Nonstandard Topologies with Bases That Consist Only of Standard Sets

Nonstandard Topologies with Bases That Consist Only of Standard Sets

PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 129, Number 7, Pages 2075{2083 S 0002-9939(00)05790-7 Article electronically published on December 4, 2000 NONSTANDARD TOPOLOGIES WITH BASES THAT CONSIST ONLY OF STANDARD SETS NADER VAKIL (Communicated by Carl G. Jockusch, Jr.) Abstract. Let X be an infinite set, D a set of pseudo-metrics on X; Ξ ⊂ ∗X; and Γ ⊂ ∗D: If ρ(a; b) is limited (finite) for every a; b 2 Ξ and every ρ 2 Γ; then, for each ρ 2 Γ; we can define a pseudo-metricρ ~ on Ξ by writingρ ~(a; b)=st(ρ(a; b)): We investigate the conditions under which the topology induced on Ξ by fρ~ : ρ 2 Γg has a basis consisting only of standard sets. This investigation produces a theory with a variety of applications in functional analysis. For example, a specialization of some of our general results will yield such classical compactness theorems as Schauder's theorem, Mazur's theorem, and Gelfand-Philips's theorem. 1. Introduction Let X; D; Ξ and Γ be as in the abstract. Suppose that ◦Ξ=fx 2 X : ∗x 2 Ξg6= ; and ◦Γ=fd 2 D : ∗d 2 Γg6= ;.Letρ(a; b) be limited (finite) for every a; b 2 Ξandρ 2 Γ, and let Γ=~ fρ~ : ρ 2 Γg whereρ ~(a; b)=st(ρ(a; b)): Let (Ξ; Γ)~ denote the uniform space whose uniformity is generated by the set Γ.~ The topology induced on Ξ by this uniformity will be referred to as Γ-topology~ on Ξ. We call Γ compatible with Ξ if the Γ-topology~ on Ξ has a basis that consists only of standard sets. In this paper we investigate the following question. Question. Under what conditions on the sets Ξ and Γ will the set Γ be compatible with Ξ? Our main results regarding this question are presented in Section (2), where we treat the question in a setting that is suitable for a wide range of applications. In Section (3), after giving several nonstandard compactness criteria for subsets of a Banach space and its dual, we use the theory developed in Section (2) to derive such classical compactness theorems as Schauder's theorem, Mazur's theorem, and Gelfand-Philips's theorem. The idea of the Γ-topology~ introduced above is an extension of the notion of S-topology introduced by Robinson [1] in the context of metric spaces, and further studied by Henson ([2], page 117) in the context of uniform spaces. What was considered in [2] was a special case of (Ξ; Γ)~ in which Ξ = ∗X and Γ=f ∗d : d 2 Dg. In this special case, we shall call the Γ-topology~ the S- topology although it was not called so in [2]. The question of existence of a basis Received by the editors June 8, 1999 and, in revised form, November 16, 1999. 2000 Mathematics Subject Classification. Primary 46S20; Secondary 54J05. Key words and phrases. Nonstandard topologies, Banach space, compactness conditions. c 2000 American Mathematical Society 2075 License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 2076 NADER VAKIL consisting only of standard sets for the S-topology was not considered by Robinson or by Henson. Topologies that are generated by a family of standard sets on ∗X should not be confused with the S-topology as defined in [1] and [2] and generalized to Γ-topology~ here. A special case of the former was first introduced in [4], page 47, and called the discrete S-topology. The idea was somewhat extended and further discussed in [3], page 197, where the relation of such topologies to compactifications of X was pointed out. In [6], starting with a topological space (X; T), Salbany and Todorov considered the topology T~ that is generated on ∗X by the collection f ∗G : G 2 T g, and used this topology to study various classical compactifications of (X; T). In [7], H. Render, starting with a uniform space (X; V) and its induced topo- logical space (X; T), studied the relation between the T~ (as defined in the above paragraph) and the S-topology (as defined on the previous page). The result he announced was that: if (X; V) is not zero-dimensional, then there does not exist a base of the S-topology consisting only of standard sets. But this statement is false. As a trivial example, consider the set X =(0; 1) with its usual unifor- + mity (i.e., the one which has the sets U = fhx; yi : jx − yj <g, 2 R ,fora basis). By definition, the S-topology associated with this uniformity has the sets S(a; )=fb 2 ∗X :stja − bj <g, a 2 ∗X; 2 R+, for a basis. It is obvious that the standard sets ∗S(x; )withx 2 X and 2 R+ also form a basis for the S-topology. This does not, however, happen in an arbitrary uniform space. The reason it happened here was that the set X =(0; 1) with its usual uniformity is a totally bounded space. In Corollary (1.3), we show that, given a uniform space (X; V), the S-topology on ∗X has a basis consisting only of standard sets if and only if (X; V) is totally bounded. Thus we first address our opening question in the general setting where it is posed. To fix the notation, let (X; D) be a uniform space with its uniformity generated by a set of pseudo-metrics D. If, for an element a 2 ∗X, ∗d(a; x)is limited for all d 2 D and all x 2 X,thenwesaythata is limited. We denote the set of all limited elements of ∗X by ltd( ∗X). For other nonstandard concepts used in this paper, we refer the reader to [3]. The reference [5] contains all of the standard concepts used in this paper. Theorem 1.1. Let Γ=f ∗d : d 2 Dg. A subset A of X is totally bounded in (X; D) if and only if ∗A ⊂ ltd( ∗X) and the Γ~-topology on ∗A has a basis consisting only of standard sets. Proof. Recall that A is totally bounded if and only if every element of ∗A is pre-near standard. Assume that A is totally bounded. Since ∗A ⊂ pns( ∗X) ⊂ ltd ( ∗X), ∗d(a; b) is limited for every d 2 D and every a; b 2 ∗A.Furthermore, for all a 2 ∗A; x 2 X; 2 R+; and d 2 D,thesets (1) S~(a; x; d; )=fb 2 ∗A :st(j ∗d(x; b) − ∗d(x; a)j) <g are open relative to the Γ-topology,~ are standard, and form a subbasis for the Γ-topology~ on ∗A. To prove they are open, fix b 2 S~(a; x; d; ). Choose δ = − st(j ∗d(x; b) − ∗d(x; a)j). Since S~(b; d; δ) ⊂ S~(b; x; d; δ), we have S~(b; d; δ) ⊂ S~(a; x; d; ). To see that they are standard, note that the expression st(j ∗d(x; b) − ∗d(x; a)j) in(1)canbewrittenas ja^(x; d) − ^b(x; d)j,where c^ : X × D ! R (c 2 ltd( ∗X)) is defined byc ^(x; d)=st(∗d(c; x)). Thusthe License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use NONSTANDARD TOPOLOGIES 2077 defining formula of S~(a; x; d; ) is equivalent to one that contains only standard constants. This proves that the set S~(a; x; d; ) is standard. Next let S~(a; d; )= fb 2 ∗A :st(∗d(a; b))<g be a subbasic element of the Γ-topology~ of ∗ 2 ∗ A.Choosex X such that st( d(a; x))< 3 . Then it is easily seen ~ ⊂ ~ that S(a; x; d; 3 ) S(a; d; ). This proves that the sets defined in (1) form a subbasis for the Γ-topology~ on ∗A. The converse is an easy consequence of the definitions. In the next two corollaries we continue to assume that Γ = f ∗d : d 2 Dg. Since, for subsets of a complete uniform space, total boundedness and relative compactness are equivalent properties, Theorem (1.1) has the following immediate corollary. Corollary 1.2. Suppose that (X; D) is a complete uniform space. A subset A of X is relatively compact in (X; D) if and only if ∗A ⊂ ltd( ∗X) and the Γ~-topology on ∗A has a basis consisting only of standard sets. Corollary 1.3. Suppose that D consists only of bounded pseudo-metrics on X: The following assertions are equivalent. (i) The space (X; D) is totally bounded. (ii) The space (∗X; Γ)~ has a basis consisting only of standard sets. (iii) The space (∗X; Γ)~ is quasi-compact and has X as a dense subset. Proof. The implication (i) =) (ii) is an immediate consequence of Theorem (1.1). Assume that (ii) holds, and let B~ be a basis of (∗X; Γ)~ that consists only of standard sets. Since every nonempty member of B~ contains a standard element, X is dense in (∗X; Γ).~ The quasi-compactness of (∗X; Γ)~ follows from the fact C~ f ∗ n 2 Bg~ if is a nonemptyT subfamily of C : X C with the finite intersection property, then C6~ = ;: For the implication (iii) =) (i), we need only observe that the denseness of X in (∗X; Γ)~ implies that every point in ∗(X; D)is pre-nearstandard. 2. Main theorem Corollary (1.2) suggests that the notion of compatibility defined in the opening paragraph of the previous section might give rise to interesting compactness con- ditions in functional analysis.

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