1. M. Nicolesco, Les fonctions polyharmoniques. Exposés sur la théorie des fonctions. IV [in French], Hermann & Cie, Paris (1936).
2. F. Gazzola, H. C. Grunau, and G. Sweers, Polyharmonic Boundary Value Problems. Positivity Preserving and Nonlinear Higher Order Elliptic Equations in Bounded Domains, Springer, Berlin (2010).
3. A. V. Bitsadze, “On some properties of polyharmonic functions” [in Russian], Differents. Uravn. 24, No. 5, 825–831 (1988).
4. V. V. Karachik, “Solvability conditions for the Neumann problem for the homogeneous polyharmonic equation,” Differ. Equ. 50, No. 11, 1449–1456 (2014).
5. B. Kh. Turmetov and R. R. Ashurov, “On solvability of the Neumann boundary value problem for a non-homogeneous polyharmonic equation in a ball,” Bound. Value Probl. 2013, Paper No. 162 (2013).