Compact implies feebly compact
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This article gives the statement and possibly, proof, of an implication relation between two topological space properties. That is, it states that every topological space satisfying the first topological space property (i.e., compact space) must also satisfy the second topological space property (i.e., feebly compact space)
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Statement
Any compact space is a feebly compact space.
Definitions used
Compact space
Further information: Compact space
A topological space is termed compact if every open cover of the space has a finite subcover.
Feebly compact space
A topological space is termed feebly compact if every locally finite collection of nonempty open subsets is finite.
Proof
Given: A compact space , a locally finite collection of nonempty open subsets
of
.
To prove: is finite.
Proof:
- There exists a point-indexed open cover
of
such that each
intersects finitely many
s: For each point
, we can find an open subset
that works by the definition of locally finite. Putting these together, there exists a point-indexed open cover.
- There exists a finite subset
of
such that the
, cover
: This follows from the previous step and the definition of compactness.
- The size of
is finite: Every
, is a nonempty open subset of
, hence it intersects at least one of the
s. Thus, to count the
s, it suffices to count the
s that intersect at least one
. For each
, the number of
s intersecting it is finite, and there are finitely many
s in
, so the total number is finite.