2.5-4 F. Riesz’s Lemma. Let Y and Z be subspaces of a normed space X (of any dimension), and suppose that Y is closed and is a proper subset of Z. Then for every real number θ in the interval (0, 1) there is a z ∈ Z such that ||z|| = 1, ||z − y|| ≧ θ for all y ∈ Y.