Fotakis, D. A., S. E. Nikoletseas, V. G. Papadopoulou, and P. G. Spirakis. 2012. “HARDNESS RESULTS AND EFFICIENT APPROXIMATIONS FOR FREQUENCY ASSIGNMENT PROBLEMS: RADIO LABELLING AND RADIO COLORING”. Computing and Informatics 20 (2):121-80. http://147.213.75.17/ojs/index.php/cai/article/view/514.