Abstract

AbstractWe define and studyλ-strict ideals in Banach spaces, which forλ=1 means strict ideals. Strict u-ideals in their biduals are known to have the unique ideal property; we prove that so also doλ-strict u-ideals in their biduals, at least forλ>1/2. An open question, posed by Godefroyet al. [‘Unconditional ideals in Banach spaces’,Studia Math.104(1993), 13–59] is whether the Banach spaceXis a u-ideal in Ba(X), the Baire-one functions inX**, exactly whenκu(X)=1; we prove that ifκu(X)=1 thenXis a strict u-ideal in Ba (X) , and we establish the converse in the separable case.

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