Zevin, A.A. (1998) “Estimates of eigenvalues of self-adjoint boundary-value problems with periodic coefficients”, Ukrains’kyi Matematychnyi Zhurnal, 50(5), pp. 632–638. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/4901 (Accessed: 14 April 2025).