This note is devoted to the study of geometric properties and the relationships between a projective space and an exponential class, both naturally associated with the positive elements in a commutative Banach algebra. Even though the motivating problem consists of understanding the geometry of the class of densities with respect to a given measure, the formulation can be carried out in general...

Let A and B be unital Banach algebras with A a subalgebra of B. Denote the algebra of all n×n matrices with entries from A by Mn(A). In this paper we prove some results concerning the open question: If A is inverse closed in B, then is Mn(A) inverse closed in Mn(B)? We also study related questions in the setting where A is a symmetric Banach *-algebra.

Let A be a complex, commutative unital Banach algebra. We introduce two notions of exponential reducibility of Banach algebra tuples and present an analogue to the Corach-Suárez result on the connection between reducibility in A and in C(M(A)). Our methods are of an analytical nature. Necessary and sufficient geometric/topological conditions are given for reducibility (respectively reducibility...

Let n ∈ N − {1}, and let A be a Banach algebra. An additive map D : A → A is called n-Jordan derivation if D(a) = D(a)a + aD(a)a + ...+ aD(a)a+ aD(a), for all a ∈ A. Using fixed point methods, we investigate the stability of n–Jordan derivations (n–Jordan ∗−derivations) on Banach algebras (C∗−algebras). Also we show that to each approximate ∗−Jordan derivation f in a C∗− algebra there correspon...

We give sufficient conditions that allow contractible (resp., reflexive amenable) Banach algebras to be finite-dimensional and semisimple algebras. Moreover, we show that any contractible (resp., reflexive amenable) Banach algebra in which every maximal left ideal has a Banach space complement is indeed a direct sum of finitely many full matrix algebras. Finally, we characterize Hermitian ∗-alg...

We consider the relationship between derivations and local power series quotients for a locally multiplicatively convex Fr echet algebra (this includes the case of a Banach algebra). In x2 we derive necessary conditions for a commutative Fr echet algebra to have a local power series quotient. Our main result here is Proposition 2.6, which shows that if the generating element has nite closed des...

The aim of this notes is to provide basic information about commutative Banach algebras. The final goal is to show that a unital, commutative complex Banach algebra A can be embedded as subalgebra of C(MA), the algebra of continuous functions on a w∗-compact setMA, known as the maximal ideal space or character space. Also, the non unital commutative complex Banach algebra can be embedded as a s...

We study when certain properties of Banach algebras are stable under ultrapower constructions. In particular, we consider when every ultrapower of A is Arens regular, and give some evidence that this is so if and only if A is isomorphic to a closed subalgebra of operators on a super-reflexive Banach space. We show that such ideas are closely related to whether one can sensibly define an ultrapo...

It is then easily verified that RG satisfies the ring axioms; in fact, RG is a linear algebra over R. (We write all groups multiplicatively, and denote group identities by 1; we also use 1 for the unit element of R if there is one.) If R, in addition to being a ring, is a Banach algebra (i.e., an algebra over the complex field K, with a submultiplicative norm which makes R a Banach space), then...

We study Banach C̃-algebras, i.e., complete ultra-pseudo-normed algebras over the ring C̃ of Colombeau generalized complex numbers. We develop a spectral theory in such algebras. We show by explicit examples that important parts of classical Banach algebra theory do not hold for general Banach C̃algebras and indicate a particular class of Banach C̃-algebras that overcomes these limitations to a lar...