TOMKINSON, M. J. Groups as the union of proper subgroups. MATHEMATICA SCANDINAVICA, [S. l.], v. 81, p. 191–198, 1997. DOI: 10.7146/math.scand.a-12873. Disponível em: https://tidsskrift.dk/math/article/view/12873. Acesso em: 14 aug. 2026.