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In Convex Analysis and Nonlinear Optimization by Borwein and Lewis there's the following exercise, stating that a closed approximately convex is convex:


In (iv) I don't understand why the polar set is a ray. The contradiction in (v) comes from $H$ being a half-space. $E$ is an euclidean(finite dimensional) normed liniar space. The polar set $H^{\circ}$ is defined as $H^{\circ}=\{\phi\in E\,|\,\langle\phi,x\rangle\leq 1, \forall x\in H\}$.

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