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C*-seminorms

Bertram Yood — 1996

Studia Mathematica

A necessary and sufficient condition is given for a*-algebra with identity to have a unique maximal C*-seminorm. This generalizes the result, due to Bonsall, that a Banach *-algebra with identity has such a*-seminorm.

On the non-existence of norms for some algebras of functions

Bertram Yood — 1994

Studia Mathematica

Let C(Ω) be the algebra of all complex-valued continuous functions on a topological space Ω where C(Ω) contains unbounded functions. First it is shown that C(Ω) cannot have a Banach algebra norm. Then it is shown that, for certain Ω, C(Ω) cannot possess an (incomplete) normed algebra norm. In particular, this is so for Ω = ℝ n where ℝ is the reals.

Transitivity for linear operators on a Banach space

Bertram Yood — 1999

Studia Mathematica

Let G be the multiplicative group of invertible elements of E(X), the algebra of all bounded linear operators on a Banach space X. In 1945 Mackey showed that if x 1 , … , x n and y 1 , … , y n are any two sets of linearly independent elements of X with the same number of items, then there exists T ∈ G so that T ( x k ) = y k , k = 1 , … , n . We prove that some proper multiplicative subgroups of G have this property.

Centralizers for subsets of normed algebras

Bertram Yood — 2000

Studia Mathematica

Let G be the set of invertible elements of a normed algebra A with an identity. For some but not all subsets H of G we have the following dichotomy. For x ∈ A either c x c - 1 = x for all c ∈ H or s u p ∥ c x c - 1 ∥ : c ∈ H = ∞ . In that case the set of x ∈ A for which the sup is finite is the centralizer of H.

Ascent, descent and roots of Fredholm operators

Bertram Yood — 2003

Studia Mathematica

Let T be a Fredholm operator on a Banach space. Say T is rootless if there is no bounded linear operator S and no positive integer m ≥ 2 such that S m = T . Criteria and examples of rootlessness are given. This leads to a study of ascent and descent whether finite or infinite for T with examples having infinite ascent and descent.

Commutators in Banach *-algebras

Bertram Yood — 2008

Studia Mathematica

The set of commutators in a Banach *-algebra A, with continuous involution, is examined. Applications are made to the case where A = B(ℓ₂), the algebra of all bounded linear operators on ℓ₂.

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