SOSKOVA, A.; SOSKOV, I. CO-SPECTRA OF JOINT SPECTRA OF STRUCTURES. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 96, p. 35–44, 2004. Disponível em: https://admin.uni-sofia.bg/index.php/fmi/article/view/159. Acesso em: 15 jan. 2025.