Motivated by the classical results by Halmos and Rokhlin on the genericity of
weakly but not strongly mixing transformations and the Furstenberg tower
construction, we show that weakly but not strongly mixing extensions on a fixed
product space with both measures non-atomic are generic. In particular, a
generic extension does not have an intermediate nilfactor.Comment: 29 pages; Lemma 7 strengthened and given a new proof; Former Lemma 6
removed; Former Lemma 8 is now Lemma 6, with a slight reformulation; Typos
fixe