1. Introduction
The concept of a continuous function in topological spaces and multifunction continuity of a basic concept in the theory of classical point set topology have received considerable attention by several authors not only in the field of functional analysis but also in other branches of applied sciences such as mathematical economics, control theory and fuzzy topology. With regard to this, several scholars have generalized the notion of continuity in (bi-) topological spaces by use of more weaker forms of open and closed sets called semiopen and semiclosed sets: [1] - [9].
The fundamental notion of semiopen sets and continuity of functions on such sets was introduced by Levine [4] in the realm of topological spaces. These concepts have then been generalized to bitopological spaces by Maheshwari and Prasad [5], and as well by Bose [10]. Berge [11] on the other hand introduced the notion of upper and lower continuous multifunctions and lately, the concept got generalized to bitopological spaces by Popa [6], in which he investigated how multifunctions preserved the conserving properties of connectedness, compactness and paracompactness between bitopological spaces.
Noiri and Popa [12] in 2000, investigated the notions of upper (lower) continuous multifunction and M-continuous function deal to Berge [11] and, Popa and Noiri [8] respectively, and extended these notions to upper and lower M-continuous multifunctions. They observed that, upper (lower) continuous multifunctions have properties similar to those of upper (lower) continuous functions and multifunctions between topological spaces. Recently, Matindih and Moyo [9] have generalized the ideas of [12] and studied upper and lower M-asymmetric semicontinuous multifunctions from which they showed that, semicontinuous multifunctions have quasi-properties to those for upper and lower continuous functions and M-continuous multifunctions between topological spaces, with the only difference that, the semiopen sets in use are bitopologgically structured.
Irresolute mappings, quasi to continuity on the other hand, have received considerable attention by various scholars. This notion of irresolute functions and their fundamental properties were first introduced and investigated in 1972 by Crossley and Hildebrand [3]. They showed that, irresolute functions are not necessarily continuous and neither are continuous functions necessarily irresolute. As a generalization to this idea, Ewert and Lipski [13], then studied the concept of upper and lower irresolute multivalued mappings, followed by Popa [14] who looked at some characterizations of upper and lower irresolute multifunctions. Popa in [1] further extended his investigation to studying the structural properties and relationship of irresolute multifunctions to continuous functions and multifunctions in topological spaces. As a generalization to Popa’s [14] idea, Matindih et al. [2] have recently studied upper and lower M-asymmetric irresolute multifunctions in bitopologgical spaces with sets satisfying certain minimal structures. They clearly showed that, upper and lower M-asymmetric irresolute multifunctions have properties similar to those of upper and lower irresolute multifunctions [1] defined between topological spaces. Further, with the aid of counter examples, it was shown that, upper and lower M-asymmetric irresolute multifunctions are respectively upper and lower M-asymmetric semicontinuous; however, the converse was not in general true.
In this present paper, we introduce and investigate the notion of M-asymmetric irresolute multifunctions as an extension from Matindih et al., [2] and, a generalization of results by Popa [1].
Our paper is organized as follows: Section 2 presents necessary concepts concerning semiopen sets, m-asymmetric semiopen sets and upper and lower M-asymmetric irresolute multifunctions [2]. In Section 3, we present and discuss characterizations of M-asymmetric irresolute multifunctions defined between bitopological spaces with sets satisfying certain minimal structure. Section 4 gives concluding remarks.
2. Preliminaries and Basic Properties
We present in this section some properties and notations to be used throughout this paper. For more details, we refer the reader to ( [2] [5] [6] [7] [9] [10] [12] [14] [15]).
By a bitopological space
, we mean a nonempty set X on which are defined two topologies
and
. The concept was first introduced by Kelly [15]. In the sequel,
or in short X will denote a bitopological space unless clearly stated. For a bitopological space
,
;
, we shall denote the interior and closure of a subset A of X with respect to the topology
by
and
respectively.
Definition 2.1. [10] [15] Let
,
;
be a bitopological space and A be any nonvoid subset of X.
1) A is said to be
-open if
; i.e.,
where
and
. The complement of an
-open set is a
-closed set.
2) The
-interior of A denoted by
(or
-
) is the union of all
-open subsets of X contained in A. Evidently, if
, then A is
-open.
3) The
-closure of A denoted by
is defined to be the intersection of all
-closed subsets of X containing A. Observe that asymmetrically,
and
.
Definition 2.2. [5] [10] Let
,
;
be a bitopological space and, let A and B be non-void subsets of X.
1) A is said to be
-semiopen in X provided there is a
-open subset O of X such that
, equivalently
. It’s complement is said to be
-semiclosed.
2) The
-semiinterior of A denoted by
-
is defined to be the union of
-semiopen subsets of X contained in A. The
-semiclosure of A denoted by
-
, is the intersection of all
-semiclosed sets of X containing A.
3) B is said to be a
-semi-neighbourhood of a point
provided there is a
-semiopen subset O of X such that
.
The family of all
-semiopen and
-semiclosed subsets of X will be denote by
and
respectively.
Definition 2.3. [7] [12] A subfamily mX of a power set
of a nonevoid set X is said to be a minimal structure (briefly m-structure) on X if both
and X lies in mX.
The pair
is called an m-space and the members of
are said to be mX-open sets.
Definition 2.4. [9] Let
,
;
be a bitopological space and mX a minimal structure on X generated with respect to mi and mj. An ordered pair
is called a minimal bitopological space.
As our minimal structure mX on X is determined by the two minimal structures mi and mj,
;
generated by the two topologies
and
respectively, we shall denote it in the sense of Matindih and Moyo [9] by mij(X) (or simply mij) and call the pair
(or
) a minimal bitopological space unless explicitly defined.
Definition 2.5. [9] A minimal structure mX on a bitoplogical space
,
;
; is said to have property (
) in the sense of Maki [7] if the union of any collection of mij(X)-open subsets of X belongs to mX.
Definition 2.6. [9] Let
,
;
be a minimal bitopological space and A a subset of X. A is said to be:
1) mij-semiopen in X if there exists an mi-open set O such that
, that is,
.
2) mij-closed in X if there exists an mi-open set O such that
whenever
, that is,
or equivalently, there exists an mi-closed set K in X such that
.
We shall denote the collection of all mij-semiopen and mij-semiclosed sets in
by
and
respectively.
Remark 2.7. [9] Let
,
;
be a minimal bitopological space.
1) if
and
, the any mij-semiopen set is
-semiopen.
2) every mij-open (resp. mij-closed) set is mij-semiopen (resp. mij-semiclosed), but the converse is not generally true, see Examples 3.5 [9].
The mij-open sets and the mij-semiopen sets are generally not stable for the union condition. However, for certain mij-structures, the class of mij-semiopen sets are stable under union of sets as in the Lemma below:
Lemma 2.8. [9] Let
,
;
be an mij-space and
be a family of subsets of X. Then, the properties below hold:
1)
provided for all
,
.
2)
provided for all
,
.
Remark 2.9. It should be generally noted that, the intersection of any two mij-semiopen sets may not be mij-semiopen in a minimal bitopological space
, as Example 3.9 of [9] illustrates.
Definition 2.10. [9] Let
,
;
be an mij-space and let O be a subset of X. Then,
1) O is an mij-semineighborhood of a point x of X if there exists an mij-semiopen subset U of X such that
.
2) O is an mij-semineighborhood of a subset A of X if there exists an mij-semiopen subset U of X such that
.
3) O is an mij-semineighbourhood which intersects a subset A of X if there exists a semiopen subset U of X such that
and
.
Definition 2.11. [9] Let
,
;
be an mij-space and A a none-void subset of X. Then, we denote and defined the mij-semiinterior and mij-semiclosure of A respectively by:
1)
,
2)
,
Remark 2.12. [9] For any bitopological spaces
;
1)
is a minimal structure on X.
2) In the following, we denote by mij(X) a minimal structure on X as a generalization of
and
. For a none-void subset A of X, if
, then by Definition 2.11;
a)
,
b)
.
Lemma 2.13. [9] For any mij-space
,
;
and nonevoid subsets A and B of X, the following properties of mij-semiclosure and mij-semiinterior holds:
1)
and
.
2)
and
provided
.
3)
,
,
and
.
4)
provided
.
5)
provided
.
6)
and
Lemma 2.14. [9] Let
,
;
be an mij-space and A be a nonevoid subset of X. For each
containing
,
if and only if
.
Lemma 2.15. [9] For an mij-space
,
;
and any none-void subset A of X, the properties below holds:
1)
,
2)
.
Lemma 2.16. [9] For an mij-space
,
;
and any none-void subset A of X, the properties below are true:
1)
.
2)
provided
. The converse to this assertion is not necessarily true as was shown in Example 3.17 of [9].
Remark 2.17. [9] For a bitopological space
,
;
the families
and
are all mij-structures of X satisfying property
.
Lemma 2.18. [9] For an mij-space
,
;
with mij satisfying property (
) and subsets A and F of X, the properties below holds:
1)
provided
.
2) If
, then
.
Lemma 2.19. [9] Let
,
;
be an mij-space with mij satisfying property
and A be a nonevoid subset of X. Then the properties given below holds:
1)
if and only if A is an mij(X)-semiopen set.
2)
if and only if X\A is an mij-semiopen set.
3)
is mij-semiopen.
4)
is mij-semiclosed.
Lemma 2.20. [9] Let
,
;
be an mij-space with mij satisfying property
and let
be an arbitrary collection of subsets of X. If
for every
, then
.
Lemma 2.21. [9] Let
,
;
be an mij-space and A a nonvoid subset of X. If mij-satisfy property
, there holds;
1)
, and
2)
.
If the property
of Make is removed in the previous Lemma, the equality does not necessarily hold, refer to Example 3.23 [9]
Lemma 2.22. [9] For a minimal bitopological space
,
;
, and any subset U of X, the properties below holds:
1)
.
2)
.
Definition 2.23. [12] A point-to-set correspondence
from a topological space X to a topological space Y such that for every point x of X,
is a nonevoid subset of Y is called a multifunction.
In the sense of Berge [11], we shall denote and define the upper inverse and lower inverse of a non-void subset H of Y with respect to a multifunction F respectively by:
Generally for
and
between Y and the power set
,
provided
. Clearly for a nonvoid subset H of Y,
and also,
For any nonvoid subsets A and H of X and Y respectively,
and
and also,
.
Definition 2.24. [13] [14] A multifunction
, between topological spaces X and Y is said to be:
1) upper irresolute at a point xo of X provided for any semiopen subset H of Y such that
, there exists a semiopen subset O of X with
such that
, whence,
.
2) lower irresolute at a point xo of X provided for any semiopen subset G of Y such that
, there exists a semiopen subset O of X with
such that
for all
, whence,
.
3) upper (resp lower) irresolute provided it is upper (resp lower) irresolute at all points xo of X.
Definition 2.25. [1] A multifunction
, between topological spaces X and Y is said to be irresolute at a point
if for any semiopen sets
such that
and
there exists a semiopen set
containing xo such that
and
for every
.
A multifunction
is irresolute if it is irresolute at every point
.
Definition 2.26. [9] Let
and
,
;
be minimal bitopological spaces. A multifunction
is said to be:
1) Upper mij-semi-continuous at some point
provided for any mij(Y)-open set V satisfying
, there is an mij(X)-semiopen set O with
for which
, whence,
.
2) Lower mij-semi-continuous at some point
provided for each mij(Y)-open set V satisfying
, we can find an mij(X)-semiopen set O with
such that for all
,
.
3) Upper (resp Lower) mij-semi continuous if it is Upper (resp Lower) mij-semi continuous at each and every point of X.
Definition 2.27. [2] A multifunction
between minimal bitopological spaces
and
,
;
said to be:
1) upper M-asymmetric irresolute at a point
provided for any mij(Y)-semiopen set H such that
, there exists an mij(X)-semiopen set O with
such that
whence,
.
2) lower M-asymmetric irresolute at a point
provided for any mij(Y)-semiopen set H that intersects
, there exists a mij(X)-semiopen set O with
such that
for all
whence,
.
3) upper (resp lower) M-asymmetric irresolute provided it is upper (resp lower) M-Asymmetric irresolute at each and every point xo of X.
3. Some Characterization on M-Asymmetric Irresolute Multifunctions
We now study a special kind of Asymmetric-multifunction F for which the inverse image of any mij-asymmetric semiopen set under F is as well an mii-asymmetric semiopen set.
Definition 3.1. A multifunction
;
, between bitopological spaces X and Y having certain minimal conditions is said to be M-Asymmetric irresolute at a point
if for any mij(Y)-semiopen sets H1 and H2 such that
and
, there exists an mij(X)-semiopen set O containing xo such that
and
for every
.
The multifunction F is M-Asymmetric irresolute if it is M-Asymmetric irresolute at every point
.
Remark 3.2. Clearly, we can noted that, provided a multifunction is both upper and lower M-Asymmetric irresolute, then it is M-asymmetric irresolute and vice-versa, as we illustrate in Example 3.3 below.
Example 3.3. Define a multifunction
by:
where
with minimal structures defined by
and
and,
with minimal structures given by
and
. Clearly, F is M-asymmetric irresolute at
and hence, is both upper and lower M-asymmetric irresolute.
Indeed,
and
for the mij(Y)-semiopen sets
and
whence,
and
. This also holds for some mij(X)-semiopen sets containing c and f respectively.
We now discuss some characterizations of M-asymmetric irresolute multifunctions and look at some of the relationship to.
Theorem 3.4. A multifunction
,
;
for which
satisfies property
, is said to be M-asymmetric irresolute at some point xo of X if and only if for any mij(Y)-semiopen sets H1 and H2 such that
and
, there holds the relation:
Proof. For necessity, suppose
for any
with
and
. By Definition 2.10 (iii) and Lemma 2.14, there exists an mij(X)-semiopen neighborhood O of xo such that
. Thus,
and
. Since the sets H1 and H2 are mij(Y)-semiopen, we have
and
for all
. Because O is mij(X)-semiopen, we infer F to be an M-asymmetric irresolute multifunction at a point xo of X.
For sufficiency, suppose F is an M-asymmetric irresolute multifunction at a point xo of X, let H1 and H2 be any mij(Y)-semiopen sets such that
and
. By Definition 3.1, there exists an mij(X)-semiopen set O with
for which
and for all
,
. And so,
and
. Because F is a multifunction,
and
so that
. Since, O is an mij(X)-semiopen set,
. Thus, since Y satisfies property
, we have by applying Lemmas 2.18, 2.19 and 2.20 that,
Theorem 3.5. Let
,
;
satisfy property
. A multifunction
is M-asymmetric irresolute at a point xo of X if and only if for any mij(X)-semiopen set O containing xo and any mij(Y)-semiopen sets H1 and H2 with
and
, there exists a nonempty mij(X)-open set
such that
and
for all
.
Proof. For necessity, let,
be a family of all mij(X)-open neighbourhoods of a point xo. Then, for any mij(X)-open set
and any mij(Y)-semiopen sets H1 and H2 with
and
, we can find some nonempty mij(X)-open set UO contained in O for which
and
for every
. Put
, then Q is an mij(x)-open set,
by Theorem 3.4 and
and
for all
. Set
, then
Hence, Q is an mij(X)-semiopen set,
and
and
for all
, whence,
and
as R is an mij(X)-semiopen set by Definition 2.6. Therefore, F is an M-asymmetric irresolute multifunction at a point xo of X.
For sufficiency, suppose F is M-asymmetric irresolute at a point xo of X, let H1 and H2 be mij(Y)-semiopen sets with
and
. Then,
by Theorem 3.4. Let O be an mij(X)-open neighbourhood of xo. From Remark 2.7 (2), we infer
and
, whence respectively
and
. Hence,
. But then,
and we then have,
. Since
we obtain from Lemma 2.14 that
. Set
. Then,
,
,
,
and so, UO is an mij(X)-open set. Consequently,
and
for all
.
Remark 3.6. Theorem 3.5 clearly, indicates that, every M-asymmetric irresolute multifunction is generally M-asymmetric semi-continuous however, the converse is nor generally true, as we shall see in Example 3.7 and Example 3.8 respectively.
Example 3.7. Recall in Example 3.3 that, the mapping
defined by:
is M-asymmetric irresolute. However, F is not M-asymmetric semicontinuous henceforth not M-asymmetric continuous. Indeed,
and
for the mij(Y)-semiopen sets
and
whence,
and
. However,
and
are mij(Y)-open sets whose inverse images are mij(X)-semiopen, implying that F is M-asymmetric semicontinuous.
Example 3.8. Define a multifunction
by:
where
on which are defined the minimal structures
and
and also
on which we have
and
. Then, F is M-asymmetric semicontinuous but not M-asymmetric irresolute respectively since,
but
and also
but,
.
Theorem 3.9. Let
,
;
satisfy property
and let
be a multifunction. Then, the properties below are equivalent:
1) F is M-asymmetric irresolute,
2) The set
is mij(X)-semiopen for every mij(Y)-semiopen sets H1 and H2;
3) The set
is mij(X)-semiclosed for any mij(Y)-semiclosed sets K1 and K2;
4) There holds the set inclusion
for any subsets E1 and E2 of Y.
5) For any given subsets V1 and V2 of Y, there holds the set inclusion
6) The relation
holds true for any given subsets Q1 and Q2 of Y.
Proof. 1. (1)
(2): Suppose (1) holds, let xo be any point of X and, let H1 and H2 be any mij(Y)-semiopen set satisfying
and
.
By definition,
and
for some mij(X)-semiopen neighborhood O of xo, containing all x. Thus,
and
and by hypothesis,
. By Theorem 3.5 and 3.9 of [2], we have,
and
respectively. Thus, it follows from Theorem 3.4 that,
Since xo is arbitrarily chosen in
, Definition 2.6 consequently implies
is an mij(X)-semiopen set.
(2)
(3): Suppose (2) holds. Let K1 and K2 be mij(Y)-semiclosed sets. Then,
and
are mij(Y)-semiopen by Lemma 2.13, and so,
and
. Thus, by Lemma 2.15, we have
and so,
. Similarly,
so that
. Thus,
But then,
. Consequently,
is an mij(X)-semiclosed set.
(3)
(4): Suppose (3) holds, let E1 and E2 be any arbitrary subsets of Y. Then
and
are mij(Y)-semiclosed sets and so,
is an mij(X)-semiclosed set. Thus,
But
and
, thus,
and
. As a result, Lemma 2.13 implies
(4)
(5): Suppose (4) holds. Since
and
are all mij(Y)-semiclosed sets for any subsets V1 and V2 of Y and by Lemma 2.21,
for any
, it follows that,
Hence,
.
(5)
(6): Suppose (5) is true. Since for each subsets Q1 and Q2 of Y,
is mij(Y)-semiopen and
, we have from Lemma 2.15 that,
Consequently,
.
(6)
(1): Suppose (6) holds, let xo be any point of X and H1 and H2 be any mij(Y)-semopen sets such that
and
. Since Y satisfies property
, we have from part (2) that,
Thus,
is an mij(X)-semiopen set containing xo. Put
, then
and
for all
. Since xo is arbitrarily chosen, F is mij-asymmetric irresolute at xo in X.
Theorem 3.10. Let
,
;
satisfy property
and let
, be an M-asymmetric irresolute multifunction at an arbitrary point
. Then, following properties holds:
1) For any arbitrary mij(Y)-semi neighbourhoods H1 and H2 with
and
, the set
is an mij(X)-semi neighbourhood of xo.
2) There exists an mij(X)-semi neighborhood O of xo such that for each mij(Y)-semi neighbourhoods H1 and H2 with
and
,
and
for all
.
Proof. 1) Let xo be any point in X and let H1 and H2 be an mij(Y)-semi neighbourhood of
with
and
. By Definition 2.10, there exists two mij(Y)-semiopen sets V1 and V2 such that
and
whence,
and
. Thus,
and, since F is M-asymmetric irresolute,
is an mij(Y)-semiopen set by Theorem 3.9 (2). Part (6) of Theorem 3.9, then implies
Because,
, we consequently infer that,
is a mij(X)-semi neighbourhood of xo.
2) From (1), clearly holds (2). Indeed, for any point
, let H1 and H2 be mij(Y)-semi neighbourhoods of
such that,
and
. Set
. From (1), O is an mij(X)-semi neighbourhood of xo and by the hypothesis,
and
for all x contained in O.
4. Conclusion
In this paper, a class of mappings called M-asymmetric irresolute multifunctions defined between bitopological structural sets satisfying certain minimal properties were introduced and investigated. M-asymmetric irresolute multifunctions were point-to-set mappings defined using M-asymmetric semiopen and semiclosed sets. Some relations between M-asymmetric semicontinuous multifunctions and M-asymmetric irresolute multifunctions were established.
Acknowledgements
The authors wish to acknowledge the support of Mulungushi University and the refereed authors for their helpful work towards this paper. They are also grateful to the anonymous peer-reviewers for their valuable comments and suggestions towards the improvement of the original manuscript.