Abstract
In the absence of the axiom of choice four versions of compactness (A-, B-, C-, and D-compactness) are investigated.
Typical results:
-
1.
C-compact spaces form the epireflective hull in Haus of A-compact completely regular spaces.
-
2.
Equivalent are:
-
(a)
the axiom of choice,
-
(b)
A-compactness = D-compactness,
-
(c)
B-compactness = D-compactness,
-
(d)
C-compactness = D-compactness and complete regularity,
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(e)
products of spaces with finite topologies are A-compact,
-
(f)
products of A-compact spaces are A-compact,
-
(g)
products of D-compact spaces are D-compact,
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(h)
powers X k of 2-point discrete spaces are D-compact,
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(i)
finite products of D-compact spaces are D-compact,
-
(j)
finite coproducts of D-compact spaces are D-compact,
-
(k)
D-compact Hausdorff spaces form an epireflective subcategory of Haus,
-
(l)
spaces with finite topologies are D-compact.
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3.
Equivalent are:
-
(a)
the Boolean prime ideal theorem,
-
(b)
A-compactness = B-compactness,
-
(c)
A-compactness and complete regularity = C-compactness,
-
(d)
products of spaces with finite underlying sets are A-compact,
-
(e)
products of A-compact Hausdorff spaces are A-compact,
-
(f)
powers X k of 2-point discrete spaces are A-compact,
-
(g)
A-compact Hausdorff spaces form an epireflective subcategory of Haus.
-
4.
Equivalent are:
-
(a)
either the axiom of choice holds or every ultrafilter is fixed,
-
(b)
products of B-compact spaces are B-compact.
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5.
Equivalent are:
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(a)
Dedekind-finite sets are finite,
-
(b)
every set carries some D-compact Hausdorff topology,
-
(c)
every T 1-space has a T 1-D-compactification,
-
(d)
Alexandroff-compactifications of discrete spaces and D-compact.
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Dedicated to My Friend Louis D. Nel on His Sixtieth Birthday
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Herrlich, H. Compactness and the axiom of choice. Appl Categor Struct 4, 1–14 (1996). https://doi.org/10.1007/BF00124110
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DOI: https://doi.org/10.1007/BF00124110