= Nilpotent space
A connected space is nilpotent when its fundamental group is nilpotent and its action on each higher homotopy group is nilpotent: the module has a finite filtration whose successive quotients have trivial group action. Simply connected spaces satisfy these conditions automatically. They are the natural setting for localization and rationalization, since fibrewise coefficient localization is compatible with the successive untwisted stages of such a filtration.
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