Erik D. Demaine Mohammad Taghi Hajiaghayi Dimitrios M. Thilikos -Approximation for Treewidth of Graphs Excluding a Graph with One Crossing as a Minor. 67-80 2002 conf/approx/2002 APPROX http://link.springer.de/link/service/series/0558/bibs/2462/24620067.htm db/conf/approx/approx2002.html#DemaineHT02