BALSLEV, Erik. The Singular Spectrum of Elliptic Differential Operators in $L^p (R_n)$. Mathematica Scandinavica, Berkeley, CA, United States, v. 19, p. 193–210, 1966. DOI: 10.7146/math.scand.a-10810. Disponível em: https://journals.msp.org/mscand/article/view/2532. Acesso em: 25 jul. 2026.