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Suppose that $B$ is a convex body (compact closed with nonempty interior), and let $$h_B(u) = \sup_{x \in B} \langle u,x\rangle$$ be its support function. Is there a nice description of the set $E :=\{x : \langle u,x\rangle = h_B(u)\}$, that is, the set of $x$ that achieve the supremum?

EDIT: Assume that $B$ is the unit ball of a norm $\|\cdot\|$. Can we relate the equality set $E$ above to the subdifferential of of $f(x) := \|x\|$?

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$\{x\in B : \langle u,x\rangle = h_B(u)\}\subset\partial B$

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  • $\begingroup$ Thanks, but I need more than that. I have edited the question with more details. $\endgroup$ – passerby51 Mar 31 '13 at 12:40

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