Baranetskij, Y., Kalenyuk, P., Kopach, M., & Solomko, A. (2019). The nonlocal boundary value problem with perturbations of mixed boundary conditions for an elliptic equation with constant coefficients. I. Carpathian Mathematical Publications, 11(2), 228–239. https://doi.org/10.15330/cmp.11.2.228-239