Characteristic Formulae for Relations with Nested Fixed Points
  A general framework for the connection between characteristic formulae and behavioral semantics is described in [2]. This approach does not suitably cover semantics defined by nested fixed points, such as the n-nested simulation semantics for n greater than 2. In this study we address this deficiency and give a description of nested fixed points that extends the approach for single fixed points in an intuitive and comprehensive way. 
Luca Aceto, Anna Ing\'olfsd\'ottir
cs.LO