IP-rigidity and eigenvalue groups
  We examine the class of increasing sequences of natural numbers which are IP-rigidity sequences for some weakly mixing probability preserving transformation. This property is closely related to the uncountability of the eigenvalue group of a corresponding non-singular transformation. We give examples, including a super-lacunary sequence which is not IP-rigid. 
Jon. Aaronson, Maryam Hosseini and Mariusz Lemanczyk
math.DS