Arens regularity and irregularity of ideals in Fourier and group algebras

Abstract

Let $\mathcal{A}$ be a weakly sequentially complete Banach algebra containing a bounded approximate identity that is an ideal in its second dual $\mathcal{A}^{\ast\ast}$, we call such an algebra a Wesebai algebra. In the present paper we examine the Arens regularity properties of closed ideals of algebras in the Wesebai class. We observe that, although Wesebai algebras are always strongly Arens irregular, a variety of Arens regularity properties can be observed within their closed ideals. After characterizing Arens regular ideals and strongly Arens irregular ideals, we proceed to particularize to the main examples of \wasabi algebras, the convolution group algebras $L^1(G)$, $G$ compact, and the Fourier algebras $A(\Gamma)$, $\Gamma$ discrete and amenable. We find examples of Arens regular ideals in $L^1(G)$ and $A(\Gamma)$, both reflexive and nonreflexive and examples of strongly Arens irregular ideals that are not in the \wasabi class. For this, we construct, in many noncommutative groups, a new class of Riesz sets which are not $\Lambda(p)$, for any $p>1$. Our approach also shows that every infinite Abelian group contains a Rosenthal set that is not $\Lambda(p)$, for any $p>0$. These latter results could be of independent interest.