Patrick N. Dowling

The optimality of James’s distortion theorems

A renorming of `1, explored here in detail, shows that the copies of `1 produced in the proof of the Kadec-Pe lczynski theorem inside nonreflexive subspaces of L1[0, 1] cannot be produced inside general nonreflexive spaces that contain copies of `1. Put differently, James’s distortion theorem producing oneplus-epsilon-isomorphic copies of `1 inside any isomorphic copy of `1 is, in a certain sense, optimal. A similar renorming of c0 shows that James’s distortion theorem for c0 is likewise optimal. James’s distortion theorems for `1, the space of absolutely summable sequences of scalars, and c0, the space of null sequences of scalars, are well-known [J]. The former states that, whenever a Banach space contains a subspace isomorphic to `1, the Banach space contains subspaces that are almost isometric to `1. Several of the authors of this article, individually and in concert, have tried to use this feature of `1 to determine if all (equivalent) renormings of `1 fail to have the fixed point property for nonexpansive mappings (the FPP); i.e. if, in any renorming of `1, there exist a nonempty, closed, bounded and convex subset C and a nonexpansive self-map T of C without a fixed point. The basis of these attempts was to use the fact that `1 in its usual norm fails to have the fixed point property and, since each renorming of `1 contains subspaces almost isometric to `1, a perturbation of the usual example would hopefully produce a nonexpansive self-map of a nonempty, closed, bounded, convex set in any renorming of `1. Similar attempts in c0 were also made. What appeared to be needed in these attempts were strengthened versions of James’s distortion theorems. To be specific, James’s theorem for `1 states that if a Banach space X with norm ‖ · ‖ contains an isomorphic copy of `1, then, for each > 0, there exists a sequence (xk) in the unit sphere of X such that (1− ) ∑∞ k=1 |tk| ≤ ‖ ∑∞ k=1 tkxk‖ ≤ ∑∞ k=1 |tk| for all (tk) ∈ `1. The proof of the theorem shows even more than the statement indicates. The sequence (xk) may be chosen to have the additional property that, if ( n) is a sequence of positive numbers decreasing to 0, then for each n, (1 − n) ∑∞ k=n |tk| ≤ ‖ ∑∞ k=n tkxk‖ ≤ ∑∞ k=n |tk| , for all (tk) ∈ `1. That is, for each δ > 0, by ignoring a finite number of terms at the beginning of the sequence (xk), one obtains copies of `1 which are (1 + δ)-isomorphic to `1. This Received by the editors May 8, 1995 and, in revised form, July 7, 1995. 1991 Mathematics Subject Classification. Primary 46B03, 46B20.

Weak compactness is equivalent to the fixed point property in c~0

A nonempty, closed, bounded, convex subset of c 0 has the fixed point property if and only if it is weakly compact.

The fixed point property for subsets of some classical Banach spaces

We prove a general result about the stability of the fixed point property in closed bounded convex subsets of certain Banach spaces. This allows us to characterize those closed bounded convex subsets of L1[0, 1] that are weakly compact. Similarly, we characterize the closed bounded convex subsets of !1 that are compact. Generalizations of these results to the setting of noncommutative L1-spaces are also obtained.

Renormings of ℓ 1 and C 0 and Fixed Point Properties

As has been noted in previous chapters, there are many geometric conditions on a Banach space strong enough to imply that the Banach space has the fixed point property. Geometric conditions such as uniform rotundity, uniform smoothness, or normal structure together with reflexivity are sufficient to imply the fixed point property. Each of these conditions also implies (or assumes in the last case) that the Banach space is reflexive.

Remarks on James's distortion theorems

If a Banach space X contains a complemented subspace isomorphic to l 1 and if e > 0, then there exists a subspace Y of X and a projection P from X onto Y such that Y is (1 + e)-isometric to l 1 and ∥ P ∥ ≤ 1 + e. A stronger result for c 0 is proved for Banach spaces whose dual unit ball is weak sequentially compact.

Some fixed point results in l 1 and c 0

When James [10] proved that neither l1 nor c0 is distortable, he provided a tool which appeared to be useful in considering the question of whether l1 or c0 could be renormed to have the !xed point property. The gist of the desired proof was that, since both spaces admit !xed point-free isometries on closed bounded convex sets and all renormings of l1 or c0 contain almost isometric copies of l1 or c0, then perturbations of the isometries would hopefully produce nonexpansive self-maps of closed bounded convex subsets without !xed points in the renormed spaces. Although the authors have not been able to bring this idea to fruition (not surprisingly perhaps given the lack of stability of subsets with the !xed point property as noted in [9]), connections between spaces containing good copies of l1 or c0 and the failure of the !xed point property have been investigated in several articles [4–6]. In this article, the authors continue to investigate James’s distortion theorems and their relationship to !xed points and to the more restrictive renormings of l1 and c0 considered in the above articles. Simply stated, James’s distortion theorems state that Banach spaces which contain isomorphic copies of l1 (respectively, c0) contain almost isometric copies of l1 (respectively, c0). In fact, the proofs that James [10] gives for these theorems shows a bit more.

Stability of Banach Space Properties In The Projective Tensor Product

Let X be a real or complex Banach space and 1 < p < ∞. We show that L p [0, 1] ⊗ X has a property (P) whenever X has property (P), provided that (P) has certain characteristics. Examples of property (P) are the Radon-Nikodym property, the analytic Radon-Nikodym property, the near Radon-Nikodym property, the complete continuity property, the analytic complete continuity property, and the property of not containing a copy of c 0.

Rosenthal sets and the Radon-Nikodym property

Let X be a complex Banach space, G a compact abelian metrizable group and A a subset of G, the dual group of G . If X has the Radon-Nikodym property and LTM(G; X) is separable, then LTM (G, X) has the Radon-Nikodym property. One consequence of this is that CA(G, X) has the Radon-Nikodym property whenever X has the Radon-Nikodym property and the Schur property and A is a Rosenthal set. A partial stability property for products of Rosenthal sets is also obtained. 1991 Mathematics subject classification {Amer. Math. Soc.): 46 B 22, 43 A 46.