Moshinsk, M., G. Lo, ola., ., and C. Villegas. “Comparison Between Perturbative and Exact Transitions Induced by an Interaction”. Revista Mexicana De Física, vol. 37, no. 2, Jan. 1990, pp. 369-81, https://rmf.smf.mx/ojs/index.php/rmf/article/view/2185.