Symmetric kernel of Rademacher multiplicator spaces

Let X be a rearrangement invariant (r.i.) function space on [0,1]. We consider the Rademacher multiplicator space (R,X) of measurable functions x such that xh ∈ X for every a.e. converging series h= anrn ∈ X, where (rn) are the Rademacher functions. We show that for a broad class of r.i. spaces X, the space (R,X) is not r.i. In this case, we identify the symmetric kernel Sym (R,X) of the Rademacher multiplicator space and study when Sym (R,X) reduces to L∞. In the opposite direction, we find new examples of r.i. spaces for which (R,X) is r.i. We consider in detail the case when X is a Marcinkiewicz or an exponential Orlicz space. © 2005 Elsevier Inc. All rights reserved.