Doron Bustan Orna Kupferman Moshe Y. Vardi A Measured Collapse of the Modal µ-Calculus Alternation Hierarchy. 522-533 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2996&spage=522 2004 conf/stacs/2004 STACS db/conf/stacs/stacs2004.html#BustanKV04