Originalversjon
Positivity (Dordrecht). 2017, 21 (4), 1625-1646, DOI: http://dx.doi.org/10.1007/s11117-017-0490-0
Sammendrag
Given an action α of a discrete group G on a unital C∗ -algebra A, we introduce a natural concept of α -negative definiteness for functions from G to A, and examine some of the first consequences of such a notion. In particular, we prove analogs of theorems due to Delorme–Guichardet and Schoenberg in the classical case where A is trivial. We also give a characterization of the Haagerup property for the action α when G is countable.