Kazuo Iwama Suguru Tamaki The Complexity of the Hajos Calculus for Planar Graphs. 2008 15 Electronic Colloquium on Computational Complexity (ECCC) http://eccc.hpi-web.de/eccc-reports/2008/TR08-011/index.html 011 db/journals/eccc/eccc15.html#IwamaT08