KRISTENSEN, L. On Secondary Cohomology Operations. MATHEMATICA SCANDINAVICA, [S. l.], v. 12, p. 57–82, 1963. DOI: 10.7146/math.scand.a-10673. Disponível em: https://www.mscand.dk/article/view/10673. Acesso em: 17 jan. 2022.