We can find a linear functional such that for all and , and since is a linear space we have for all , and considering positive/ negative, we get for all , and for all . Now consider .
Suppose X is a real normed space, B is a non-empty open convex subset of X, E is a linear subspace of X, B don't intersect with E.
Then there exists a linear functional f, such that f(x)=0 when x belong to E, and f(x)>0 when x belong to B.
i don't know how to use Hahn-Banach Seperation Theorem to get this.Who can help me?Thx a lot.