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On Si-Metacompact Spaces ∗ ¤ On Si-metacompact Spaces M. Sarsak, M. Ganster and M. Steiner Abstract The primary purpose of this paper is to introduce and study new variations of metacompactness by utilizing semi-open sets. 1 Introduction and Preliminaries Let (X; ¿) be a topological space. We shall denote the closure and the interior of a subset A of (X; ¿) by clA and intA, respectively. A subset A of (X; ¿) is called ®-open (or an ®-set [6]), if A ⊆ int(cl(intA)), semi-open [4] if A ⊆ cl(intA), preopen [5] if A ⊆ int(clA) and regular closed if A = cl(intA). Clearly, ®-sets and regular closed sets are semi-open. Njastad [6] pointed out that the family of all ®-sets of (X; ¿), denoted by ¿ ®, is a topology on X ¯ner than ¿. Reilly and Vamanamurthy observed in [7] that a subset A of (X; ¿) is an ®-set if and only if it is semi-open and preopen. We denote the family of all semi-open (resp. preopen, regular closed) sets in (X; ¿) by SO(X; ¿) (resp. PO(X; ¿), RC(X; ¿)). Remark 1.1 [3] Let (X; ¿) be a space. Then (¿ ®)® = ¿ ®, SO(X; ¿ ®) = SO(X; ¿), PO(X; ¿ ®) = PO(X; ¿) and RC(X; ¿ ®) = RC(X; ¿). Throughout this paper, N and R denote the set of natural numbers and the set of real num- bers, respectively. The Stone-Cech compacti¯cation of N is denoted by ¯N. The cardinality of a set A will be denoted by jAj . No separation axioms are assumed unless stated explicitly. For concepts not de¯ned in our paper we refer the reader to [2]. ¤2000 Math. Subject Classi¯cation | 54D20. Key words and phrases | S1-metacompact, S2-metacompact, S3-metacompact, ®-open, semi-open. 1 2 Si-metacompact spaces De¯nition 1 A space (X; ¿) is called (i) S1-metacompact if every semi-open cover has a point-¯nite open re¯nement, (ii) S2-metacompact if every semi-open cover has a point-¯nite semi-open re¯nement, (iii) S3-metacompact if every open cover has a point-¯nite semi-open re¯nement. De¯nition 2 A space (X; ¿) is called (i) globally disconnected [1] if every semi-open set is open, (ii) nodec [10] (or an ®-space) if every nowhere dense subset is closed, or equivalently (see [6]), if ¿ = ¿ ® . Clearly, if (X; ¿) is globally disconnected then all the notions in De¯nition 1 coincide and are equivalent to metacompactness. In general, we have the following obvious implications: S1-metacompact ) S2-metacompact + + metacompact ) S3-metacompact It will be seen, however, that none of the implications above can be reversed, and that S2-metacompactness and metacompactness are independent of each other. We shall ¯rst characterize globally disconnected spaces and S1-metacompact spaces. Proposition 2.1 For a space (X; ¿) the following are equivalent: 1) (X; ¿) is globally disconnected, 2) every semi-open cover of (X; ¿) has an open re¯nement. Proof. 1) ) 2) is clear. To show that 2) ) 1) let A 2 SO(X; ¿) and let U=intA . Then U = fU [ fxg : x 2 clUg [ fXn clUg is a semi-open cover of (X; ¿). By assumption, there exists an open re¯nement of U , say V . Suppose that A is not open, i.e. there is an x 2 A n U . Pick V 2 V such that x 2 V . Then V ⊆ U [ fxg ⊆ A and thus x 2 U, a contradiction. Hence A is open. 2 Corollary 2.2 A space (X; ¿) is S1-metacompact if and only if it is metacompact and globally disconnected. Recall that a space (X; ¿) is said to be extremally disconnected if the closure of each open set is open, or equivalently, if each regular closed set is clopen. Jankovic [3] observed that (X; ¿) is extremally disconnected if and only if SO(X; ¿) ⊆ PO(X; ¿) . The next (known) result follows easily from the fact that ¿ ® = SO(X; ¿) \ PO(X; ¿) [7] and from Remark 1.1. Remark 2.3 For a space (X; ¿) the following are equivalent: (i) (X; ¿) is extremally disconnected, (ii) (X; ¿ ®) is extremally disconnected, (iii) SO(X; ¿) ⊆ ¿ ® , (iv) (X; ¿ ®) is globally disconnected. Remark 2.4 (i) A space (X; ¿) is globally disconnected if and only if it is extremally disconnected and nodec. (ii) If (X; ¿) is extremally disconnected then the concepts of (X; ¿ ®) being metacompact, S1-metacompact, S2-metacompact and S3-metacompact coincide. Remark 2.5 The notions of (X; ¿) being metacompact and of (X; ¿ ®) being metacompact are, in general, independent of each other. In Corollary 2.10 we provide an example of a compact, thus metacompact, space (X; ¿) such that (X; ¿ ®) fails to be metacompact. On the other hand, if (X; ¿) denotes the space in Theorem 2.14(2) then (X; ¿) fails to be metacompact while (X; ¿ ®) is metacompact. Clearly, every discrete space is S1-metacompact. To provide an example of a non-discrete S1-metacompact space, we consider X = N[fpg where p denotes a free ultra¯lter on N. We take as a base for the topology ¿ all sets of the form U [ fpg where U 2 p , together with all singletons fng; n 2 N . It is very well known (see e.g. [9], Ex. 114) that (X; ¿) is perfectly normal, extremally disconnected and metacompact. Clearly every nowhere dense set is closed, so (X; ¿) is nodec and thus globally disconnected. Hence (X; ¿) is S1-metacompact by Corollary 2.2. 3 Remark 2.6 Spaces which fail to be S1-metacompact exist in profusion. The usual space R of reals is metacompact but not S1-metacompact. ¯N is a compact T2 space which is not S1-metacompact as N [ fpg with p 2 ¯N n N is a semi-open subset of ¯N that is not open. We shall now consider S2-metacompact spaces and S3-metacompact spaces. First of all, by ® Remark 1.1 we have that (X; ¿) is S2-metacompact if and only if (X; ¿ ) is S2-metacompact. Example 2.7 Let X¤ = X [ fpg denote the 1-point-compacti¯cation of an in¯nite discrete space X, where p2 = X. Let U be a semi-open cover of X¤. Pick S 2 U such that p 2 S. Then S = fpg [ A where A is an in¯nite subset of X. Clearly, V = fSg [ ffxg : x 2 X n Ag ¤ is a disjoint semi-open re¯nement of U. Thus X is a compact Hausdor® S2-metacompact space. In addition, it is easily seen that X¤ fails to be globally disconnected, hence cannot be S1-metacompact. Next we recall the following set-theoretic construction that has been observed in [8]. Lemma 2.8 [8] Let X be an in¯nite set with jXj = ¹ and let X = A [ B where jAj = ¹, jBj = º < ¹ and A \ B = ; . For each x 2 A let Cx = B [ fxg . Then C = fCx : x 2 Ag is a cover of X . Suppose that D is a re¯nement of C . For each x 2 A there exists Dx 2 D such that x 2 Dx ⊆ Cx . If jDxj > 1 for each x 2 A then there exists an element z 2 B such that z lies in at least ¹ elements of the re¯nement D . Theorem 2.9 Let (X; ¿) be an in¯nite space possessing a dense open subset D such that jDj < jXj . Then (X; ¿) fails to be S2-metacompact. Proof. Let A = X nD and B = D . Utilizing Lemma 2.8 we obtain a semi-open cover C that does not have a semi-open point ¯nite re¯nement. Thus (X; ¿) fails to be S2-metacompact. Corollary 2.10 (i) ¯N fails to be S2-metacompact. (ii) Let σ denote the topology of ·N, the Katetov extension of N, and let ¿ be the topology ® of ¯N. It is well known that σ = ¿ . Clearly ·N fails to be S2-metacompact. Since ¯N is known to be extremally disconnected, by Remark 2.4, ·N even fails to be S3-metacompact. 4 Theorem 2.11 Let (X; ¿) be an in¯nite space possessing a closed nowhere dense subset N and a dense subset B such that jBj < jNj ¸ @0 . Then (X; ¿) fails to be S2-metacompact. Proof. For each x 2 N let Sx = (X n N) [ fxg . Then U = fSx : x 2 Ng is a semi-open cover of X . Suppose that V is a semi-open re¯nement of U. For each x 2 N there exists Tx 2 V such that x 2 Tx. Clearly Tx ⊆ Sx for x 2 N, and Tx 6= Ty for distinct points x; y 2 N . Now, fintTx : x 2 Ng is a family of jNj nonempty open subsets of (X; ¿). Since B is dense and jBj < jNj ¸ @0, there must be a point z 2 B which is contained in in¯nitely many sets Tx. Thus V cannot be point-¯nite and (X; ¿) fails to be S2-metacompact. Corollary 2.12 The usual space R fails to be S2-metacompact (let N be the Cantor set, B the set of rationals and apply Theorem 2.11). In the following we exhibit a useful method for generating new topological spaces from given spaces. Let (Y; σ) be a topological space, Z be an in¯nite set disjoint from Y and let X = Y [ Z . We de¯ne a topology ¿ on X in the following way: every set S ⊆ Y with S 2 σ is open in (X; ¿) and a basic neighbourhood of z 2 Z is of the form fzg [ Y . Clearly, (Y; σ) is a dense and open subspace of (X; ¿). It is easily checked that for S 6= ; we have that S 2 SO(X; ¿) if and only if S \ Y is a nonempty semi-open subset of (Y; σ).
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