Extensions of Normed Algebras

Thomas William Dawson

Published 2003-08-25Version 1

We review and analyse techniques from the literature for extending a normed algebra, A to a normed algebra, B, so that B has interesting or desirable properties which A may lack. For example, B might include roots of monic polynomials over A. These techniques have been important historically for constructing examples in the theory of Banach algebras. We construct new examples in this way. Elsewhere we contribute to the related programme of determining which properties of an algebra are shared by certain extensions of it. Similarly, we consider the relations between the topological spaces, M(A) and M(B), of closed, maximal ideals of A and B respectively. For example, it is shown that if B is one of the types of 'algebraic extensions' of A constructed in the thesis and M(B) has trivial first Cech-cohomology group then so has M(A). The invertible group of a normed algebra is studied in Chapter 4; it is shown that if a Banach algebra, A, has dense invertible group then so has every integral extension of A. The context for this work is also explained: some new results characterising trivial uniform algebras by means of approximation by invertible elements are given. We show how these results partially answer a famous, open problem of Gelfand. Results in Chapter 4 lead to the conjecture that a uniform algebra is trivial if the group of exponentials of its elements is dense in the algebra. We investigate this conjecture in Chapter 5. In the search for a counterexample, we construct and establish some properties of `logarithmic extensions' of a regular uniform algebra.