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## Homework Statement

There exists no function f: ℝ

^{n}[itex]\rightarrow[/itex] ℝ so that for some point a [itex]\in[/itex] ℝ

^{n}, we have D

_{v}f(a) > 0 for all nonzero vectors v [itex]\in[/itex] ℝ

^{n}.

**2. The attempt at a solution**

No quite sure where to go with this one, any help would be great!