The Circular Chromatic Number of the Mycielskian of Mt(Kn)

As a natural generalization of chromatic number of a graph, the circular chromatic number of graphs (or the star chromatic number) was introduced by A.Vince in 1988. Let $M^t(G)$ denote the $t$th iterated Mycielski graph of $G$. It was conjectured by Chang, Huang and Zhu(Discrete mathematics,205(1999), 23-37) that for all $n\ge t+2,{\chi }_c(M^t(K_n))=\chi (M^t(K_n))=n+t\mathrm{.}$ In 2004, D.D.F. Liu proved the conjecture when $t\ge 2$, $n\ge 2^{t-1}+2t-2$. In this paper,we show that the result can be strengthened to the following: if $t\ge 4$,$n\ge {11/12}2^{t-1}+2t+{1/3}$,then$\chi_c(M^t(K_n))=\chi(M^t(K_n))$.