Representation Theory, Hopf Algebras, Rings and Algebas

Since the first optical instruments were invented, the idea that the visible image of an object under observation depends on tools of observation became commonly assumed in physics. A way of formalizing this idea in mathematics is the construction that assigns to an arbitrary object A in a category K its envelope Env Ω Φ A in a given class of morphisms (a class of representations) Ω with respect to a given class of morphisms (a class of observation tools) Φ. It turns out that if we take a sufficiently wide category of topological algebras as K, then each choice of the classes Ω and Φ defines a "projection of functional analysis into geometry," and the standard "geometric disciplines," like complex geometry, differential geometry, and topology, become special cases of this construction. This gives a formal scheme of "categorical construction of geometries" with many interesting applications, in particular, "geometric generalizations of the Pontryagin duality" (to the classes of noncommutative groups). In this paper we describe this scheme in topology and in differential geometry.

Since the first optical instruments were invented, an idea that the visible image of an object under observation depends on tools of observation became commonly assumed in physics. A way of formalizing this idea in mathematics is the construction that assigns to an arbitrary object A in a category K its envelope Env Ω Φ A in a given class of morphisms (a class of representations) Ω with respect to a given class of morphisms (a class of observation tools) Φ. It turns out that if we take a sufficiently wide category of topological algebras as K, then each choice of the classes Ω and Φ defines a "projection of functional analysis into geometry," and the standard "geometric disciplines," like complex geometry, differential geometry, and topology, become special cases of this construction. This gives a formal scheme of "categorical construction of geometries" with many interesting applications, in particular, "geometric generalizations of the Pontryagin duality" (to the classes of noncommutative groups). In this paper we describe this scheme in topology and in differential geometry.

We suggest a generalization of Pontryagin duality from the category of commutative, complex Lie groups to the category of (not necessarily commutative) Stein groups with algebraic connected component of identity. In contrast to the other similar generalizations, in our approach the enveloping category consists of Hopf algebras (in a proper symmetrical monoidal category).

The Structure Theorem for Hopf modules states that if a bialgebra H is a Hopf algebra (i.e. it is endowed with a so-called antipode) then every Hopf module M is of the form M coH ⊗ H, where M coH denotes the space of coinvariant elements in M. Actually, it has been shown that this result characterizes Hopf algebras: H is a Hopf algebra if and only if every Hopf module M can be decomposed in such a way. The main aim of this paper is to extend this characterization to the framework of quasi-bialgebras by introducing the notion of preantipode and by proving a Structure Theorem for quasi-Hopf bimodules. We will also establish the uniqueness of the preantipode and the closure of the family of quasi-bialgebras with preantipode under gauge transformation. Then, we will prove that every Hopf and quasi-Hopf algebra (i.e. a quasi-bialgebra with quasi-antipode) admits a preantipode and we will show how some previous results, as the Structure Theorem for Hopf modules, the Hausser-Nill theorem and the Bulacu-Caenepeel theorem for quasi-Hopf algebras, can be deduced from our Structure Theorem. Furthermore, we will investigate the relationship between the preantipode and the quasi-antipode and we will study a number of cases in which the two notions are equivalent: ordinary bialgebras endowed with trivial reassociator, commutative quasi-bialgebras, finite-dimensional quasi-bialgebras.

Let G be a finitely generated group, and let kG be its group algebra over a field of characteristic 0. A Taylor expansion is a certain type of map from G to the degree completion of the associated graded algebra of kG which generalizes the Magnus expansion of a free group. The group G is said to be filtered-formal if its Malcev Lie algebra is isomorphic to the degree completion of its associated graded Lie algebra. We show that G is filtered-formal if and only if it admits a Taylor expansion, and derive some consequences.