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If $f: U \to \mathbb{R}$ is differentiable in a open and bounded $U \subset \mathbb{R}^m$ and, for all $a \in \overline{U}-U$ we have $$f(x) \to 0, \ x \to a ;$$ Then exist a $x_0 \in U$ such that $f'(x_0)=0$.

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    So...what's the question?2012-10-18

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