Il'kiv, V. S., and B. I. Ptashnik. “Representation and Investigation of Solutions of a Nonlocal Boundary-Value Problem for a System of Partial Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal, vol. 48, no. 2, Feb. 1996, pp. 184-9, https://umj.imath.kiev.ua/index.php/umj/article/view/5346.