The Modal mu-calculus Alternation Hierarchy is Strict

@inproceedings{Bradfield1996TheMM,
  title={The Modal mu-calculus Alternation Hierarchy is Strict},
  author={Julian Bradfield},
  booktitle={CONCUR},
  year={1996}
}
One of the open questions about the modal mu-calculus is whether the alternation hierarchy collapses; that is, whether all modal fixpoint properties can be expressed with only a few alternations of least and greatest fixpoints. In this paper, we resolve this question by showing that the hierarchy does not collapse. @ 1998 Elsevier Science B.V. All rights reserved 

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