A certain type of functor on a category of coactions of a locally compact group on
-algebras
is introduced and studied. These functors are intended to help in the study of the
crossed-product functors that have been recently introduced in relation to the
Baum–Connes conjecture. The most important coaction functors are the ones
induced by large ideals of the Fourier–Stieltjes algebra. It is left as an open problem
whether the “minimal exact and Morita compatible crossed-product functor” is
induced by a large ideal.