Algebraic Characterization of Uniquely Vertex Colorable Graphs
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The study of graph vertex colorability from an algebraic perspective has introduced novel techniques and algorithms into the field. For instance, it is known that $k$-colorability of a graph $G$ is equivalent to the condition $1 \in I_{G,k}$ for a certain ideal $I_{G,k} \subseteq \k[x_1, ..., x_n]$. In this paper, we extend this result by proving a general decomposition theorem for $I_{G,k}$. This theorem allows us to give an algebraic characterization of uniquely $k$-colorable graphs. Our results also give algorithms for testing unique colorability. As an application, we verify a counterexample to a conjecture of Xu concerning uniquely 3-colorable graphs without triangles.
15 pages, 2 figures, print version, to appear J. Comb. Th. Ser. B
15 pages, 2 figures, print version, to appear J. Comb. Th. Ser. B