IBRAHIM, S. E.-S. . ON THE ESSENTIAL SPECTRA OF GENERAL DIFFERENTIAL OPERATORS. Tamkang Journal of Mathematics, [S. l.], v. 30, n. 2, p. 105–126, 1999. DOI: 10.5556/j.tkjm.30.1999.4213. Disponível em: https://journals.math.tku.edu.tw/index.php/TKJM/article/view/4213. Acesso em: 18 may. 2024.