Title:

The second dual of a Banach algebra

Let A be a Banach algebra over a field IF that is either the real field IR or the complex field ℂ, and let A' be its first dual space and A" its second dual space. R. Arens in 1950, gave a way of defining two Banach algebra products on A" , such that each of these products is an extension of the original product of A when A is naturally embedded in A" . These two products mayor may not coincide. Arens calls the multiplication in A regular provided these two products in A" coincide. Perhaps the first important result on the Arens second dual, due essentially to Shermann and Takeda, is that any C*algebra is Arens regular and the second dual is again a C*algebra. Indeed if A is identified with its universal representation then A" may be identified with the weak operator closure of Ahat. In a significant paper Civin and Yood, obtain a variety of results. They show in particular that for a locally compact Abelian group G ,Ll(G) is Arens regular if and only if G is finite. (Young showed that this last result holds for arbitrary locally compact groups.) Civin and Yood also identify certain quotient algebras of [Ll(G)]". PakKen Wong proves that Ahat is an ideal in A" when A is a semisimple annihilator algebra, and this topic has been taken up by S. Watanabe to show that [L 1 (G)]hat is ideal in [L 1 (G) ]" if and only if G is compact and [M (G)]hat is an ideal in [M(G)]" if and only if G is finite. One shoulu also note in this context the well known fact that if E is a reflexive Banach space with the approximation property and A is the algebra of compact operators on E, (in particular A is semisimple annihilator algebra) then A" may be identified with BL(E). S.J. Pym [The convolution of functionals on spaces of bounded functions, Proc. London Math. Soc., (3) 15 (1965)] has proved that A is Arens regular if and only if every linear functional on A is weakly almost periodic. A general study of those Banach algebras which are Arens regular has been done by N.J. Young and Craw and Young. But in general, results and theorems about the representations of A" are rather few. In Chapter One we investigate some relationships between the Banach algebra A and its second dual space. We also show that if A" is a C*algebra, then * is invariant on A. In Chapter Two we analyse the relations between certain weakly compact and compact linear operators on a Banach algebra A, associated with the two Arens products defined on A". We clarify and extend some known results and give various illustrative examples. Chapter Three is concerned with the second dual of annihilator algebras. We prove in particular that the second dual of a semisimple annihilator algebra is an annihilator algebra if and only if A is reflexive. We also describe in detail the second dual of various classes of semisimple annihilator algebras. In Chapter Four, we particularize some of the problems in Chapters Two and Three to the Banach algebra ℓ1 (S) when S is a semigroup. We also investigate some examples of ℓ1(S) in relation to Arens regularity. Throughout we shall assume familiarity with standard Banach algebra ideas; where no definition is given in the thesis we intend the definition to be as in Bonsall and Duncan. Whenever possible we also use their notation.
