Strong Edge Coloring of Outerplane Graphs with Independent Crossings

The strong chromatic index of a graph is the minimum number of colors needed in a proper edge coloring so that no edge is adjacent to two edges of the same color. An outerplane graph with independent crossings is a graph embedded in the plane in such a way that all vertices are on the outer face and two pairs of crossing edges share no common end vertex. It is proved that every outerplane graph with independent crossings and maximum degree Δ has strong chromatic index at most 4Δ − 6 if Δ ≥ 4, and at most 8 if Δ ≤ 3. Both bounds are sharp.