Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

On an example of neighborhood.

  1. Oct 14, 2012 #1
    Hi folks, as I was reviewing the metric space section in Amann- Escher textbook, I came across the following example of neighborhood:
    "For [itex]\left[0,1\right][/itex] with the metric induced from [itex]R[/itex], [itex]\left[\frac{1}{2},1\right][/itex] is a neighborhood of 1, but not of [itex]\frac{1}{2}[/itex]."
    However I can't point out the exactly "r">0 satisfying [itex]B_{[0,1]}(1,r)[/itex][itex]\subseteq[0,1][/itex].
  2. jcsd
  3. Oct 14, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Won't any r < 1/2 do?
  4. Oct 14, 2012 #3
    [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].
  5. Oct 15, 2012 #4
    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].
    Thank guys.
  6. Oct 15, 2012 #5


    User Avatar
    Science Advisor

    It does! What makes you think there are any numbers in [1/2, 1] that are not in [1/2, 1]?
  7. Oct 15, 2012 #6
    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook