BERGENDAL, Gunnar. On spectral functions belonging to an elliptic differential operator with variable coefficients. Mathematica Scandinavica, Berkeley, CA, United States, v. 5, p. 241–254, 1957. DOI: 10.7146/math.scand.a-10501. Disponível em: https://journals.msp.org/mscand/article/view/2826. Acesso em: 25 jul. 2026.