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...

Let k be a field, H a cocommutative bialgebra, A a commutative left H-module algebra, Hom(H,A) the k-algebra of the k-linear maps from H to A under the convolution product, Z(H,A) the submonoid of Hom(H,A) whose elements satisfy the cocycle condition and G any subgroup of the monoid Z(H,A). We give necessary and sufficient conditions for the projectivity and flatness over the graded ring of nor...

The results in this paper were motivated by the case when the inverse semigroup is the McAlister monoid MX on a set X. In [1] we considered some very large convolution algebras on MX , including some C ∗-algebras. Our main focus in [1] was deciding on the primitivity of the algebras, and one key tool was the use of generalized Rukolaine idempotents. To define these very large convolution algebr...

The question of whether a hypercyclic operator T acting on Fréchet algebra X admits or not an vectors (but 0) has been addressed in the recent literature. In this paper we give new criteria and characterisations context convolution operators H(C) backward shifts general sequence algebra. Analogous questions arise for stronger properties like frequent hypercyclicity. trend sufficient condition w...

A closed subalgebra of a Banach algebra is called maximal if it is not contained in any larger proper closed subalgebra. Let G be a discrete abelian topological group and L its group-algebra, i.e. L is the Banach algebra of functions/on G with E*e<J |/(X)| < °° and multiplication defined as convolution. What are the maximal subalgebras of L? The complete answer is not known even when G is the g...

Let Gr be the affine Grassmannian for a connected complex reductive group G. Let CG be the complex vector space spanned by (equivalence classes of) Mirković-Vilonen cycles in Gr. The Beilinson-Drinfeld Grassmannian can be used to define a convolution product on MV-cycles, making CG into a commutative algebra. We show, in type A, that CG is isomorphic to C[N], the algebra of functions on the uni...

We give a general method for computing the cyclic cohomology of crossed products by étale groupoids, extending the Feigin-Tsygan-Nistor spectral sequences. In particular we extend the computations performed by Connes, Brylinski, Burghelea and Nistor for the convolution algebra C∞ c (G) of an étale groupoid, removing the Hausdorffness condition and including the computation of hyperbolic compone...

A hypergroup is roughly speaking a locally compact Hausdorff space which has enough structure so that a convolution on the corresponding vector space of Radon measures makes it a Banach algebra. Hypergroups generalize in many ways topological groups. In this paper we extend to compact not necessarily commutative hypergroups some basic techniques on multipliers set forth for compact groups in He...

We study Translation functors and Wall-Crossing functors on infinite dimensional representations of a complex semisimple Lie algebra using D -modules. This functorial machinery is then used to prove the Endomorphism-theorem and the Structure-theorem, two important results established earlier by W. Soergel in a totally different way. Other applications to the category O of Bernstein-GelfandGelfa...