# Strong edge colouring conjecture

A strong edge-colouring of a graph is a edge-colouring in which every colour class is an induced matching; that is, any two vertices belonging to distinct edges with the same colour are not adjacent. The strong chromatic index is the minimum number of colours in a strong edge-colouring of .

**Conjecture**

The conjectured bounds would be sharp. When is even, expanding each vertex of a -cycle into a stable set of size yields such a graph with edges in which the largest induced matching has size . A similar construction achieves the bound when is odd.

Greedy colouring the edges yields . Using probabilistic methods, Molloy and Reed~[MoRe97] proved that there is a positive constant such that, for sufficiently large , every graph with maximum degree has strong chromatic index at most .

The greedy bound proves the conjecture for . For , the conjectured bound of 10 was proved independently by Hor\'ak, He, and Trotter[HHT] and by Andersen [A]. For , the conjectured bound is 20, and Cranston [C] proved that 22 colours suffice.

For a bipartite graph , Faudree et al. [FGST] conjectured that . This is implied by the stronger conjecture due to Kaiser.

**Conjecture**Let be a bipartite graph such that every vertex in has degree at most and every vertex in has degree at most . Then .

## Bibliography

[A] L. D. Andersen. The strong chromatic index of a cubic graph is at most 10. Discrete Math., 108(1-3):231--252, 1992.

[C] D. W. Cranston. Strong edge-coloring of graphs with maximum degree 4 using 22 colors. Discrete Math., 306(21):2772--2778, 2006.

[FGST] R. J. Faudree, A. Gyárfás, R. H. Schelp, and Zs. Tuza. Induced matchings in bipartite graphs. Discrete Math., 78(1-2):83--87, 1989.

[HHT] P. Horák, Q. He, and W. T. Trotter. Induced matchings in cubic graphs. J. Graph Theory, 17(2):151--160, 1993.

* indicates original appearance(s) of problem.