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Let $ \displaystyle{ S^2 = \{(x,y,z) \in \mathbb R ^3 : x^2 + y^2 + z^2 =1 \} }$ and $ \displaystyle { U= \{ (x,y) \in \mathbb R ^2 : x^2 + y^2 < 1 \}}$. Consider the functions

$ \displaystyle{ f_1,f_2: U \to S^2 }$ where $ \displaystyle{ f_1 = (u,v, \sqrt{1-u^2 -v^2}) }$ and $ \displaystyle{ f_2 = (u,v, -\sqrt{1-u^2 -v^2}) }$.

Prove that $ f_2 ^{-1} \circ f_1$ is differentiable.

Do I have to find $ f_2 ^{-1} \circ f_1$ or I can prove that is differentiable without finding the function ?

Can you give some help?

Thanks in advance!

Edit: I still haven't made any progress. Any ideas? Thank you!

Sorry I made a mistake. It is $ f(U_1) \cup f(U_2) = S^2 - \{ (x,y,z) \in \mathbb R ^3 : z=0 \} $. So $ f_2 ^{-1} \circ f_1 $ is differentiable on this set.

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    @JuanSimões: Sorry you are right! I have edit the question.2012-05-23

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The original formulation of your question was quite ok. After the edit the answer (to the new question) is easy, it is just the composition of two functions which are differentiable (you need some basic knowledge about functions between submanifolds).

The map you defined in the edit extends, however, quite obviously, to all of $S^2$, by defining $f(x,y,0)= (x,y,0)$, and it makes sense to ask whether this extended map is continuous or even differentiable as a map $S^2\rightarrow S^2$.

Since I'm suspicious that this is homework I won't answer this question, but in order to figure this out you may want to ask yourself what this map is doing geometrically and whether it might extend even to $\mathbb{R}^3$ to a continuous or even differentiable map.

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    Namely: $f$ is the restriction of $F(x,y,z)=(x,y,-z)$ to the sphere.2013-06-18