Zevin, A. A. “Estimates of Eigenvalues of Self-Adjoint Boundary-Value Problems With Periodic Coefficients”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 50, no. 5, May 1998, pp. 632–638,
https://umj.imath.kiev.ua/index.php/umj/article/view/4901
.