1. D. Bauer, H.J. Broersma, H.J. Veldman, Not every 2-tough graph is hamiltonian, preprint 1997.
2. Graph Theory with Applications;Bondy,1976
3. Tough graphs and hamiltonian circuits;Chvátal;Discrete Math.,1973
4. Toughness and the existence of k-factors;Enomote;J. Graph Theory,1985
5. A. Kaneko, personal communication, 1998.