ZEVIN, A. A. Estimates of eigenvalues of self-adjoint boundary-value problems with periodic coefficients. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 50, n. 5, p. 632–638, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4901. Acesso em: 14 apr. 2025.