Are Maps on a Sphere Homotopic if They Avoid Antipodal Points?

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Discussion Overview

The discussion revolves around the homotopy of continuous maps from the n-sphere to itself, specifically focusing on the condition that these maps avoid antipodal points. Participants explore the implications of this condition on the existence of a homotopy between two such maps.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant asserts that if two continuous maps ##f,g:S^n\rightarrow S^n## satisfy ##f(x) \neq -g(x)## for all ##x\in S^n##, then they are homotopic, seeking a method to prove this.
  • Another participant suggests using a straight line homotopy defined as ##H(x,t) = f(x) + t(g(x) - f(x))## and notes the importance of the antipodal condition in this context.
  • A further contribution proposes a modification of the homotopy to ensure it remains on the sphere, specifically using the formula ##H(x,t) = \frac{f(x) + t(g(x) - f(x))}{\|f(x) + t(g(x) - f(x))\|}##, questioning the implications of the denominator potentially vanishing.
  • One participant points out that both ##f(x)## and ##g(x)## are elements of ##S^{n}## and suggests taking the norm of both sides to analyze the situation.
  • A later reply expresses appreciation for the insight provided by another participant, indicating a collaborative atmosphere.

Areas of Agreement / Disagreement

Participants appear to be exploring the problem collaboratively, but there is no consensus on the proof or the implications of the antipodal condition, indicating that the discussion remains unresolved.

Contextual Notes

Participants have not fully resolved the mathematical steps necessary to rigorously prove the homotopy condition, particularly regarding the behavior of the homotopy when the denominator approaches zero.

R136a1
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If I take two arbitrary continuous maps ##f,g:S^n\rightarrow S^n## such that ##f(x) \neq -g(x)## for any ##x\in S^n##, then ##f## and ##g## are homotopic.

How do I show this result? I really don't see how to use the condition that ##f## and ##g## never occupy two antipodal points. Any hint would be appreciated.
 
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Consider the straight line homotopy ##H(x,t) = f(x) + t(g(x) - f(x))##. Try to force the straight line homotopy onto the sphere; you'll see how the constraint on ##f(x)## and ##g(x)## not occupying antipodal points comes into play.
 
Thanks a lot, miss!

So, my idea is to take

H(x,t) = \frac{f(x) + t(g(x) - f(x))}{\|f(x) + t(g(x) - f(x))\|}

I guess the constraint on ##f## and ##g## comes into play because we don't want the denominator to vanish? But I have troubles proving this rigorously. Assume that the denominator is ##0##, then

(t-1)f(x) = tg(x)

I'm pretty stuck now!
 
##f(x)## and ##g(x)## are both elements of ##S^{n}##; take the norm of both sides.
 
WannabeNewton said:
##f(x)## and ##g(x)## are both elements of ##S^{n}##; take the norm of both sides.

Wow, I didn't think of that! Why are you so smart?
 

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