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Compactness and the axiom of choice

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Abstract

In the absence of the axiom of choice four versions of compactness (A-, B-, C-, and D-compactness) are investigated.

Typical results:

  1. 1.

    C-compact spaces form the epireflective hull in Haus of A-compact completely regular spaces.

  2. 2.

    Equivalent are:

  3. (a)

    the axiom of choice,

  4. (b)

    A-compactness = D-compactness,

  5. (c)

    B-compactness = D-compactness,

  6. (d)

    C-compactness = D-compactness and complete regularity,

  7. (e)

    products of spaces with finite topologies are A-compact,

  8. (f)

    products of A-compact spaces are A-compact,

  9. (g)

    products of D-compact spaces are D-compact,

  10. (h)

    powers X k of 2-point discrete spaces are D-compact,

  11. (i)

    finite products of D-compact spaces are D-compact,

  12. (j)

    finite coproducts of D-compact spaces are D-compact,

  13. (k)

    D-compact Hausdorff spaces form an epireflective subcategory of Haus,

  14. (l)

    spaces with finite topologies are D-compact.

  1. 3.

    Equivalent are:

  2. (a)

    the Boolean prime ideal theorem,

  3. (b)

    A-compactness = B-compactness,

  4. (c)

    A-compactness and complete regularity = C-compactness,

  5. (d)

    products of spaces with finite underlying sets are A-compact,

  6. (e)

    products of A-compact Hausdorff spaces are A-compact,

  7. (f)

    powers X k of 2-point discrete spaces are A-compact,

  8. (g)

    A-compact Hausdorff spaces form an epireflective subcategory of Haus.

  1. 4.

    Equivalent are:

  2. (a)

    either the axiom of choice holds or every ultrafilter is fixed,

  3. (b)

    products of B-compact spaces are B-compact.

  1. 5.

    Equivalent are:

  2. (a)

    Dedekind-finite sets are finite,

  3. (b)

    every set carries some D-compact Hausdorff topology,

  4. (c)

    every T 1-space has a T 1-D-compactification,

  5. (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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