Arvind Gupta Jan van den Heuvel Ján Manuch Ladislav Stacho Xiaohong Zhao On the Complexity of Ordered Colorings. 832-847 2008 22 SIAM J. Discrete Math. 2 http://dx.doi.org/10.1137/060676222 db/journals/siamdm/siamdm22.html#GuptaHMSZ08