Baranetskij, Ya.O., et al. “The Nonlocal Boundary Value Problem With Perturbations of Mixed Boundary Conditions for an Elliptic Equation With Constant Coefficients. I”. Carpathian Mathematical Publications, vol. 11, no. 2, Dec. 2019, pp. 228-39, doi:10.15330/cmp.11.2.228-239.