Positive Herz-Schur multipliers and approximation properties of crossed products

Journal article, 2018

For a C*-algebra A and a set X we give a Stinespring-type characterisation of the completely positive Schur A -multipliers on K(ℓ^2(X))⊗A . We then relate them to completely positive Herz-Schur multipliers on C ∗ -algebraic crossed products of the form A⋊ α,r G , with G a discrete group, whose various versions were considered earlier by Anantharaman-Delaroche, Bedos and Conti, and Dong and Ruan. The latter maps are shown to implement approximation properties, such as nuclearity or the Haagerup property, for A⋊ α,r G .