Kramers-Kronig relations and sum rules of negative refractive index mediaK.-E. Peiponen, V. Lucarini, E. M. Vartiainen and J. J. SaarinenEur. Phys. J. B, 41 1 (2004) 61-65DOI: https://doi.org/10.1140/epjb/e2004-00294-6