Hahn-Banach theorem

The Hahn-Banach theorem is a central tool in functional analysis; it shows that there are "enough" continuous linear functionals defined on every normed vector space to make the study of the dual space interesting.

The most general formulation of the theorem needs some preparations. If V is a vector space over the scalar field K (either the real numbers R or the complex numbers C), we call a function N : V -> R sublinear if N(ax + by) ≤ |a| N(x) + |b| N(y) for all x and y in V and all scalars a and b in K. Every norm on V is sublinear, but there are other examples.

Now let U be a subspace of V and let φ : U -> K be a [linear function]? such that |φ(x)| ≤ N(x) for all x in U. Then the Hahn-Banach theorem states that there exists a linear map ψ : V -> K which extends φ (meaning ψ(x) = φ(x) for all x in U) and which is dominated by N on all of V (meaning |ψ(x)| ≤ N(x) for all x in V.

The extension ψ is in general not uniquely specified by φ and the proof gives no method as to how to find ψ: it depends on Zorn's lemma.