A new vertex coloring heuristic and corresponding chromatic number

One method to obtain a proper vertex coloring of graphs using a reasonable number of colors is to start from any arbitrary proper coloring and then repeat some local re-coloring techniques to reduce the number of color classes. The Grundy (First-Fit) coloring and color-dominating colorings of graphs are two well-known such techniques. The color-dominating colorings are also known and commonly referred as {\rm b}-colorings. But these two topics have been studied separately in graph theory. We introduce a new coloring procedure which combines the strategies of these two techniques and satisfies an additional property. We first prove that the vertices of every graph $G$ can be effectively colored using color classes say $C1, \ldots, Ck$ such that $(i)$ for any two colors $i$ and $j$ with $1\leq i< j \leq k$, any vertex of color $j$ is adjacent to a vertex of color $i$, $(ii)$ there exists a set ${u1, \ldots, uk}$ of vertices of $G$ such that $uj\in Cj$ for any $j\in {1, \ldots, k}$ and $uk$ is adjacent to $uj$ for each $1\leq j \leq k$ with $j\not= k$, and $(iii)$ for each $i$ and $j$ with $i\not= j$, the vertex $uj$ has a neighbor in $Ci$. This provides a new vertex coloring heuristic which improves both Grundy and color-dominating colorings. Denote by $z(G)$ the maximum number of colors used in any proper vertex coloring satisfying the above properties. The $z(G)$ quantifies the worst-case behavior of the heuristic. We prove the existence of ${Gn}{n\geq 1}$ such that $\min {\Gamma(Gn), b(Gn)} \rightarrow \infty$ but $z(Gn)\leq 3$ for each $n$. For each positive integer $t$ we construct a family of finitely many colored graphs ${\mathcal{D}}t$ satisfying the property that if $z(G)\geq t$ for a graph $G$ then $G$ contains an element from ${\mathcal{D}}_t$ as a colored subgraph. This provides an algorithmic method for proving numeric upper bounds for $z(G)$.