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A NULLSTELLENSATZ FOR AMOEBAS

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A CHARACTERIZATION OF SUBSPACES AND<br />

QUOTIENTS OF REFLEXIVE BANACH SPACES<br />

WITH UNCONDITIONAL BASES<br />

W. B. JOHNSON and BENTUO ZHENG<br />

Abstract<br />

We prove that the dual or any quotient of a separable reflexive Banach space with the<br />

unconditional tree property (UTP) has the UTP. This is used to prove that a separable<br />

reflexive Banach space with the UTP embeds into a reflexive Banach space with an<br />

unconditional basis. This solves several longstanding open problems. In particular,<br />

it yields that a quotient of a reflexive Banach space with an unconditional finitedimensional<br />

decomposition (UFDD) embeds into a reflexive Banach space with an<br />

unconditional basis.<br />

1. Introduction<br />

It has long been known that Banach spaces with unconditional bases as well as their<br />

subspaces are much better behaved than general Banach spaces and that many of the<br />

reflexive spaces (including L p (0, 1), 1

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