Petr A. Golovach, Bernard Lidický, Daniël Paulusma. ::::L::::(2, 1, 1)-Labeling Is NP-Complete for Trees. In Jan Kratochvíl, Angsheng Li, Jirí Fiala, Petr Kolman, editors, Theory and Applications of Models of Computation, 7th Annual Conference, TAMC 2010, Prague, Czech Republic, June 7-11, 2010. Proceedings. Volume 6108 of Lecture Notes in Computer Science, pages 211-221, Springer, 2010. [doi]
Abstract is missing.