Relative discrete spectrum of W*-dynamical systems

A definition of relative discrete spectrum of noncommutative W*-dynamical systems is given in terms of the basic construction of von Neumann algebras, motivated from three perspectives: Firstly, as a complementary concept to relative weak mixing of W*-dynamical systems. Secondly, by comparison with the classical (i.e. commutative) case. And, thirdly, by noncommutative examples.