Teplins’kyi, O. Y. (1998). Asymptotic solution of one-dimensional spectral boundary-value problems with rapidly varying coefficients: The case of multiple spectra. Ukrains’kyi Matematychnyi Zhurnal, 50(10), 1399–1408. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/4825