Polynomial reflexivity in Banach spaces

We ask when the space ofN-homogeneous analytic polynomials on a Banach space is reflexive. This turns out to be related to whether polynomials are weakly sequentially continuous, and to the geometry of spreading models. For example, if these spaces are reflexive for allN, no quotient of the dual space may have a spreading model with an upperq-estimate, and every bounded holomorphic function on the unit ball has a Taylor series made up of weakly sequentially continuous polynomials (we assume the approximation property). Alencar, Aron and Dineen [AAD] gave the first example of some properties of a polynomially reflexive space (usingT*, the original Tsirelson space); we show that these properties and others are shared by all polynomially reflexive spaces. This paper forms a portion of the Ph. D. dissertation of the author, under the supervision of W. B. Johnson.     

A remark on bases and reflexivity in Banach spaces

LetX be a Banach space with a basis. It is proved that if (a) all bases ofX are shrinking, or (b) all bases ofX are boundedly complete, thenX is reflexive. Research supported by Grant AF EOAR 66-18. This is part of the author’s Ph.D. thesis prepared at The Hebrew University of Jerusalem, under the supervision of Professor A. Dvoretzky and Professor J. Lindenstrauss. The author wishes to thank Professor Lindenstrauss for his valuable remarks.     

A note on integral-convexity in Banach spaces

In the present paper we focus on a generalization of the notion of integral convexity. This concept, introduced in [J.Y. Wang, Y.M. Ma, The integral convexity of sets and functionals in Banach spaces, J. Math. Anal. Appl. 295 (2004) 211-224] by replacing, in the definition of classical notion of convexity, the sum by the integral, has interesting applications in optimal control problems. By using, instead of Bochner integral, a more general vector integral, that of Pettis, we obtain some results on integral-extreme points of subsets of a Banach space stronger than those given in [J.Y. Wang, Y.M. Ma, The integral convexity of sets and functionals in Banach spaces, J. Math. Anal. Appl. 295 (2004) 211-224]. Finally, a natural example coming from measure theory is included, in order to reflect the relationships between different kinds of integral convexity.     

A note on compactness in Banach spaces

Two simple concepts are presented for constructing necessary and sufficient compactness criteria in Banach spaces. The concepts are applied to the spaces lip(T), C(T), and L_p(µ), where T is a compact metric space.     

A note on holomorphic approximation in Banach spaces

Given a complex Banach space X and a holomorphic function f on its unit ball B, we discuss the problem whether f can be approximated, uniformly on smaller balls, by functions g holomorphic on all of X. Research partially supported by NSF grant DMS0700281.     

A note on a selector theorem in Banach spaces

The existence of a strongly measurable function taking its values in a variable subset of a separable and reflexive Banach space was shown in Ref. 1. Here, we show the collection of all such functions to be weakly compact in itself.     

A note on subsymmetric renormings of Banach spaces

We revisit the concept of a subsymmetric norm and construct a subsymmetric renorming of a Banach space with a subsymmetric basis. As a by-product of our work we introduce the concept of a lower symmetric basis and investigate its connection with subsymmetric bases and subsymmetric renormings.     

A note on ball-covering property of Banach spaces

By a ball-covering B of a Banach space X, we mean that B is a collection of open (or closed) balls off the origin whose union contains the unit sphere SX of X; and X is said to have the ball-covering property (BCP) provided it admits a ball-covering by countably many balls. In this note we give a natural example showing that the ball-covering property of a Banach space is not inherited by its subspaces; and we present a sharp quantitative version of the recent Fonf and Zanco renorming result saying that if the dual X^∗ of X is w^∗ separable, then for every ε>0 there exist a (1+ε)-equivalent norm on X, and an R>0 such that in this new norm SX admits a ball-covering by countably many balls of radius R. Namely, we show that R=R(ε) can be taken arbitrarily close to (1+ε)/ε, and that for X=?₁[0,1] the corresponding R cannot be equal to 1/ε. This gives the sharp order of magnitude for R(ε) as ε→0.     

A note on causality in reflexive Banach spaces.

In this short note we discuss the causality of linear closable operators in reflexive Banach spaces. We show that in general causality is not preserved under closure procedures. We shall give an alternative definition of causality for closable operators by means of a certain continuity poperty that carries over to the closure. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Property of reflexivity for multiplication operators on Banach function spaces

In this paper, we give conditions under which the powers of the multiplication operator \(M_{z}\) are reflexive on a Banach space of functions analytic on a plane domain.     

A note on weighted Banach spaces of holomorphic functions

For every open subset G of and for every continuous, strictly positive weight v on G, the Banach space of all the holomorphic functions f on G such that vanishes at infinity on G, endowed with the natural weighted sup-norm, is isomorphic to a closed subspace of the Banach space c₀; hence it is reflexive if and only if it is finite dimensional.Received: 30 September 2002     

A note on the convergence of martingales in Banach function spaces

We shall give a necessary and sufficient condition for uniformly integrable martingales to converge in the norm topology in a Banach function space. This yields a new characterization of the probabilisticA _p -condition introduced by Izumisawa and Kazamaki [4].     

A note on weakly Lindelöf determined Banach spaces

We prove that weakly Lindelöf determined Banach spaces are characterized by the existence of a “full” projectional generator. Some other results pertaining to this class of Banach spaces are given.     

A note on reflexivity and nonconvexity

In this short note we prove that a Banach space X is reflexive if, and only if, the Eisenfeld–Lakshmikantham measure of nonconvexity in X satisfies the Cantor property. Using this characterization, some results in best approximation and fixed point theory for reflexive Banach spaces are generalized by removing convexity requirements.     

A note on convergence in Banach spaces of cotype p

Let B be a separable Banach space. The following is one of the results proved in this paper. The Banach space B is of cotype p if and only if

1. d_n, n 1, has no subsequence converging in probability, and

2. ∑_{n 1}|a_n|^p < ∞ whenever ∑_{n 1}a_nd_n converges almost surely are equivalent for every sequence d_n, n 1, of symmetric independent random elements taking values in B.