Vlasii, O. D., and B. I. Ptashnik. “A Problem With Nonlocal Conditions for Partial Differential Equations With Variable Coefficients”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 53, no. 10, Oct. 2001, pp. 1328-36, https://umj.imath.kiev.ua/index.php/umj/article/view/4352.