We study the existence of solutions of the system
 N. A. Asif, I. Talib: Existence of Solutions to a Second Order Coupled System with Nonlinear Coupled Boundary Conditions. American Journal of Applied Mathematics, Special Issue: Proceedings of the 1st UMT National Conference on Pure and Applied Mathematics (1st UNCPAM 2015) 3 (3-1) (2015) 54-59.
 N. A. Asif, I. Talib, C. Tunc: Existence of solution for first-order coupled system with nonlinear coupled boundary conditions. Boundary Value Problems 2015 (1) (2015) 134.
 C. Bereanu, J. Mawhin: Nonhomogeneous boundary value problems for some nonlinear equations with singular ∅-Laplacian. J. Math. Anal. Appl. 352 (2009) 218-233.
 P.G. Bergmann: Introduction to the Theory of Relativity. Dover Publications, New York (1976).
 H. Brezis, J. Mawhin: Periodic solutions of the forced relativistic pendulum. Differential Integral Equations 23 (2010) 801-810.
 D. Franco, D. O'Regan: Existence of solutions to second order problems with nonlinear boundary conditions. In: Proceedings of the Fourth International Conference on Dynamical Systems and Differential Equations. (2002) 273-280.
 D. Franco, D. O'Regan: A new upper and lower solutions approach for second order problems with nonlinear boundary conditions. Arch. Inequal. Appl. 1 (2003) 423-430.
 D. Franco, D. O'Regan, J. Perán: Fourth-order problems with nonlinear boundary conditions. Journal of Computational and Applied Mathematics 174 (2005) 315-327.