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Kernel Constructions and Borel Sets by A KERNEL CONSTRUCTIONS AND BOREL SETS BY A. H. STONE(i) 1. Introduction. A well-known theorem, essentially due to Cantor, states that every topological space X has a "perfect kernel" K; K is the largest dense-in- itself subset of X, K is closed, and X — K is scattered. Analogues of this situation occur frequently. Recently the author had occasion to use a "non-locally-separable kernel" [9, Lemma 7]; and further work on (nonseparable) Borel sets has led to a need for further generalizations. Hence we here study a rather inclusive class of kernel constructions^). The main result (Theorem 4 below) is essentially an extension of the Cantor-Bendixson theorem ; roughly speaking, if X (for simpli- city) is metric, the complement of its "non-locally-P kernel" is the union of a countable family of closed sets, each locally P, and having other desirable proper- ties^). We also obtain (Theorem 7) a useful criterion for this kernel to be empty. The results are applied to give extensions of two well-known theorems: one, due to Banach, about sets which are locally of first category, and the other, due to Montgomery, about sets which are locally Borel. We also apply them to charac- terize two classes of metric spaces: those which are both absolutely Gs and absolutely Fa, and those of which every subspace is both absolutely Gô and ab- solutely Fa. Other applications to the theory of Borel sets will be made in a sub- sequent paper. Our main interest is in metric spaces, but most of the results are formulated more generally since the proofs are no simpler in the metric case. The author is indebted to the referee for several helpful suggestions. Notation. Let P be a class (or "property", not necessarily topological) of topological spaces. We shall assume throughout that P is "hereditary" in the sense that if X e P then, for every closed subspace A of X, A e P. If every subspace of (XeP) is also a member of P, we say that P is completely hereditary. For example, P might be the class of all compact spaces, or of all absolutely Borel metric spaces, or of all spaces of (covering) dimension ;£ n. Relevant examples Presented to the Society, March 27,1962; received by the editors March 28,1962. (!) The preparation of this paper was supported by the National Science Foundation, Grant NSF-G19883. (2) Not every kernel of importance is included, as for instance the "dimensional kernel" (see [5, p. 186] and the beginning of §4 below). A more general class of kernels is described in [4, pp. 164-166]. For a quite different method of kernel construction, see [3]. (3) The "classical" Cantor-Bendixson theorem has one further feature which cannot be extended, namely that the perfect kernel of a separable metric space X can be obtained by countably many operations, i.e., in the notation of §2 below, one can take a* to be countable. This feature will persist when X has a countable base, but not in general (even if X is metric). 58 License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use KERNEL CONSTRUCTIONSAND BOREL SETS 59 of completely hereditary classes are : (1) all spaces of weight less than a given cardinal; (2) all metric spaces of dimension :g n; (3) all subspaces of measure 0 of a fixed (complete) measure space. If si is any class of spaces or sets, we define a completely hereditary class, denoted by (si), as follows: (si) = {X| X c A for some Aesi). A neighborhood of a point x in a space X is any set whose interior contains x. A neighborhood in X is a neighborhood of some point of X. A space X is locally P if each xeX has a neighborhood belonging to P. Clearly. (1.1) If Pis completely hereditary, or if X is regular, and if X is locally P, then each xe X has arbitrarily small neighborhoods belonging to P. We say that X is nowhere locally P if no neighborhood in X belongs to P. Thus, if P is completely hereditary, this is equivalent to requiring that no nonempty open subset of X belongs to P. In a metric space X, with metric p, U(x, S) denotes the neighborhood {y\yeX,p(x,y)<ô}, where xeX and ô > 0. 2. Existence and elementary properties of kernels. Theorem 1. For any (hereditary) class P, given any space X there exists a closed subset K = K(P, X) of X such that (i) K is nowhere locally P, (ii) whenever H is a closed subset of X which is nowhere locally P,H <=X. Thus K(P, X) is the largest closed subset of X which is nowhere locally P; we call it the "non-locally-P kernel of X(4). Proof. Define transfinite sequences {Fx}, {Gx}, of subsets of X as follows: Suppose Fp Gß defined for all ordinals ß < a. Then (a) iia = O,Fx = X,Gx = 0; (b) if a is a limit ordinal, Fx = f]{Fß \ ß < a}, Gx = 0 ; (c) if a = y + 1, put Gx = set of all xeFy such that some neighborhood of x in Fy belongs to P, and define Fx = Fy —Ga. Clearly F0 =>Fx =>• • •, and Gx is open relative to Fy in case (c), so that Fx is closed relative to Fr Thus in all cases Fx is closed in X. The decreasing transfinite se- quence {Fx} must become ultimately constant; for some a* we thus have Fx, = Fx.+1 = ••• = K, say(3). If some x e K has a neighborhood in K which belongs to P, then x e Gx,+1 and therefore x£Fx,+1 = K, which is impossible, proving (i). If H is closed and nowhere locally P, we show by transfinite induction that H cz Fx. Assume this true for all ß < a. In cases (a) and (b) it trivially follows that H <=Fx. In case (c), suppose xeHr\Gx; then x has a neighborhood U in Fy such that U e P. Since H is a closed subset of Fy, U n H is a neighborhood of x in H and belongs to P, contrary to our hypothesis on H. Thus H r\Gx = 0, proving H er Fx. Hence H c Fx, = K. (4) The proof which follows is essentially that in [4, p. 166], but is given in full as it is needed for reference later. License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 60 A. H. STONE [April Corollary. If further P is completely hereditary, we may omit the require- ment in (ii) that H be closed. (That is, K is now the largest nowhere locally P subset of X.) This is clear from the proof; alternatively, one easily sees that if if is nowhere locally P (where P is completely hereditary) then so is H. The requirement of complete hereditariness is essential for this corollary, as the following example shows. Let X be the unit interval, P the class of all compact spaces; there is no largest nowhere locally compact subset of X (as one sees by first observing that both it and its complement would have to be dense). We shall refer to the sets Fx, Gx constructed above (in the proof of Theorem 1) as FX(P, X), GX(P,X). When the family P can be understood from the context, we abbreviate them to Fa(X), GX(X),and abbreviate K(P, X) to K(X). The follo- wing properties are then easily verified by transfinite induction. (2.1) FX(X) is a closed subset of X, and Fx(Fß(X)) = Fß+x(X). (2.2) If Y is a closed subset of X, FX(Y) c Y nFx(X). (2.3) If P is completely hereditary and Y <=X, then Fx(Y) <= Y n FX(X). (2.4) If either P is completely hereditary or X is regular, and if Y is an open subset of X, then Fx(Y)=Yr>FX(X). From these properties the next assertions follow immediately. (2.5) K(X) is a closed subset of X. (2.6) K(K(X)) = K(X). (2.7) If Y is a closed subset of X, K(X) = K( Y). If P is completely hereditary, then (2.T) K(X) Z3K(Y) if X =>Y. (2.8) If Y is an open subset of X, K(Y) = Y nK(X) providing that either P is completely hereditary or X is regular. In particular, K(X — K(X)) = 0. The above properties of kernels are characteristic. We have : Theorem 2. Suppose that a mapping K (of 2s in 2s), defined for all sub- spaces X of a fixed space S, satisfies (2.5)-(2.8). Then there exists a hereditary family P such that, for all X c S, K(X) = K(P, X). If further (2.7') is satisfied, P may be taken to be completely hereditary. Take P to be the family of all X <=S for which K(X) = 0 ; this is hereditary, by (2.7). Write H(X) = K(P,X) for each X c S; we verify that H(X) = K(X). If V = X - K(X), then V is open in X, and K(V) = 0 by (2.8); as H(X) is nowhere locally P, we must have V n H(X) = 0, proving H(X) cz K(X). To prove equality we have only to show that the closed subset K(X) of X is nowhere locally P.
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