Herz-Schur multipliers of dynamical systems

We extend the notion of a Herz-Schur multiplier to the setting of non-commutative dynamical systems: given a C*-algebra A, a locally compact group G, and an action α of G on A, we define transformations on the reduced crossed product of A by α which, in the case A=C, reduce to the classical Herz-Schur multipliers. We introduce Schur A-multipliers, establish a characterisation that generalises the classical descriptions of Schur multipliers, and present a transference theorem in the new setting, identifying isometrically the Herz-Schur multipliers of the dynamical system (A,G,α) with the invariant part of the Schur A-multipliers. We discuss special classes of Herz-Schur multipliers, in particular, those associated to a locally compact abelian group G and its canonical action on the C*-algebra C⁎(Γ) of the dual group Γ.