turkmath.org

A group functor L is a structure which, among other features, associates a finite group G with an abelian group L(G). For example, L(G) might be the ordinary character ring or Burnside ring or cohomology ring of G. In an old version of the theory pioneered by James Green, advantage was made of the fact that, in many applications, L(G) is a ring. Later progress has often ignored that extra structure. We shall comment on some renewed interest in Green's approach, updated to incorporate some more recent developments.