Kramers-Kronig relations and sum rules of negative refractive index media K.-E. Peiponen, V. Lucarini, E. M. Vartiainen and J. J. Saarinen Eur. Phys. J. B, 41 1 (2004) 61-65 Published online: 30 September 2004 DOI: 10.1140/epjb/e2004-00294-6