property (E.A).
There are maps which do not satisfy the property (E.A).
Example 1.3. [6]. Let X = [2, âˆž) endowed with the usual metric. Define S, T : X â†’ X by Sx = x+1 and Tx = 2x+1, for all x âˆˆ X. Suppose S and T satisfy the property (E.A), then there exists a sequence {x_{ n } } in X satisfying \underset{x\xe2\u2020\u2019\mathrm{\xe2\u02c6\u017e}}{\text{lim}}S{x}_{n}=\underset{x\xe2\u2020\u2019\mathrm{\xe2\u02c6\u017e}}{\text{lim}}T{x}_{n}=t, for some t âˆˆ X. Therefore \underset{x\xe2\u2020\u2019\mathrm{\xe2\u02c6\u017e}}{\text{lim}}{x}_{n}=t1 and \underset{x\xe2\u2020\u2019\mathrm{\xe2\u02c6\u017e}}{\text{lim}}{x}_{n}=\frac{t1}{2}. Then t = 1, which is a contradiction since 1 âˆ‰ X. Hence S and T do not satisfy the property (E.A).
The class of maps satisfying property (E.A) contains the class of the wellknown compatible maps (see Jungck [8]) as well as the class of noncompatible maps. The property (E.A) is very useful in the study of fixed points of nonexpansive maps. In fact the property (E.A) ensure the existence of a coincidence point for a pair of nonexpansive type maps in a metric space [7].
For the completeness, we recall the certain definitions and examples from [4].
Definition 1.4. [4]. Let \mathfrak{V}=\left(\mathfrak{V},+\right) be an additive abelian group and \mathcal{B}=\left(\mathcal{B},+,.{,}^{\mathrm{\xe2\u20ac\xb2}}\right) a Boolean algebra. The set with two operations namely 'addition' and 'scalar multiplication' is said to be a Boolean vector space over (or simply, a \mathcal{B}\text{vector} space) if for all x,y\xe2\u02c6\u02c6\mathfrak{V} and a,b\xe2\u02c6\u02c6\mathcal{B},
(i) a(x + y) = ax + ay;
(ii) (ab)x = a(bx) = b(ax);
(iii) 1x = x; and
(iv) if ab = 0, then (a + b)x = ax + bx.
The elements of and will be denoted respectively, by x, y, z and a, b, c (with or without indices); the zero of and also nullelement of will both be denoted by 0, while the universal element (= 0') of will be denoted by 1.
Example 1.5. [4]. Let be any Boolean algebra and be the additive group of the corresponding Boolean ring; then is a \mathcal{B}\text{vector} space if we define: For a\xe2\u02c6\u02c6\mathcal{B} and x\xe2\u02c6\u02c6\mathfrak{V}, ax = the (Boolean) product of a and x in .
Example 1.6. [4]. Let R be any Ring with unity element 1 and let denotes the set of all the central idempotents of R; then it is known that \left(\mathcal{B},\xe2\u02c6\xaa,\xe2\u02c6\copyright {,}^{\mathrm{\xe2\u20ac\xb2}}\right) is a Boolean algebra, where, by definition, a âˆª b = a + b  ab, a âˆ© b = ab and a' = 1  a. If is the additive group of the ring R, and for a\xe2\u02c6\u02c6\mathcal{B} and x\xe2\u02c6\u02c6\mathfrak{V}, ax = the product of a and x in R, then is a Boolean vector space over \left(\mathcal{B},\xe2\u02c6\xaa,\xe2\u02c6\copyright {,}^{\mathrm{\xe2\u20ac\xb2}}\right).
Definition 1.7. [4]. A Boolean vector space over a Boolean algebra is said to be normed (or simply, normed) if and only if there exists a map â•‘.â•‘ (called norm): \mathfrak{V}\xe2\u2020\u2019\mathcal{B} such that

(i)
â•‘xâ•‘ = 0 if and only if x = 0, and

(ii)
â•‘axâ•‘ = aâ•‘xâ•‘ for all a\xe2\u02c6\u02c6\mathcal{B} and x\xe2\u02c6\u02c6\mathfrak{V}.
In view of [[4], Corollary 3.2] we note the following.
Let be a normed vector space and \mathfrak{V}\xc3\u2014\mathfrak{V}\xe2\u2020\u2019\mathcal{B} then d(x, y) = â•‘x  yâ•‘ defines a Boolean metric on , i.e.,

(i)
d(x, y) = 0 if and only if x = y;

(ii)
d(x, y) = d(y, x) and

(iii)
d(x, z) < d(x, y) + d(y, z).
Definition 1.8. [4]. Let be a Ïƒcomplete (= countably complete) Boolean algebra. If {a_{ n } } is a sequence of elements of , we define:
and if
then we say that a_{ n } converges to a, and will be written as a_{ n } â†’ a. A sequence {a_{ n } } in is a Cauchy sequence if and only if d*(a_{ n }, a_{ m } ) â†’ 0, where d* is the Boolean metric on defined by d*(a, b) = a'b + ab'.
Definition 1.9. [4]. If {x_{ n } } is a sequence of elements of , we say that x_{ n } â†’ x\left(x\xe2\u02c6\u02c6\mathfrak{V}\right) if and only if â•‘x_{ n }  xâ•‘ â†’ 0; and a sequence {x_{ n } } in is Cauchy if and only if â•‘x_{ n }  x_{ m } â•‘ â†’ 0.
The following definition is the consequence of Definitions 1.1 and 1.4.
Definition 1.10. Let be a normed Boolean vector space and S,T:\mathfrak{V}\xe2\u2020\u2019\mathfrak{V}. Then the maps S and T are said to satisfy the property (E.A) if there exits a sequence {x_{ n } } in such that
2 Main results
Let be a normed Boolean vector space and T:\mathfrak{V}\xe2\u2020\u2019\mathfrak{V}. A point z\xe2\u02c6\u02c6\mathfrak{V} is called a fixed point of T, if Tz = z. The point z is called a coincidence point of S,T:\mathfrak{V}\xe2\u2020\u2019\mathfrak{V}, if Sz = Tz and a common fixed point, if z = Sz = Tz.
For the sake of brevity, we shall use the following denotations:
Let Î¦ denotes the class of all functions \mathrm{\xcf\u02c6}:\mathcal{B}\xe2\u2020\u2019\mathcal{B} satisfying:
(i) Ïˆ is continuous;
(ii) Ïˆ(a) < a'.
Example 2.1. Let A be a nonempty set and the class of all subsets of A with three set operation âˆ©, âˆª, ' (union, intersection, and complement). Then defines a Boolean algebra. Now, let \mathrm{\xcf\u2020}:\mathcal{B}\xe2\u2020\u2019\mathcal{B} be the function defined by
where 1 denotes the universal element of . Then Ï† âˆˆ Î¦.
Now we obtain a coincidence theorem for a pair of selfmaps on a normed Boolean vector space.
Theorem 2.2. Letbe a normed Boolean vector space andS,T:\mathfrak{V}\xe2\u2020\u2019\mathfrak{V}such that
(A) S\mathfrak{V}\xe2\u0160\u2020T\mathfrak{V}
(B) the maps S and T satisfy the property (E.A);
(C) d(Sx, Sy) = Ïˆ(M(x, y)) for allx,y\xe2\u02c6\u02c6\mathfrak{V}, where Ïˆ âˆˆ Î¦.
If S\mathfrak{V} or T\mathfrak{V} is a complete subspace of then S and T have a coincidence in .
Further, S and T have a unique common fixed point provided that SSu = Su and S and T commute at the coincidence point.
Proof. Since the maps S and T satisfy the property (E.A) there exits a sequence {x_{ n } } in such that
Suppose T\mathfrak{V} is a complete subspace of then there exists a point u\xe2\u02c6\u02c6\mathfrak{V} such that Tu = t.
Using (C), we get
Making n â†’ âˆž, we obtain d(Su, Tu) = Ïˆ(d(Su, Tu)) < (d(Su, Tu))'. Which follows that Su = Tu and u is a coincidence point of S and T.
Further, if SSu = Su, and the maps S and T commute at their coincidence point u then Su = STu = TSu and Su is a common fixed point of S and T.
The case in which S\mathfrak{V} is a complete subspace of , the condition S\mathfrak{V}\xe2\u0160\u2020T\mathfrak{V} implies that there exists a point u\xe2\u02c6\u02c6\mathfrak{V} such that Tu = t and the previous proof works.
To prove the uniqueness of common fixed point, we suppose z_{1}, z_{2} are two common fixed points of S and T. Then Sz_{1} = Tz_{1} = z_{1} and Sz_{2} = Tz_{2} = z_{2}. Using the condition (C)
Which follows that z_{1} = z_{2}. â–¡
Corollary 2.3. Let be a complete normed Boolean vector space and S:\mathfrak{V}\xe2\u2020\u2019\mathfrak{V} such that
(I) S satisfies the property (E.A)
(II) d(Sx, Sy) = Ïˆ(m(x, y)) for allx,y\xe2\u02c6\u02c6\mathfrak{V}, where Ïˆ âˆˆ Î¦.
Then S has a unique fixed point.
Proof. This comes from Theorem 2.2 when T is an identity map on . â–¡
Theorem 2.4. Letbe a normed Boolean vector space andS,T:\mathfrak{V}\xe2\u2020\u2019\mathfrak{V}such that
(A) S\mathfrak{V}\xe2\u0160\u2020T\mathfrak{V}
(B) the maps S and T satisfy the property (E.A);
(C) d(Sx, Sy) = Ïˆ(G(x, y)) for allx,y\xe2\u02c6\u02c6\mathfrak{V}, where Ïˆ âˆˆ Î¦.
If S\mathfrak{V} or T\mathfrak{V} is a complete subspace of then S and T have a coincidence in .
Further, S and T have a unique common fixed point provided that SSu = Su and S and T commute at the coincidence point.
Proof. Since the maps S and T satisfy the property (E.A) there exits a sequence {x_{ n } } in such that
Suppose T\mathfrak{V} is a complete subspace of then there exists a point u\xe2\u02c6\u02c6\mathfrak{V} such that Tu = t.
Using (C), we get
Making n â†’ âˆž, we obtain d(Su, Tu) = Ïˆ(d(Su, Tu)) < (d(Su, Tu))'. Which follows that Su = Tu and u is a coincidence point of S and T.
Further, if SSu = Su, and the maps S and T commute at their coincidence point u then Su = STu = TSu and Su is a common fixed point of S and T.
The case in which S\mathfrak{V} is a complete subspace of , the condition S\mathfrak{V}\xe2\u0160\u2020T\mathfrak{V} implies that there exists a point u\xe2\u02c6\u02c6\mathfrak{V} such that Tu = t and the previous proof works.
To prove the uniqueness of common fixed point, we suppose z_{1}, z_{2} are two common fixed points of S and T. Then Sz_{1} = Tz_{1} = z_{1} and Sz_{2} = Tz_{2} = z_{2}. Using the condition (C)
Which follows that z_{1} = z_{2}. â–¡
Corollary 2.5. Let be a complete normed Boolean vector space and S:\mathfrak{V}\xe2\u2020\u2019\mathfrak{V} such that
(I) S satisfies the property (E.A);
(II) d(Sx, Sy) = Ïˆ(g(x, y)) for allx,y\xe2\u02c6\u02c6\mathfrak{V}, where Ïˆ âˆˆ Î¦.
Then S has a unique fixed point.
Proof. This comes from Theorem 2.4 when T is an identity map on . â–¡
Now we present an example to illustrate our results.
Example 2.6. Let A be a nonempty set and the class of all subsets of A. Then the class with three set operation +, â€¢, ' (union, intersection, and complement) defines a Boolean algebra. Further, this class with the set operation "exclusiveor addition" âŠ• (symmetric difference of sets) defines a Boolean ring. Let \mathfrak{V}=\left(\mathfrak{V},\xe2\u0160\u2022\right) be the additive abelian group of this Boolean ring. For a in and x in , we define ax = a â€¢ x (the Boolean) product of a and x in . Then is a Boolean vector space over .
Let S,T:\mathfrak{V}\xe2\u2020\u2019\mathfrak{V} be selfmaps defined by
Let \mathrm{\xcf\u02c6}:\mathcal{B}\xe2\u2020\u2019\mathcal{B} defined by Ï†(a) = a  1 for all a\xe2\u02c6\u02c6\mathcal{B}, where '1' is the universal element of .
Now there exists a sequence {x_{ n } } in defined by x_{ n }= Î¾ for all n = 1, 2,..., such that
Further, S\mathfrak{V}\xe2\u0160\u201aT\mathfrak{V} and
where â•‘â€¢â•‘ is any norm defined on . Thus all the hypotheses of Theorem 2.2 are satisfied and Î¾ is a common fixed point of S and T.
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Mishra, S., Pant, R. & Murali, V. Fixed point theorems for a class of maps in normed Boolean vector spaces. Fixed Point Theory Appl 2012, 47 (2012). https://doi.org/10.1186/16871812201247
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DOI: https://doi.org/10.1186/16871812201247