PEETRE, Jaak; RUS, Ioan A. Sur la positivité de la fonction de Green. Mathematica Scandinavica, Berkeley, CA, United States, v. 21, p. 80–89, 1967. DOI: 10.7146/math.scand.a-10847. Disponível em: https://journals.msp.org/mscand/article/view/2476. Acesso em: 25 jul. 2026.