Abstract. The aim of this paper is to study the set L 2 X of all L 2 -summand vectors of a real Banach space X. We provide a characterization of L 2 -summand vectors in smooth real Banach spaces and a general decomposition theorem which shows that every real Banach space can be decomposed as an L 2 -sum of a Hilbert space and a Banach space without nontrivial L 2 -summand vectors. As a consequence, we generalize some results and we obtain intrinsic characterizations of real Hilbert spaces.
BackgroundWe say that a Banach space X is smooth if every point x in X satisfies that there exists a unique f in X * such that f = x and f (x) = x 2 . If X is a smooth Banach space and x is in X, then we will denote this f by J X (x). The mapping J X : X −→ X * is usually called the duality mapping. Excellent books for consulting the duality mapping are [7] and [8].A closed subspace M of a real Banach space X is said to be an L 2 -summand subspace if there exists another closed subspace N of X verifying X = (M ⊕ N ) 2 ; in other words, m + n 2 = m 2 + n 2 for every m in M and every n in N . The linear projection π M of X onto M that fixes the elements of M and maps the elements of N to {0} is called the L 2 -summand projection of X onto M . Notice that N is uniquely determined, and hence so is π M . Good references for L 2 -summand subspaces are [2] and [3].A vector e of a real Banach space X is an L 2 -summand vector if Re is an L 2 -summand subspace. Furthermore, if e = 0, then there exists a functional e * in X * , which is called the L 2 -summand functional of e, such that e * = 1/ e , e * (e) = 1 and π Re (x) = e * (x) e for every x in X. We want to recall two relevant results about L 2 -summand vectors:(1