CHRISTENSEN, Erik; IVAN, Cristina. Sums of two-dimensional spectral triples. Mathematica Scandinavica, Berkeley, CA, United States, v. 100, n. 1, p. 35–60, 2007. DOI: 10.7146/math.scand.a-15015. Disponível em: https://journals.msp.org/mscand/article/view/682. Acesso em: 24 jul. 2026.