Is the Intersection of Infinite Non-Empty Open Subsets Empty?

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Discussion Overview

The discussion revolves around the intersection of infinite non-empty open subsets in R^n as the dimension n tends to infinity. Participants explore whether such intersections can be empty or not, considering various examples and conditions.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions whether the intersection of nested non-empty open subsets in R^n is empty as n approaches infinity.
  • Another participant asserts that it is not necessarily empty.
  • A different viewpoint suggests that it can be empty, providing an example with the sets A_n = (0, 1/n), where the intersection results in 0.
  • Another example is presented where the sets are defined as closed intervals from 0 to 1 + 1/n, leading to a non-empty intersection of the closed interval from 0 to 1.
  • Participants express some confusion regarding the thread title, which was initially about "Union of non-empty sets."

Areas of Agreement / Disagreement

Participants do not reach a consensus, as multiple competing views remain regarding whether the intersection can be empty or not, depending on the specific sets considered.

Contextual Notes

Some assumptions about the nature of the sets (open vs. closed, compactness) and their properties are not fully explored, leaving the discussion open-ended regarding the conditions under which the intersection may or may not be empty.

Bachelier
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If you take the intersection of non empty open subsets in Rn as n tends to infinity, such that

U_1 \supseteq U_2 \supseteq U_3...

Is it empty?
 
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not necessarily
 
but it can. right?
 
An example of that in R would be the sequence of sets:

A_n=\left(0,\frac{1}{n}\right)

Then

\bigcap_{n=1}^{\infty}A_n=0

In general if your sets are compact and nested, then the intersection will never be empty.
 
Bachelier said:
but it can. right?

Sure it can happen. Here is a case where it does not. Let Ui be the closed interval of real numbers from zero to 1 + 1/n. The infinite intersection is the closed interval from zero to 1.
 
Great. Thanks.
 
The fact that this thread was titled "Union of non-empty sets" was a bit confusing!
 
HallsofIvy said:
The fact that this thread was titled "Union of non-empty sets" was a bit confusing!


I posted the question right before I went to bed. LOL :-p
 

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