Bases of rearrangement invariant spaces

Abstract

We prove that if E is a rearrangement-invariant space, then a boundedly complete basis exists in E, if and only if one of the following conditions holds: 1) E is maximal and E ≠ L1[0, 1]; 2) a certain (any) orthonormal system of functions from L∞[0, 1], possessing the properties of the Schauder basis for the space of continuous on [0, 1] functions with the norm L∞, represents a boundedly complete basis in E. As a corollary, we state the following assertion: Any (certain) orthonormal system of functions from L∞[0, 1], possessing the properties of the Schauder basis for the space of continuous on [0, 1] functions with the norm L∞, represents a spanning basis in a separable rearrangement-invariant space E, if and only if the adjoint space E* is separable. We prove that in any separable rearrangement-invariant space E the Haar system either forms an unconditional basis, or a strongly conditional one. The Haar system represents a strongly conditional basis in a separable rearrangement-invariant space, if and only if at least one of the Boyd indices of this space is trivial.