Basic Theory of Tower Systems

Abstract

For a Banach space $X$, we study the completed direct limit of its even iterated duals along the canonical embeddings and the bounded inverse limit of the corresponding odd duals. We give explicit constructions, establish their duality relation, and describe the comparison maps associated with iterated dualization. We then examine several assertions in three preprints of Ugle\v{s}i\'c, giving counterexamples concerning dimension stability and the commutation of bidualization with direct limits. The finite quotient shape (the category whose objects are the finite-dimensional quotients, with all continuous linear maps between them) is classified by the algebraic dimension of the continuous dual. We also compute the dimensions of the finite iterated duals and the completed tower of $\ell^{\infty}(\mathbb{N})$, and prove that, inside the tower space, the canonical copy of a higher layer need not be the range of an $M$-projection. Throughout, uncompleted limits, Banach completions, and the different meanings of isomorphism are distinguished explicitly.