I. Althöfer, G. Das, D. Dobkin, D. Joseph, and J. Soares, On sparse spanners of weighted graphs, Discrete & Computational Geometry, vol.6, issue.1, pp.81-100, 1993.
DOI : 10.1007/BF02189308

L. Cai, NP-completeness of minimum spanner problems, Discrete Applied Mathematics, vol.48, issue.2, pp.187-194, 1994.
DOI : 10.1016/0166-218X(94)90073-6

L. Cai and D. G. , Tree Spanners, SIAM Journal on Discrete Mathematics, vol.8, issue.3, pp.359-387, 1995.
DOI : 10.1137/S0895480192237403

S. L. Hakimi and S. Yau, Distance matrix of a graph and its realizability, Quarterly of Applied Mathematics, vol.22, issue.4, pp.305-317, 1964.
DOI : 10.1090/qam/184873

G. Kortsarz, On the hardness of approximating spanners. to appear at APPROX'98, 1998.

S. Khanna and R. Motwani, Towards a syntactic characterization of PTAS, Proceedings of the twenty-eighth annual ACM symposium on Theory of computing , STOC '96, pp.329-337, 1996.
DOI : 10.1145/237814.237979

G. Kortsarz and D. Peleg, Generating sparse 2?spanners, Proceedings 3rd Scandinavian Workshop on Algorithm Theory, SWAT'92, pp.301-309, 1992.

A. Mansfield, Determining the thickness of graphs is NP-hard, Math. Proc. Camb. Phil
DOI : 10.1137/0210055

H. Christos, Papadimitriou and Mihalis Yannakakis. Optimization, approximation, and complexity classes, Journal of Computer and System Sciences, vol.43, pp.425-440, 1991.

D. Peleg and A. A. Schäffer, Graph spanners, Journal of Graph Theory, vol.5, issue.1, pp.99-116, 1989.
DOI : 10.1002/jgt.3190130114

D. Peleg and J. D. Ullman, An optimal synchronizer for the hypercube, Proceedings 1987 6th ACM Symposium on Principles of Dist. Comp., Vancouver, pp.77-85, 1987.

J. Soares, Graph spanners: a survey, Congressus Numerantium, vol.89, pp.225-238, 1992.

G. Venkatesan, U. Rotics, M. S. Madanlal, J. A. Makowsky, and C. Pandu-rangan, Restrictions of Minimum Spanner Problems, Information and Computation, vol.136, issue.2, pp.143-164, 1997.
DOI : 10.1006/inco.1997.2641