Say the two paths $f_0$ and $f_1$ ae homotopic. Then $(1-t)f_0+tf_1$ is the homotopy between the two paths.

Say $f_0,f_1\in\Bbb{R^2}$, and there is a point $(a,b)$ in $f_0$. How can we find which point in $f_1$ is $(a,b)$ mapped to? Is there a unique mapping for each point in $f_0$?


  • $\begingroup$ Did I annoy you with my drawing? ;-) $\endgroup$
    – Christoph
    Commented Apr 23, 2014 at 8:28
  • $\begingroup$ @Christoph- Haha no thanks a lot that was a fantastic answer! It's just that I can't upvote, as I'm not a regular member. $\endgroup$ Commented Apr 23, 2014 at 13:49

1 Answer 1


I suppose you want to know where $(a,b)$ travels during the homotopy from $f_0$ to $f_1$? If $f_0$ is an injective path, so it has no self-intersections, the answer is $f_1(f_0^{-1}(a,b))$.

If $f_0$ is not injective, $(a,b)$ might have more than one preimage and theres no unique answer. Consider the following homotopic curves:

Two homotopic curves


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