From semi-total to equitable total colorings, and their induced partitions into perfect and total perfect codes

Abstract

Independently posed by Behzad and Vizing, the Total Coloring Conjecture asserts that the total chromatic number of a connected simple graph $G$ is either $Δ(G)+1$ or $Δ(G)+2$, where $Δ(G)$ is the largest degree of a vertex of $G$. Deciding whether a cubic graph $G$ has total chromatic number $Δ(G)+1$, even for bipartite cubic graphs, is NP-hard. The problem persists even for equitable total colorings. Williams and Holroyd gave a condition for total colorings via the introduction of semi-total colorings. In the present work, focus is set on obtaining, by means of a variation of Kempe's 1879 graph-coloring algorithm, equitable total colorings out of semi-total colorings of: symmetric cubic graphs, cage graphs and expansions of cubic graphs. It is noted that total colorings of the 3-cube and subsequent polygon prisms exist having perfect codes as their vertex color classes, as well as semi-total colorings of Möbius ladders having total perfect codes as their vertex color classes, only one of which, in $K_{3,3}$, being equitable; in addition, a partition into two perfect-codes and one total-perfect code is given by the vertex color classes of the 90-vertex Foster graph. On the other hand, a semi-total coloring of the 42-vertex $(5,6)$-cage yields its decomposition into three copies of the Heawood graph via its edges of same color endvertices and seven remaining 6-cycles.