@article{Len-Vul-Haz-1992,
    author={Marc Lenoir and M. Vullierme-Ledard and Christophe Hazard },
    title={Variational formulations for the determination of resonant 
           states in scattering problems },
    journal={SIAM Journal on Mathematical Analysis },
    year={1992 },
    volume={23 },
    pages={579--608},
}