COTILTED COALGEBRAS AND TAME COMODULE TYPE

We introduce the notion of a cotilting C-comodule and a cotilting procedure for coalgebras C over a field K, by a modification of the well-known tilting theory of finite-dimensional K-algebras and its categorical gener- alizations. Among other things, we construct a pair of cotilting functors of Brenner-Butler type for coalgebras and we use them to construct new classes of tame coalgebras from the hereditary coalgebras of tame comodule type. We also prove that a connected hereditary K-coalgebra C of tame comodule type is of discrete comodule type if and only if dim_K C is infinite.