The Second Duals of Beurling Algebras

Let $A$ be a Banach algebra, with second dual space $A""$. We propose to study the space $A""$ as a Banach algebra. There are two Banach algebra products on $A""$, denoted by $/,/Box/,$ and $/,/Diamond/,$. The Banach algebra $A$ is Arens regular if the two products $/Box$ and $/Diamond$ coincide on $A""$. In fact, $A""$ has two topological centers denoted by $/mathfrak{Z}^{(1)}_t(A"")$ and $/mathfrak{Z}^{(2)}_t(A"")$ with $A /subset /mathfrak{Z}^{(j)}_t(A"")/subset A""/;/,(j=1,2)$, and $A$ is Arens regular if and only if $/mathfrak{Z}^{(1)}_t(A"")=/mathfrak{Z}^{(2)}_t(A"")=A""$. At the other extreme, $A$ is strongly Arens irregular if $/mathfrak{Z}^{(1)}_t(A"")=/mathfrak{Z}^{(2)}_t(A"")=A$. We shall give many examples to show that these two topological centers can be different, and can lie strictly between $A$ and $A""$.We shall discuss the algebraic structure of the Banach algebra $(A"",/,/Box/,)V$; in particular, we shall seek to determine its radical and when this algebra has a strong Wedderburn decomposition. We are also particularly concerned to discuss the algebraic relationship between the two algebras $(A"",/,/Box/,)$ and $(A"",/,/Diamond/,)$. Most of our theory and examples will be based on a study of the weighted Beurling algebras $L^1(G,/omega)$, where $/omega$ is a weight function on the locally compact group $G$. The case where $G$ is discrete and the algebra is ${/ell}^{/,1}(G, /omega)$ is particularly important.We shall also discuss a large variety of other examples. These include a weight $/omega$ on $/mathbb{Z}$ such that $/ell^{/,1}(/mathbb{Z},/omega)$ is neither Arens regular nor strongly Arens irregular, and such that the radical of $(/ell^{/,1}(/mathbb{Z},/omega)"", /,/Box/,)$ is a nilpotent ideal of index exactly $3$, and a weight $/omega$ on $/mathbb{F}_2$ such that two topological centers of the second dual of $/ell^{/,1}(/mathbb{F}_2, /omega)$ may be different, and that the radicals of the two second duals may have different indices of nilpotence.