  For any finite group G, we define a bivariant functor from the Dress category
of finite G-sets to the conjugation biset category, whose objects are subgroups
of G, and whose morphisms are generated by certain bifree bisets. Any additive
functor from the conjugation biset category to abelian groups yields a Mackey
functor by composition. We characterize the Mackey functors which arise in this
way.

Group Theory;Algebraic Topology