Shift invariant preduals of $\ell_1(\Z)$

TL;DR

This paper constructs and analyzes a large family of shift-invariant preduals of ll_1() that turn it into a dual Banach algebra, revealing diverse structures beyond the classical c_0().

Contribution

It explicitly constructs uncountably many shift-invariant preduals of ll_1(), including non-isometric and non-_0() examples, and develops a framework linking these preduals to semigroup compactifications.

Findings

All constructed preduals are Banach space isomorphic to c_0().

Many preduals are non-isometric and not Banach space isomorphic to c_0().

Preduals correspond to certain semigroup compactifications of .

Abstract

The Banach space $\ell_1(\Z)$ admits many non-isomorphic preduals, for example, $C(K)$ for any compact countable space $K$, along with many more exotic Banach spaces. In this paper, we impose an extra condition: the predual must make the bilateral shift on $\ell_1(\Z)$ weak$^*$-continuous. This is equivalent to making the natural convolution multiplication on $\ell_1(\Z)$ separately weak*-continuous and so turning $\ell_1(\Z)$ into a dual Banach algebra. We call such preduals \emph{shift-invariant}. It is known that the only shift-invariant predual arising from the standard duality between $C_0(K)$ (for countable locally compact $K$) and $\ell_1(\Z)$ is $c_0(\Z)$. We provide an explicit construction of an uncountable family of distinct preduals which do make the bilateral shift weak$^*$-continuous. Using Szlenk index arguments, we show that merely as Banach spaces, these are all isomorphic to $c_0$. We then build some theory to study such preduals, showing that they arise from certain semigroup compactifications of $\Z$. This allows us to produce a large number of other examples, including non-isometric preduals, and preduals which are not Banach space isomorphic to $c_0$.