Idempotents of double Burnside algebras, L-enriched bisets, and   decomposition of p-biset functors
  Let R be a (unital) commutative ring, and G be a finite group with order invertible in R. We introduce new idempotents in the double Burnside algebra RB(G,G), indexed by conjugacy classes of minimal sections of G, i.e. pairs (T,S) of subgroups of G, where S is a normal subgroup of T contained in the Frattini subgroup of T. These idempotents are orthogonal, and their sum is equal to the identity. It follows that for any biset functor F over R, the evaluation F (G) splits as a direct sum of specific R-modules indexed by minimal sections of G, up to conjugation. The restriction of these constructions to the biset category of p-groups, where p is a prime number invertible in R, leads to a decomposition of the category of p-biset functors over R as a direct product of categories F\_L indexed by atoric p-groups L up to isomorphism. We next introduce the notions of L-enriched biset and L-enriched biset functor for an arbitrary finite group L, and show that for an atoric p-group L, the category F\_L is equivalent to the category of L-enriched biset functors defined over elementary abelian p-groups. Finally, the notion of vertex of an indecomposable p-biset functor is introduced (when p is invertible in R), and, when R is a field of characteristic different from p, the objects of the category F\_L are characterized in terms of vertices of their composition factors. 
Serge Bouc (LAMFA)
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