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Fort space

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In mathematics, there are a few topological spaces named after M. K. Fort, Jr.

Fort space

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Fort space[1] is defined by taking an infinite set X, with a particular point p in X, and declaring open the subsets A of X such that:

  • A does not contain p, or
  • A contains all but a finite number of points of X.

The subspace has the discrete topology and is open and dense in X. The space X is homeomorphic to the one-point compactification of an infinite discrete space.

Modified Fort space

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Modified Fort space[2] is similar but has two particular points. So take an infinite set X with two distinct points p and q, and declare open the subsets A of X such that:

  • A contains neither p nor q, or
  • A contains all but a finite number of points of X.

The space X is compact and T1, but not Hausdorff.

Fortissimo space

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Fortissimo space[3] is defined by taking an uncountable set X, with a particular point p in X, and declaring open the subsets A of X such that:

  • A does not contain p, or
  • A contains all but a countable number of points of X.

The subspace has the discrete topology and is open and dense in X. The space X is not compact, but it is a Lindelöf space. It is obtained by taking an uncountable discrete space, adding one point and defining a topology such that the resulting space is Lindelöf and contains the original space as a dense subspace. Similarly to Fort space being the one-point compactification of an infinite discrete space, one can describe Fortissimo space as the one-point Lindelöfication[4] of an uncountable discrete space.

See also

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Notes

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  1. ^ Steen & Seebach, Examples #23 and #24
  2. ^ Steen & Seebach, Example #27
  3. ^ Steen & Seebach, Example #25
  4. ^ "One-point Lindelofication".

References

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