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Let G be a simple undirected graph.

I found some examples of connected graphs G with line graphs containing no hamilton cycle, but none of them was $2$-connected.

  • Are there $2$-connected graphs with a line graph containing no hamilton-cycle ?
  • If yes, what is the smallest ? (Due to my search, it should have more than $7$ vertices)
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  • $\begingroup$ Since line graphs tend to have a hamilton-cycle, I wonder if there is a classification of graphs with a line graph without a hamilton cycle. $\endgroup$ – Peter Oct 2 '14 at 10:01
  • $\begingroup$ Have you checked this paper sciencedirect.com/science/article/pii/S0893965912001036 dealing with a conjecture of Thomassen, stating that every $4$-connected line graph is hamiltonian? Its not really what you're asking for but it may offer some insight. $\endgroup$ – Jernej Oct 2 '14 at 10:09
  • $\begingroup$ I did not read the paper, but I noticed the conjecture. $\endgroup$ – Peter Oct 2 '14 at 10:11
  • $\begingroup$ Is the line graph of a $2$-connected graph always $2$-connected ? $\endgroup$ – Peter Oct 2 '14 at 10:13
  • $\begingroup$ Hm.. what is surely true is that if $G$ is $2$-edge connected then $L(G)$ is $2$-connected. This may give a clue for counterexamples. $\endgroup$ – Jernej Oct 2 '14 at 10:19
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Here is a counterexample on (incidentally) 8 vertices. I guess this can be generalized.enter image description here

The idea is to only look for $2$-connected non-hamiltonian graphs since the line graph of a hamiltonian graph is always hamiltonian.

A natural question now is whether one can find a different kind of such graphs (not Theta graphs)

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  • $\begingroup$ The line graph of this graph is from the type I am particular interested and for which I know no name. I call them "strange graphs". Every vertex is start vertex for some hamiltonian path, but the graph contains no hamilton cycle. The line graphs might help to classify those graphs. $\endgroup$ – Peter Oct 2 '14 at 10:21
  • $\begingroup$ Did you check that it is the least example ? $\endgroup$ – Peter Oct 2 '14 at 11:14
  • $\begingroup$ @Peter Yes it is the least example. $\endgroup$ – Jernej Oct 2 '14 at 11:44

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