Abstract

In this paper we investigate two conjectures proposed in (Graphs Combin. 13 (1997) 305–314). The first one is uniquely totally colorable (UTC) conjecture which states: Empty graphs, paths, and cycles of order 3 k, k a natural number, are the only UTC graphs. We show that if G is a UTC graph of order n, then Δ⩽ n/2+1, where Δ is the maximum degree of G. Also there is another question about UTC graphs that appeared in (Graphs Combin. 13 (1997) 305–314) as follows: If a graph G is UTC, is it true that in the proper total coloring of G, each color is used for at least one vertex? We prove that if G is a UTC graph of order n and in the proper total coloring of G, there exists a color which did not appear in any vertex of G, then G is a Δ-regular graph and n/2⩽ Δ⩽ n/2+1.

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