KALIMBETOV, B.T.; ESKARAEVA, B.I. Contrast structure in equations with zero spectrum of limit operator and irreversible spectral value of the kernel. Bulletin of the Karaganda University. Mathematics Series, [S. l.], v. 78, n. 2, p. 56–64, 2015. Disponível em: http://mts.buketov.edu.kz/index.php/mathematics-vestnik/article/view/24. Acesso em: 7 sep. 2025.