On-line coloring and cliques covering for KK s,t -free graphs

The problem of on-line coloring of an arbitrary graphs is known to be hard. Here we consider the problem of on-line coloring in the simplified situation where the input graph is KK_s,t-free. We show that the on-line coloring algorithm with the First Fit strategy of proposed by Capponi and Pilotto in 1] is the best one for KK_1,t-free graphs (t≥3). A.Capponi and C.Pilotto have shown that this algorithm achieves a competitive ratio of t−1 while we show that it is the best possible. However for the family of KK_s,t-free graphs (s≥2, t≥2) there exists no on-line coloring algorithm with a competitive function. The problem of an on-line cliques covering for these families is hard. There exists no on-line cliques covering algorithm with a competitive function for the family of KK_s,t-free graphs (s≥ 1, t≥3). The additional assumption that the input graph is given in a connected way does not help a lot and does not change our results described above.