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Journal of the London Mathematical Society 2000 62(1):239-252; doi:10.1112/S0024610700001150
© 2000 by London Mathematical Society
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© The London Mathematical Society

Disjointly Strictly-Singular Inclusions between Rearrangement Invariant Spaces

A. García Del Amo, Francisco L. Hernández, Víctor M. Sánchez and Evgueni M. Semenov

Department of Mathematics, Faculty of Sciences, University of Salamanca 37008 Salamanca, Spain, garciada{at}gugu.usal.es
Department of Mathematical Analysis, Faculty of Mathematics, Complutense University 28040 Madrid, Spain, pacoh{at}eucmax.sim.ucm.es
Department of Mathematical Analysis, Faculty of Mathematics, Complutense University 28040 Madrid, Spain, Victor_Sanchez{at}Mat.UCM.Es
Department of Mathematics, Voronezh State University Voronezh 394693, Russia, root{at}func.vsu.ru

Received 8 September 1998. Revision received 7 May 1999.

A linear operator between two Banach spaces X and Y is strictly-singular (or Kato) if it fails to be an isomorphism on any infinite dimensional subspace. A weaker notion for Banach lattices introduced in [8] is the following one: an operator T from a Banach lattice X to a Banach space Y is said to be disjointly strictly-singular if there is no disjoint sequence of non-null vectors (xn)nisinN in X such that the restriction of T to the subspace [(xn){infty}n=1] spanned by the vectors (xn)nisinN is an isomorphism. Clearly every strictly-singular operator is disjointly strictly-singular but the converse is not true in general (consider for example the canonic inclusion Lq[0, 1]{rightarrowhook}Lp[0, 1] for 1≤p<q<{infty}). In the special case of considering Banach lattices X with a Schauder basis of disjoint vectors both concepts coincide. The notion of disjointly strictly-singular has turned out to be a useful tool in the study of lattice structure of function spaces (cf. [7–9]). In general the class of all disjointly strictly-singular operators is not an operator ideal since it fails to be stable with respect to the composition on the right.

The aim of this paper is to study when the inclusion operators between arbitrary rearrangement invariant function spaces E[0, 1] {equiv} E on the probability space [0, 1] are disjointly strictly-singular operators.


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