KRISTENSEN, Leif. On Secondary Cohomology Operations. Mathematica Scandinavica, Berkeley, CA, United States, v. 12, p. 57–82, 1963. DOI: 10.7146/math.scand.a-10673. Disponível em: https://journals.msp.org/mscand/article/view/2650. Acesso em: 25 jul. 2026.