BALSLEV, Erik. The Singular Spectrum of Elliptic Differential Operators in $L^p (R_n)$. MATHEMATICA SCANDINAVICA, [S. l.], v. 19, p. 193–210, 1966. DOI: 10.7146/math.scand.a-10810. Disponível em: https://tidsskrift.dk/math/article/view/10810. Acesso em: 14 aug. 2026.