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# Article

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Keywords:
traceable number; upper traceable number; Hamiltonian number
Summary:
For a nontrivial connected graph $G$ of order $n$ and a linear ordering $s\: v_1, v_2, \ldots , v_n$ of vertices of $G$, define $d(s) = \sum _{i=1}^{n-1} d(v_i, v_{i+1})$. The traceable number $t(G)$ of a graph $G$ is $t(G) = \min \lbrace d(s)\rbrace$ and the upper traceable number $t^+(G)$ of $G$ is $t^+(G) = \max \lbrace d(s)\rbrace ,$ where the minimum and maximum are taken over all linear orderings $s$ of vertices of $G$. We study upper traceable numbers of several classes of graphs and the relationship between the traceable number and upper traceable number of a graph. All connected graphs $G$ for which $t^+(G)- t(G) = 1$ are characterized and a formula for the upper traceable number of a tree is established.
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