Interval incidence coloring of bipartite graphs

In this paper1 we study the problem of interval incidence coloring of bipartite graphs. We show the upper bound for interval incidence coloring number (χii) for bipartite graphs χii≤2Δ, and we prove that χii=2Δ holds for regular bipartite graphs. We solve this problem for subcubic bipartite graphs, i.e. we fully characterize the subcubic graphs that admit 4, 5 or 6 coloring, and we construct a linear time exact algorithm for subcubic bipartite graphs. We also study the problem for bipartite graphs with Δ=4 and we show that 5-coloring is easy and 6-coloring is hard (NP-complete). Moreover, we construct an O(nΔ3.5logΔ) time optimal algorithm for trees.