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Looking for the proof of the lemma asserting that the conical surface (envelope) is a closed space. Thank you.

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    Please give more details, so that we know what you have in mind. Also, writing complete words is usually a great idea! :)2011-01-08
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    If by "conical surface" you mean this: http://en.wikipedia.org/wiki/Conical_surface and if by "closed" you mean this: http://en.wikipedia.org/wiki/Closed_set then the lemma is false and there is no proof. You need some additional condition like having a closed directrix.2011-01-08
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    Please see my answer below.2011-01-08

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