STRELICAS, Š.; DEGUTIS, J. On the method demonstrating existence of eigenvalues for a differential operator with boundary conditions dependent on parameter. Lithuanian Mathematical Journal, [S. l.], v. 11, n. 3, p. 683–690, 1971. DOI: 10.15388/LMJ.1971.21024. Disponível em: https://www.journals.vu.lt/LMJ/article/view/21024.. Acesso em: 1 jul. 2024.