S. G. Krein, “The behavior of solutions of elliptic problems when the region is varied,” Studia Mathematica,31, 411–428 (1968).
Zh.-L. Lions and É. Madzhenes, Homogeneous Boundary Value Problems and Their Applications [Russian translation], Mir, Moscow (1971).
S. Agmon, A. Duglis, and L. Nirenberg, Bounds near the Boundary for the Solutions of Elliptic Partial Differential Equations for General Boundary Conditions [Russian translation], Vol. 1, IL, Moscow (1962).
V. M. Babich, “The problem of the extension of functions,” Uspekhi Matem. Nauk,8, No. 2 (1953).