DOWNEY, R.; GREENBERG, N.; HAMMATT, E. A Hierarchy of Computably Enumerable Degrees II: Some Recent Developments and New Directions. New Zealand Journal of Mathematics, [S. l.], v. 52, p. 175–231, 2021. DOI: 10.53733/133. Disponível em: https://nzjmath.org/index.php/NZJMATH/article/view/133. Acesso em: 8 dec. 2021.