Let k be a field and let G be a finite group. There is a canonical element in the Hochschild cohomology of the Tate cohomology gamma(G) is an element of HH3,-1 (H) over cap*(G,k) with the following property. Given a graded (H) over cap*(G,k)-module X, the image of gamma(G) in Ext((H) over cap*(G,k))(3,-1) (X,X) vanishes if and only if X is isomorphic to a direct summand of (H) over cap*(G, M) for some kG-module M.
The description of the realizability obstruction works in any triangulated category with direct sums. We show that in the case of the derived category of a differential graded algebra A, there is also a canonical element of Hochschild cohomology HH3,-1 H*(A) which is a predecessor for these obstructions.
- HOMOLOGICAL FINITENESS CONDITIONS
- UNIVERSAL TODA BRACKETS
- BROWN REPRESENTABILITY
- INFINITE GROUPS
- HOMOTOPY PAIRS