Linear Hahn–Banach extension operators

摘要: Given any subspace N of a Banach space X, there is M containing and the same density character as N, for which exists linear Hahn–Banach extension operator from M* to X*. This result was first proved by Heinrich Mankiewicz [4, Proposition 3.4] using some deeper results Model Theory. More precisely, they used version Lowenheim–Skolem theorem due Stern [11], in turn relies on Keisler–Shelah theorems Previously Lindenstrauss [7], finite dimensional lemma compactness argument, obtained this reflexive spaces. We shall show that leads directly general result, without appeal