Uncountability of neighborhoods in R^n

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In summary, the conversation discusses whether all neighborhoods in R^n form an uncountable set and asks for a proof. It is mentioned that some bases for the topology on R^n are uncountable, while others are countably infinite. The conversation then suggests trying to prove the set of all balls around any single point is uncountable. Finally, it is clarified that the set in question is {x in R : |x-y|<r}, which is confirmed to be uncountable.
  • #1
variety
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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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  • #2
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.
 
  • #3
How about just a neighborhood of the real line centered at x with radius r>0.
 
  • #4
Try proving the the set of all balls around any single point is uncountable, and think about it's relationship to your problem.
 
  • #5
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?
 
  • #6
Yeah sorry I wasn't that clear. I just want to know if the set {x in R : |x-y|<r} is uncountable.
 
  • #7
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 [tex]f(x) = y + r \tanh(x)[/tex].)
 

1. What does "uncountability of neighborhoods in R^n" mean?

The uncountability of neighborhoods in R^n refers to the fact that there are infinitely many points in a neighborhood in n-dimensional space (R^n). This means that it is impossible to assign a unique number or label to each point in the neighborhood.

2. How is the concept of "uncountability" related to neighborhoods in R^n?

The concept of uncountability is related to neighborhoods in R^n because it describes the infinite number of points within a neighborhood in n-dimensional space. This infinite number of points cannot be counted or enumerated in a finite way.

3. Why is the uncountability of neighborhoods in R^n important in mathematics?

The uncountability of neighborhoods in R^n is important in mathematics because it is a fundamental concept in understanding the nature of infinite sets. It is also a key concept in topology, which is a branch of mathematics that studies the properties of spaces and continuous transformations.

4. Can the uncountability of neighborhoods in R^n be proven?

Yes, the uncountability of neighborhoods in R^n can be proven using various mathematical techniques. One approach is to use the concept of Cantor's diagonal argument, which shows that the cardinality (size) of the set of real numbers is greater than that of the set of natural numbers. Since neighborhoods in R^n contain real numbers, they are also uncountable.

5. How does the uncountability of neighborhoods in R^n relate to the concept of infinity?

The uncountability of neighborhoods in R^n is closely related to the concept of infinity. A neighborhood in R^n contains an infinite number of points, which cannot be counted or enumerated in a finite way. This is similar to the concept of infinity, where the number of elements in an infinite set is also uncountable and cannot be represented by a finite number.

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