BURENKOV, V.; TUYEN, B. T. On spectral stability problem for a pair of self-adjoint elliptic differential operators on bounded open sets. Eurasian Mathematical Journal, [S. l.], v. 10, n. 3, p. 84–88, 2019. DOI: 10.32523/2077-9879-2019-10-3-84-88. Disponível em: https://emj.enu.kz/index.php/main/article/view/158. Acesso em: 20 oct. 2026.