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This question already has an answer here:

This question is different as I seek to know a particular context of this subject i.e. wrt 3d printing. enter image description here

It is important for me to know what this means.

" So, projection onto the first coordinate is a continuous, and invertible, mapping from the upper arc to the open interval (−1,1):"

Why is it important ?

It's like doing a self motivated research on this subject. Bump into a doubt and ask here for clarification. How else could I do it ?

ref : http://en.wikipedia.org/wiki/Manifold#Motivational_examples

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marked as duplicate by gandalf3, Ray Mairlot, VRM, Chebhou, p2or Apr 1 '15 at 12:09

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