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  • Now recall that a cozero element of

    2021-11-30

    Now recall that a cozero Angiotensin Fragment 1-7 acetate mg of a frame L is an element of the form for some continuous real-valued function h in L. Equivalently, is a cozero element if and only if there exists an such that and . This is the pointfree counterpart to the notion of a cozero set for ordinary continuous real-valued functions. For more information on the cozero map we refer to [5]. As usual, Coz L will denote the cozero lattice of all cozero elements of L.
    In the sequel, we refer to a disjoint whenever for every . Recall that by a discrete in L it is meant a collection for which there is a cover C of L such that for each , for all i with possibly one exception. Note that any discrete system is clearly disjoint: if there is a cover C such that for every , for all i except possibly one, then for every and , that is, .
    Universality: Kowalsky's Hedgehog Theorem Recall from [10] that a family of frame homomorphisms is said to be separating in case for every .
    We need now to recall the following result ([10, Theorem 3.7]), stated here in frame-theoretical terms. Let be a family of frame homomorphisms. Then there is a frame homomorphism such that, for each i, the diagram commutes, where is the ith injection map. The map e need not be a quotient map, but one has the following:
    Furthermore, given a fixed class of frames, a frame T in is said to be universal in this class ([8]) if for every there exists a frame homomorphism from T onto L. We now have the following:
    This is the pointfree extension of Kowalsky's Hedgehog Theorem ([17]) that shows that every metrizable space is embeddable into a countable cartesian power of the metric hedgehog space.
    Being metrizable, the hedgehog frame is collectionwise normal [22, Theorem 2.5] (see also [24], Theorem 2 and its Corollary). Recall that collectionwise normality is a stronger variant of normality introduced by A. Pultr in [22]: while a frame L is normal whether for any satisfying there exist disjoint such that , it is collectionwise normal if for each co-discrete system there is a discrete such that for every . Here, by a co-discrete it is meant a collection for which there is a cover C such that for each , for all i with possibly one exception. More generally, for a cardinal , we say that L is κ-collectionwise normal if it satisfies the definition of collectionwise normality for sets I with cardinality . Hence collectionwise normality is κ-collectionwise normality for any cardinality κ. If are two cardinalities, then λ-collectionwise normality implies κ-collectionwise normality. Hence, κ-collectionwise normality implies normality for every κ.
    We start with a characterization of normality that will be useful in our study:
    We need now to recall some facts and notation about sublocales ([19]): An is a sublocale of L iff the embedding is a localic map. This means that S is closed under arbitrary infima and moreover for every and . The set of all sublocales of L forms a coframe (i.e., the dual of a frame) under inclusion, in which arbitrary infima coincide with intersections, 1 is the bottom element and L is the top element. There are two special classes of sublocales: the closed ones, defined as for each , and the open ones, defined as for each . The -sublocales are the countable joins of closed sublocales in ([16]). Any sublocale S of a frame L is a frame with meets and Heyting operation as in L but joins may differ. Denoting by the left adjoint of the embedding , we have In the particular case of an -sublocale one gets from the formula for the joins in the coframe of sublocales that It then follows easily that:
    The next result is the extension to locales of the classical result, due to Urysohn [25], that any -subspace of a normal space is normal.
    The following lemma identifies the difference between normality and κ-collectionwise normality.
    The next lemma is the counterpart of Lemma 6.2 for κ-collectionwise normality.