Section 28: Problem 4 Solution
Working problems is a crucial part of learning mathematics. No one can learn topology merely by poring over the definitions, theorems, and examples that are worked out in the text. One must work part of it out for oneself. To provide that opportunity is the purpose of the exercises.
James R. Munkres
Limit point compactness implies countable compactness for a
-space. Let
be a limit point compact
-space and
be a countable open covering of
such that there is no finite subcovering of
. Let
. Note that for every
,
does not cover
, but for every
there is minimal
such that
. Let
. For each
let
. This defines an infinite subset of
that must have a limit point
. But then the neighborhood
of
contains only finite number of elements in the sequence. And for each of them different from
we can find a neighborhood of
that does not contain it (here we use the
-property — consider Example 1 to see why it does not work in general). The finite intersection of all these neighborhoods with
is a neighborhood of
that does not contain any point of the sequence different from
. This contradicts the fact that
is a limit point of the sequence.Countable compactness implies limit point compactness. Suppose
is a countably compact space and
is an infinite subset of
. There is a countably infinite subset
and every limit point of
is a limit point of
as well. If no point in
is a limit point of
then every point in
has a neighborhood that does not contain any other points in
, and the countable collection of such neighborhoods covers
. Since each set in the collection covers one point of
only and
is infinite, there is no finite subcollection covering
. Therefore,
is not closed (a closed subset of a countably compact space is countably compact: add the complement of the subset to any its covering) and there is a limit point of
.