Jianer Chen Yang Liu 0002 Songjian Lu Directed Feedback Vertex Set Problem is FPT. 2007 Structure Theory and FPT Algorithmics for Graphs, Digraphs and Hypergraphs http://drops.dagstuhl.de/opus/volltexte/2007/1233 conf/dagstuhl/2007P7281 db/conf/dagstuhl/P7281.html#ChenLL07