OSPANOV, K. Discreteness and estimates of spectrum of a first order difference operator. Eurasian Mathematical Journal, [S. l.], v. 9, n. 2, p. 89–94, 2018. DOI: 10.32523/2077-9879-2018-9-2-89-94. Disponível em: https://emj.enu.kz/index.php/main/article/view/119. Acesso em: 21 oct. 2026.