Why Is \left[\frac{1}{2},1\right] a Neighborhood of 1 in \left[0,1\right]?

  • Context: Graduate 
  • Thread starter Thread starter funcalys
  • Start date Start date
  • Tags Tags
    Example
Click For Summary

Discussion Overview

The discussion revolves around the concept of neighborhoods in metric spaces, specifically examining why the interval \(\left[\frac{1}{2},1\right]\) is considered a neighborhood of the point 1 within the closed interval \(\left[0,1\right]\). Participants explore the conditions under which a set can be classified as a neighborhood and clarify misunderstandings related to the definition.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the specific radius \(r > 0\) that would satisfy the condition \(B_{[0,1]}(1,r) \subseteq [0,1]\).
  • Another participant suggests that any \(r < \frac{1}{2}\) would suffice for the neighborhood around 1.
  • A different participant asserts that \(\left[\frac{1}{2}, 1\right]\) is indeed a neighborhood of 1, proposing \(r = \frac{1}{2}\) as a valid radius, while also noting it is not a neighborhood of \(\frac{1}{2}\).
  • One participant expresses confusion about the requirement for the ball centered at 1 to lie completely within the interval \(\left[\frac{1}{2}, 1\right]\).
  • Another participant humorously questions the logic behind thinking there could be numbers in \(\left[\frac{1}{2}, 1\right]\) that are not in the same interval.
  • A participant reiterates the focus on the closed unit interval, clarifying that points outside this interval, such as 1.1 or 1.2, are not relevant to the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the understanding of neighborhoods, with some clarifying concepts while others remain confused about the definitions and implications. The discussion does not reach a consensus on the initial confusion regarding the neighborhood definition.

Contextual Notes

Some participants appear to have misunderstandings regarding the definitions of neighborhoods in metric spaces, particularly in relation to the closed interval \(\left[0,1\right]\) and the implications of the radius chosen for the neighborhood.

funcalys
Messages
30
Reaction score
1
Hi folks, as I was reviewing the metric space section in Amann- Escher textbook, I came across the following example of neighborhood:
"For \left[0,1\right] with the metric induced from R, \left[\frac{1}{2},1\right] is a neighborhood of 1, but not of \frac{1}{2}."
However I can't point out the exactly "r">0 satisfying B_{[0,1]}(1,r)\subseteq[0,1].
:confused:
 
Physics news on Phys.org
Won't any r < 1/2 do?
 
[1/2, 1] is a neighborhood of 1. In this case, r=1/2. Any element, of the ball with a radius of 1/2 centered at 1, has a distance less than 1/2 from 1.

[1/2,1] is not a neighborhood of 1/2. This is because any ball with a radius of r>0 centered at 1/2 contains some elements that are not in [1/2, 1].
 
guess I misunderstood some of the concept in the first place, I thought the ball centered at 1 must completely lie in the interval [1/2,1].
:D.
Thank guys.
 
It does! What makes you think there are any numbers in [1/2, 1] that are not in [1/2, 1]?
 
funcalys said:
guess I misunderstood some of the concept in the first place, I thought the ball centered at 1 must completely lie in the interval [1/2,1].
:D.
Thank guys.

So, for this example, we are not concerned with the entire real line, just the closed unit interval. So, I think that you are probably considering points like 1.1 and 1.2 (for example) to be lying in this ball. However, for this example you can just think about those points as not existing because we only care about points in the closed unit interval.
 

Similar threads

  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
Replies
8
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 26 ·
Replies
26
Views
2K
Replies
5
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K