Aznabayev, D., A. Bekbaev, I. Ishmukhamedov, V. Korobov, and A. Turmaganbet. “Nonrelativistic Ionization Energy Levels of a Helium Atom”. Physical Sciences and Technology, vol. 2, no. 1, Oct. 2016, https://phst.kaznu.kz/index.php/journal/article/view/63.