Hamiltonian paths in iterated line graphs

For integer $n$, the $n$-iterated line graph $L^n(G)$ of an undirected graph $G$ is defined to be $L(L^{n-1}(G))$, where $L^1(G)$ is the line graph $L(G)$ of $G$. In this paper we introduce hamiltonian path index. Hamiltonian path index, denoted by $h_p(G)$, is the minimum number $n$ such that $L^n(G)$ contains a hamiltonian path. We show that hamiltonian path index of $G$ exists for any graph $G$ and we set the exact value of hamiltonian path index for trees and discuss the problem about graphs with hamiltonian 2-connected blocks.