Sadish Sadasivam Huaming Zhang NP-Completeness of <i>st</i>-Orientations for Plane Graphs. 298-309 2009 FCT http://dx.doi.org/10.1007/978-3-642-03409-1_27 conf/fct/2009 db/conf/fct/fct2009.html#SadasivamZ09