MATTILA, K. On proper boundary points of the spectrum and complemented eigenspaces. MATHEMATICA SCANDINAVICA, [S. l.], v. 43, p. 363–368, 1978. DOI: 10.7146/math.scand.a-11790. Disponível em: https://www.mscand.dk/article/view/11790. Acesso em: 3 may. 2024.