arXiv subjects

D. W. B. Somerset

Publications and source records attributed to D. W. B. Somerset.

7 recordsLinked to original sources

arXiv2021

The structure of k_ω spaces and a question of Arhangel'skii's

In 2010 a question of Arhangel'skii's highlighted a gap in the knowledge of k_ω-spaces. His specific question had in fact been answered by Siwiec in 1976, but the highlighted gap still remains. We introduce the simple idea of pure quotient maps, extend Morita's theorem to these, and use Fell's topology to show that every quotient map onto a k_ω-space can be 'purified'; and thus fill the gap, elucidate the structure of k_ω-spaces, and obtain a fuller answer to Arhangel'skii's question.

math.GN↗

arXiv2012

Spectral synthesis in the multiplier algebra of a C_0(X)-algebra

Let A be a C_0(X)-algebra. Then the multiplier algebra M(A) is a C(Y)-algebra in a natural way, where Y is the Stone-Cech compactification of X. Each x in X gives rise to an ideal J_x of A and an ideal H_x of M(A). The ideal J_x is contained in H_x, and H_x is contained in the strict closure of J_x in M(A). This paper studies the problem of determining when H_x is equal to the strict closure of J_x. THis can be interpreted as a problem of spectral synthesis type.

math.OA↗

arXiv2000

Spectral synthesis for Banach Algebras II

This paper continues the study of spectral synthesis and the topologies $τ_{\infty}$ and $τ_r$ on the ideal space of a Banach algebra, concentrating particularly on the class of Haagerup tensor products of C$^*$-algebras. For this class, it is shown that spectral synthesis is equivalent to the Hausdorffness of $τ_{\infty}$. Under a weak extra condition, spectral synthesis is shown to be equivalent to the Hausdorffness of $τ_r$.

math.OA↗

arXiv1999

Spectral synthesis and topologies on ideal spaces for Banach *-algebras

This paper continues the study of spectral synthesis and the topologies $τ_{\infty}$ and $τ_r$ on the ideal space of a Banach algebra, concentrating on the class of Banach $^*$-algebras, and in particular on $L^1$-group algebras. It is shown that if a group G is a finite extension of an abelian group then $τ_r$ is Hausdorff on the ideal space of $L^1(G)$ if and only if $L^1(G)$ has spectral synthesis, which in turn is equivalent to $G$ being compact. The result is applied to nilpotent groups, [FD]$^-$-groups, and Moore groups. An example is given of a non-compact, non-abelian group G for which $L^1(G)$ has spectral synthesis. It is also shown that if G is a non-discrete group then $τ_r$ is not Hausdorff on the ideal lattice of the Fourier algebra A(G).

math.OA↗

arXiv1999

Non-regularity for Banach function algebras

Let $A$ be a unital Banach function algebra with character space $Φ_A$. For $x\in Φ_A$, let $M_x$ and $J_x$ be the ideals of functions vanishing at $x$, and in a neighbourhood of $x$, respectively. It is shown that the hull of $J_x$ is connected, and that if $x$ does not belong to the Shilov boundary of $A$ then the set $\{y\inΦ_A: M_x\supseteq J_y\}$ has an infinite, connected subset. Various related results are given.

math.FA↗

arXiv1998

A note on ideal spaces of Banach Algebras

In a previous paper the second author introduced a compact topology on the space of closed ideals of a unital Banach algebra A. If A is separable then this topology is either metrizable or else neither Hausdorff nor first countable. Here it is shown that this topology is Hausdorff if A is the algebra of once continuously differentiable functions on an interval, but that if A is a uniform algebra then this topology is Hausdorff if and only if A has spectral synthesis. An example is given of a strongly regular, uniform algebra for which every maximal ideal has a bounded approximate identity, but which does not have spectral synthesis.

math.FA↗

arXiv1998

Strong regularity for uniform algebras

A survey is given of the work on strong regularity for uniform algebras over the last thirty years, and some new results are proved, including the following. Let A be a uniform algebra on a compact space X and let E be the set of all those points x of X such that A is not strongly regular at x. If E has no non-empty, perfect subsets then A is normal, and X is the character space of A. If X is either the interval or the circle and E is meagre with no non-empty, perfect subsets then A is trivial. These results extend Wilken's work from 1969. It is also shown that every separable Banach function algebra which has character space equal to either the interval or the circle and which has a countably-generated ideal lattice is uniformly dense in the algebra of all continuous functions.

math.FA↗