Hans L. Bodlaender Thomas Wolle Arie M. C. A. Koster Contraction and Treewidth Lower Bounds. 5-49 2006 10 J. Graph Algorithms Appl. 1 http://jgaa.info/accepted/2006/BodlaenderWolleKoster2006.10.1.pdf db/journals/jgaa/jgaa10.html#BodlaenderWK06