1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Topology Question

  1. Apr 26, 2010 #1
    For each n in N give examples of subspaces of R^n, which are homotopy equivalent but NOT homeomorphic to each other.

    Give reasons for your answer.

    I'm working along the lines of open and closed intervals in R and balls in R^n with n>1. Although I'm struggling with the reasoning.

    Any help would be great.
  2. jcsd
  3. Apr 26, 2010 #2
    That should work. What are you struggling with?
  4. Apr 26, 2010 #3
    Im struggling with the arguement. As I said im working along the lines of balls homotopy equivalent to points. So for example in R^2 the closed unit disk is clearly not homemorphic to a single point, it fails as a bijection. But they are homotopy equivalent. But I cant see how there are continous functions mapping from one to the other?

    I can see to go from the disk to a point you just make every point in the disk equal to a single point, I think thats continuous? But how would you go the other way.

  5. Apr 26, 2010 #4
    How you get a continuous map from a point to a disk? Just map that point anywhere you want! The center of the disk is probably the most convenient. Let's call this map f, and the map that takes the entire disk to the point g.

    Then you have to show that f g: disk -> disk and g f: pt -> pt are homotopic to the identity map on their respective receiving spaces. Can you do that?
  6. Apr 26, 2010 #5
    Ah, okay, I was missing the fact that it need not be bijective, so as you say it can go anywhere.
    Last edited: Apr 26, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook