Puch, F., and R. Baquero. “Optical Conductivity the Optical Conductivity Resonance from an Exact Description of the Electronic States Around the Fermi Energy”. Revista Mexicana De Física, vol. 52, no. 4, Jan. 2006, pp. 301-0, https://rmf.smf.mx/ojs/index.php/rmf/article/view/3465.