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Compactness and Its Consequences
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Heine-Borel Theorem
In Euclidean space, a subset is compact if and only if it is closed and bounded.
Limit Point Compactness
A space is limit point compact if every infinite subset has a limit point.
Continuous Functions Preserve Compactness
If is continuous and is compact, then is compact.
Covering Spaces and Compactness
If a covering space is compact, its base space is compact as well.
Bolzano-Weierstrass Theorem
Every bounded sequence in has a convergent subsequence.
Tychonoff's Theorem
The product of any collection of compact spaces is compact in the product topology.
Compactification
The process or result of making a non-compact space into a compact space by adding 'points at infinity' or otherwise extending its topology.
Local Compactness
A space is locally compact if every point has a compact neighborhood.
Definition of Compactness
A topological space is compact if every open cover has a finite subcover.
Compactness in Product Spaces (Projection Maps)
Projection maps from a compact product space to its factors are open and continuous.
Compact-Open Topology
A topology on the set of continuous functions between two topological spaces where the basic open sets are defined by compact subsets of the domain and open subsets of the codomain.
Compact Subsets of Hausdorff Spaces
In a Hausdorff space, every compact subset is closed.
Sequential Compactness
A topological space is sequentially compact if every sequence has a convergent subsequence.
Alexandroff Extension
A one-point compactification of a non-compact topological space that adds a single 'point at infinity' to make it compact.
Lebesgue Number Lemma
For every open cover of a compact metric space, there exists a positive number such that any subset of diameter less than is contained in some member of the cover.
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