  We study equivariant linear maps between finite-dimensional matrix algebras,
as introduced by Bhat. These maps satisfy an algebraic property which makes it
easy to study their positivity or k-positivity. They are therefore particularly
suitable for applications to entanglement detection in quantum information
theory. We characterize their Choi matrices. In particular, we focus on a
subfamily that we call (a, b)-unitarily equivariant. They can be seen as both a
generalization of maps invariant under unitary conjugation as studied by Bhat
and as a generalization of the equivariant maps studied by Collins et al. Using
representation theory, we fully compute them and study their graphical
representation, and show that they are basically enough to study all
equivariant maps. We finally apply them to the problem of entanglement
detection and prove that they form a sufficient (infinite) family of positive
maps to detect all k-entangled density matrices.
