Real Polar Sets
Definition
Given a nonempty set $A \subset \R ^d$, the
polar (or
polar set) of $A$ is the
set
\[
\Set{c \in \R ^d}{c^\top x \leq 1 \quad \text{for all } x
\in A}
\]
Notation
We denote the polar of the set $A$ by $A^\circ$.