KOSTENKO, A. S. A spectral analysis of some indefinite differential operators. Methods of Functional Analysis and Topology, [S. l.], v. 12, n. 2, p. 157–169, 2006. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/310. Acesso em: 16 dec. 2025.