LARMORE, L. L.; THOMAS, E. Group Extensions and Principal Fibrations. Mathematica Scandinavica, Berkeley, CA, United States, v. 30, p. 227–248, 1972. DOI: 10.7146/math.scand.a-11078. Disponível em: https://journals.msp.org/mscand/article/view/2248. Acesso em: 25 jul. 2026.