| [1] | L. Cesari, R. Kannan. Solutions of nonlinear hyperbolic equations at resonace. Nonlinear Analysis, 1982, 6(8): 751-805. | 
                     
                                
                                    
                                        | [2] | M. Schechter. Bounded resonance problems for semilinear elliptic equations. Nonlinear Analysis, 1995, 24(10): 1471-1482. | 
                     
                                
                                    
                                        | [3] | S. Solimini. On the solvability of some elliptic partial differential equations with the linear part at resonance. Journal of Mathematical Analy- sis and Applications, 1986, 117: 138-152. | 
                     
                                
                                    
                                        | [4] | M. Willem. Periodic solutions of wave equations with jumping nonlinearities. Journal of Differential Equations, 1980, 36(1): 20-27. | 
                     
                                
                                    
                                        | [5] | J. Berkovits, V. Mustonen. On nonresonance for systems of semilinear wave equations. Nonlinear Analysis, 1997, 29(6): 627-638. | 
                     
                                
                                    
                                        | [6] | S. Rybicki. Periodic solutions of vibrating strings. Degree theory approach. Annali di Matematica Pura ed Applicata, 2001, 179(1): 197-214. | 
                     
                                
                                    
                                        | [7] | J. M. Coron. Periodic solutions of a nonlinear wave equation without assumption of monotonicity. Mathematische Annalen, 1983, 262(2): 273-285. | 
                     
                                
                                    
                                        | [8] | P. L. Felmer, R. F. Manasevich. Periodic solutions of a coupled system of telegraph-wave equations. Journal of Mathematical Analy- sis and Applications, 1986, 116(1): 10-21. | 
                     
                                
                                    
                                        | [9] | R. Iannacci, M. N. Nkashama. Nonlinear boundary value problems at resonance. Nonlinear Analysis, 1987, 11(4): 455-473. | 
                     
                                
                                    
                                        | [10] | R. Iannacci, M. N. Nkashama. Nonlinear elliptic partial differential equations at resonance: Higher eigenvalues. Nonlinear Analysis, 1995, 25(5): 455-471. | 
                     
                                
                                    
                                        | [11] | M. R. Grossinho, M. N. Nkashama. Periodic solutions of parabolic and telegraph equations with asymmetric nonlinearities. Nonlinear Analysis, 1998, 33(2): 187-210. | 
                     
                                
                                    
                                        | [12] | J. K. Kim, N. H. Pavel. Existence and regularity of weak periodic solutions of the 2-D wave equation. Nonlinear Analysis, 1998, 32(7): 861-870. | 
                     
                                
                                    
                                        | [13] | J. K. Kim, N. H. Pavel. $L^\infty$-optimal control of the periodic 1-D wave equation with x-dependent coefficients. Nonlinear Analysis, 1998, 33(1): 25-39. | 
                     
                                
                                    
                                        | [14] | V. Barbu, N. H. Pavel. Periodic solutions to nonlinear one dimensional wave equation with x-dependent coefficients. Transactions of the Am- erican Mathematical Society, 1997, 349(5): 2035-2048. | 
                     
                                
                                    
                                        | [15] | J. Mawhin. Compacite, monotonie et convexite dans l’Etude des problems aux limites semi-lineaires. Quebec: Universite de Sherbrooke, 1981. |