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Another countable dense subset problem

by radou
Tags: countable, dense, subset
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Nov19-10, 12:51 PM
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radou's Avatar
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1. The problem statement, all variables and given/known data

This one seems pretty simple - that's exactly why I want to check it.

One needs to show that if X has a countable dense subset, then every collection of disjoint open sets in X is countable.

3. The attempt at a solution

Let U be a collection of disjoint open sets in X, and let A be a countable subset of X which is dense in X.

Let Ui be any member of U. Then Ui contains an element of A, since if x is in Ui, x is either in A or a limit point of A. If x is a limit point of A, choose a basis element of the topology of X which contains x and is contained in Ui. Then this basis element intersects A in some point other than x. Hence, Ui contains an element of A.

Since any two members of U are disjoint, they will contain different elements from A. This gives a one-to-one correspondence between the countable set A and the family U, so U is countable.
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Nov19-10, 12:55 PM
micromass's Avatar
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Completely correct!!
Nov19-10, 01:03 PM
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radou's Avatar
P: 3,220
Excellent, thanks!

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