Rolf Bardeli Michael Clausen Andreas Ribbrock A covering problem that is easy for trees but <i>I</i>-complete for trivalent graphs. 2855-2866 2008 156 Discrete Applied Mathematics 15 http://dx.doi.org/10.1016/j.dam.2007.11.021 db/journals/dam/dam156.html#BardeliCR08