Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
4661366 | Topology and its Applications | 2007 | 4 Pages |
Abstract
For any space X, denote by dis(X) the smallest (infinite) cardinal κ such that κ many discrete subspaces are needed to cover X. It is easy to see that if X is any crowded (i.e. dense-in-itself) compactum then dis(X)⩾m, where m denotes the additivity of the meager ideal on the reals. It is a natural, and apparently quite difficult, question whether in this inequality m could be replaced by c. Here we show that this can be done if X is also hereditarily normal.Moreover, we prove the following mapping theorem that involves the cardinal function dis(X). If is a continuous surjection of a countably compact T2 space X onto a perfect T3 space Y then .
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