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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 .