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On graphs in which any pair of colour classes but one induces a tree. (English) Zbl 0874.05023
For $$m\geq 3$$, let $${\mathcal F}_m$$ be the family of graphs $$G$$ that possesses an independent set partition $$\{A_1,\ldots,A_m\}$$ such that the subgraphs of $$G$$ induced by $$A_i\cup A_j$$ are trees except one, which is a forest having two components. In this paper it is shown that for each $$G$$ of order $$n$$ in $${\mathcal F}_m$$, $$t(G)\leq f(n,m)=\frac{1}{3}(3n-2m) {{m-1} \choose {2}}-m+2$$, where $$t(G)$$ denotes the number of triangles in $$G$$. Let $$\rho(G)=f(n,m)-t(G)$$. The graphs in $${\mathcal F}_m$$ with $$\rho(G)=0$$ and the graphs in $${\mathcal F}_3$$ with $$\rho(G)=1$$ are characterized. By applying the first characterization, it is deduced that a graph $$G$$ of order $$n\geq m$$ is in $${\mathcal F}_m$$ with $$\rho(G)=0$$ iff its chromatic polynomial is given by $$\lambda(\lambda-1)\cdots(\lambda-m+3)(\lambda-m+2)^2(\lambda-m+1)^{n-m}$$. By applying the second characterization it is shown that the graphs obtained from the wheels of even order by deleting two consecutive spokes are uniquely determined by their chromatic polynomials, which solves partially Problem 4 in K. M. Koh and K. L. Teo [Graphs Comb. 5, No. 3, 259-285 (1990; Zbl 0727.05023)].

##### MSC:
 05C15 Coloring of graphs and hypergraphs 05C75 Structural characterization of families of graphs 05C38 Paths and cycles 05C35 Extremal problems in graph theory
##### Keywords:
tree; chromatic polynomial; wheel; chromatically unique graph
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##### References:
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