Support Varieties and Representation Type of Self-Injective Algebras

We use the theory of varieties for modules arising from Hochschild cohomology to give an alternative version of the wildness criterion of Bergh and Solberg: If a finite dimensional self-injective algebra has a module of complexity at least 3 and satisfies some finiteness assumptions on Hochschild cohomology, then the algebra is wild. We show directly how this is related to the analogous theory for Hopf algebras that we developed. We give applications to many different types of algebras: Hecke algebras, reduced universal enveloping algebras, small half-quantum groups, and Nichols (quantum symmetric) algebras.Comment: 21 page

Some Problems in the Representation Theory of Simple Modular Lie Algebras

The finite-dimensional restricted simple Lie algebras of characteristic p > 5 are classical or of Cartan type. The classical algebras are analogues of the simple complex Lie algebras and have a well-advanced representation theory with important connections to Kazhdan-Lusztig theory, quantum groups at roots of unity, and the representation theory of algebraic groups. We survey progress that has been made towards developing a representation theory for the restricted simple Cartan-type Lie algebras, discuss comparable results in the classical case, formulate a couple of conjectures, and pose a dozen open problems for further study.Comment: References updated; a few minor changes made in this versio

Representation theory and projective geometry

We give an elementary introduction to our papers relating the geometry of rational homogeneous varieties to representation theory. We also describe related work and recent progress.Comment: 37 pages with picture

Support varieties and representation type of small quantum groups

In this paper we provide a wildness criterion for any finite dimensional Hopf algebra with finitely generated cohomology. This generalizes a result of Farnsteiner to not necessarily cocommutative Hopf algebras over ground fields of arbitrary characteristic. Our proof uses the theory of support varieties for modules, one of the crucial ingredients being a tensor product property for some special modules. As an application we prove a conjecture of Cibils stating that small quantum groups of rank at least two are wild.Comment: 14 pages; minor revisions; to appear in Int. Math. Res. No

Action Type Geometrical Equivalence of Representations of Groups

For every variety of algebras and every algebras in these variety we can consider an algebraic geometry. Algebras may be many sorted (not necessarily one sorted) algebras. A set of sorts is fixed for each variety. This theory can be applied to the variety of representations of groups over fixed commutative ring with unit. We consider a representation as two sorted algebra. We concentrate on the case of the action type algebraic geometry of representations of groups. In this case algebraic sets are defined by systems of action type equations and equations in the acting group are not considered. This is the special case, which cannot be deduced from the general theory. In this paper the following basic notions are introduced: action type geometrical equivalence of two representations, action type quasi-identity in representations, action type quasi-variety of representations, action type Noetherian variety of representations, action type geometrically Noetherian representation, action type logically Noetherian representation.Comment: 35 page