Teplins’kyi, O. Y. “Asymptotic Solution of One-Dimensional Spectral Boundary-Value Problems With Rapidly Varying Coefficients: The Case of Multiple Spectra”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 10, Oct. 1998, pp. 1399–1408, https://umj.imath.kiev.ua/index.php/umj/article/view/4825.