Uncountability of neighborhoods in R^n

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Quick question: Is it true that all neighborhoods in R^n form an uncountable set? It seems obvious to me that the answer is yes, but there is no proof in my analysis book and I can't think of one.
 
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variety said:
Quick question: Is it true that all neighborhoods in R^n form an uncountable set? It seems obvious to me that the answer is yes, but there is no proof in my analysis book and I can't think of one.
Some bases for the topology on R^n are uncountable, and some bases are countably infinite. It depends on which one you have chosen.
 
How about just a neighborhood of the real line centered at x with radius r>0.
 
Try proving the the set of all balls around any single point is uncountable, and think about it's relationship to your problem.
 
Wait, what exactly are you asking? Do you want to prove that the set {y : |x-y|<r} is uncountable? Or do you want to prove the collection of all such neighborhoods is uncountable?
 
Yeah sorry I wasn't that clear. I just want to know if the set {x in R : |x-y|<r} is uncountable.
 
Well, that is just your open interval (y - r, y + r), which is indeed uncountable. (A bijection from the real numbers to this interval is f(x) = y + r \tanh(x).)
 
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