Klyus, I. S., and B. I. Ptashnik. “A Multipoint Problem for Pseudodifferential Equations”. Ukrains’kyi Matematychnyi Zhurnal, vol. 55, no. 1, Jan. 2003, pp. 22-29, https://umj.imath.kiev.ua/index.php/umj/article/view/3884.