Abstract
We introduce an equivariant version of the weak expectation property (WEP) at the level of operator modules over completely contractive Banach algebras [Formula: see text]. We prove a number of general results — for example, a characterization of the [Formula: see text]-WEP in terms of an appropriate [Formula: see text]-injective envelope, and also a characterization of those [Formula: see text] for which [Formula: see text]-WEP implies WEP. In the case of [Formula: see text], we recover the [Formula: see text]-WEP for [Formula: see text]-[Formula: see text]-algebras in recent work of Buss–Echterhoff–Willett [A. Buss, S. Echterhoff and R. Willett, The maximal injective crossed product, to appear in Ergodic Theory Dynam. Systems, https://doi.org/10.1017/etds.2019.25 ]. When [Formula: see text], we obtain a dual notion for operator modules over the Fourier algebra. These dual notions are related in the setting of dynamical systems, where we show that a [Formula: see text]-dynamical system [Formula: see text] with [Formula: see text] injective is amenable if and only if [Formula: see text] is [Formula: see text]-injective if and only if the crossed product [Formula: see text] is [Formula: see text]-injective. Analogously, we show that a [Formula: see text]-dynamical system [Formula: see text] with [Formula: see text] nuclear and [Formula: see text] exact is amenable if and only if [Formula: see text] has the [Formula: see text]-WEP if and only if the reduced crossed product [Formula: see text] has the [Formula: see text]-WEP.
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