Lino, J., and G. Torres. “Theoretical Study of $e^-$ - He Scattering Using the Schwinger Variational Principle With Plane Waves As a Trial Basis Set”. Revista Mexicana De Física, vol. 50, no. 5, Jan. 2004, pp. 427-0, https://rmf.smf.mx/ojs/index.php/rmf/article/view/3297.