see Equations (5) – (7) of . Clearly, the connection between the coordinates, the metric components and the topology is more complicated in the case of S3 and S2 × S1 than in the case of T3. We shall not go into a detailed discussion of these issues here, but rather refer the interested reader to  and  for a further discussion. A brief explanation of the origin of the name (in terms of polarizations of gravitational waves) as well as a different characterization is provided at the bottom of [70, p. 65].
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