Zvereva, M. B., Pokornyi, Y. V., & Shabrov, S. A. (2008). On extension of the Sturm-Liouville oscillation theory to problems with pulse parameters.
Ukrains’kyi Matematychnyi Zhurnal,
60(1), 95–99.
https://umj.imath.kiev.ua/index.php/umj/article/view/3140