Can an infinite subset be dense in a finite complement topology?

[Oxymoron]

My problem is:

Consider an infinite set X with the finite complement topology. I want to show that any infinite subset A of X is dense in X.

Now, I can show that every point of X is a limit point of A.

Can this help me in any way to show that A is dense in X. Or could someone provide me with an alternative method.

By the way, my understanding of dense is that a subset of a set is dense if its closure coincides with the set.

 

[Hurkyl]

Let me rephrase your question:

If every point of X is a limit point of A, then is A dense in X?

 

[Oxymoron]

Yes, that is my question.

 

[Hurkyl]

So what do the definitions say?

 

[Oxymoron]

Wait a sec, if every point of X is a limit point of A, then A is dense in X!

 

[matt grime]

SO, A is any infinite subset and you want to find the smallest closed set containing A. Since there are three kinds of closed set:

the empty set

a set containing a finite number of points

all of X


isn't the answer obvious? I mean, whichof those can contain A, given A is infinite?

 

[Oxymoron]

isn't the answer obvious? I mean, whichof those can contain A, given A is infinite?

is it all of X?

 

[matt grime]

well, can you name a finite (or empty set) that contains an infinite subset?

 

[Oxymoron]

nope, I can't.

So am I correct in thinking that by proving that every point of X is the limit point of A, then A is dense?

 

[Hurkyl]

Wait a sec, if every point of X is a limit point of A, then A is dense in X!

That is correct -- the point of my first post was to eliminate the unnecessary stuff, hoping you could see this when that's all that's left.

However, the approach matt has mentioned is a much easier way to do this problem... and is a fairly important theme to understand in general.

 

[Oxymoron]

I agree, Matt's method was MUCH easier.

 

[HallsofIvy]

Once again- look at the DEFINITION of "dense"!