| Abstract | Around1970 it was discovered that measure preserving actions of Z2 on probability spaces can have remarkably differentproperties from Z-actions, i.e. from actions of single transformations. Many of these differences can be describedas ‘rigidity phenomena’ and manifest themselves in the scarcity of certain objects for Z2-actions which are abundant for Z-actions: they may have unexpectedly few invariant probability measures, invariant sets, isomorphisms or automorphisms. |
