Let (G, P) be a quasi-lattice ordered group. In [2] we constructed a universal covariant representation (A,U) for (G, P) in a way that avoids some of the intricacies of the other approaches in [11] and [8]. Then we showed if (G, P) is amenable, true representations of (G, P) generate C∗-algebras which are canonically isomorphic to the C∗-algebra generated by the universal covariant representation. In this paper, we discuss characterizations of amenability in a comparatively simple and natural way to introduce this formidable property. Amenability of (G, P) can be established by investigating the behavior of ΦU on the range of a positive, faithful, linear map rather than the whole algebra.
A G−graded R−module is called flexible if Mg = RgMe for every g ∈ G. In this paper, we study the relationship between a flexible module and the graded ring R through different aspects. On one hand, we distinguish the flexible modules from other graded modules by characterizing the influence of the e-component of a flexiblemodule on the graded module itself. On the other hand, we extend the class covered by flexible graded modules to include free and projective modules in a comparatively simple manner.
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