The number of elements in the mutation class of a quiver of type .
On the lattice of cotilting modules.
Subcategories of the derived category and cotilting complexes
We show that there is a one-to-one correspondence between basic cotilting complexes and certain contravariantly finite subcategories of the bounded derived category of an artin algebra. This is a triangulated version of a result by Auslander and Reiten. We use this to find an existence criterion for complements to exceptional complexes.
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