Families without Friedberg but with positive numberings in the Ershov hierarchy. Вестник КазНУ. Серия математика, механика, информатика, [S. l.], v. 69, n. 2, p. 34–38, 2011. Disponível em: https://bm.kaznu.kz/index.php/kaznu/article/view/192. Acesso em: 5 июл. 2026.