R. Balakrishnan and K. Ranganathan (2012). A textbook of graph theory. Second Edition, Universitext. Springer, New York.
F. Okamoto, E. Salehi and P. Zhang (2010). A checkerboard problem and modular colorings of graphs. Bull. Inst. Combin. Appl.. 58, 29-47
F. Okamoto, E. Salehi and P. Zhang (2010). A solution to the checkerboard problem. Int. J. Comput. Appl. Math.. 5, 447-458