Philipp Weis Neil Immerman Structure Theorem and Strict Alternation Hierarchy for FO<sup>2</sup> on Words. 2006 Circuits, Logic, and Games http://drops.dagstuhl.de/opus/volltexte/2007/975 conf/dagstuhl/2006P6451 db/conf/dagstuhl/P6451.html#WeisI06