Hans L. Bodlaender John R. Gilbert Ton Kloks Hjálmtyr Hafsteinsson Approximating Treewidth, Pathwidth, and Minimum Elimination Tree Height. 1-12 1991 conf/wg/1991 WG db/conf/wg/wg91.html#BodlaenderGKH91 http://dx.doi.org/10.1007/3-540-55121-2_1