| [1] | Fucik S, Mawhin J.Generated periodic solution of nonlinear telegraph equation[J].Nonlinear Anal,1978, 2(5):609-617. doi:  10.1016/0362-546X(78)90008-1 | 
		
				| [2] | Kim W S.Doubly-periodic boundary value problem for nonlinear dissipative hyperbolic equations[J].J Math Appl, 1990,145(1):1-6. | 
		
				| [3] | Kim W S.Multiple doubly periodic solutions of semilinear dissipative hyperbolic equations[J].J Math Anal Appl, 1996,197(2):735-748. doi:  10.1006/jmaa.1996.0049 | 
		
				| [4] | Mawhin J.Periodic solution of nonlinear telegraph equations[A].In:Bedlarek A R,Cesari L,Eds.Dynamical Systems[C].New York:Academic Press,1977. | 
		
				| [5] | Ortega R,Robles-Perez A M.A maximum principle for periodic solutions of the telegraph equations[J]. J Math Anal Appl, 1998,221(2):625-651. doi:  10.1006/jmaa.1998.5921 | 
		
				| [6] | Berkovits J,Mustonuen V.On nonresonance for system of semilinear wave equations[J].Nonlinear Anal, 1997, 29(6):627-638. doi:  10.1016/S0362-546X(96)00067-3 | 
		
				| [7] | An Y.Periodic solutions of telegraph-wave coupled system at nonresonance[J].Nonlinear Anal, 2001, 46(4):525-533. doi:  10.1016/S0362-546X(00)00127-9 | 
		
				| [8] | Li Y.Positive doubly periodic solutions of nonlinear telegraph equations[J].Nonlinear Anal, 2003,55(3):245-254. doi:  10.1016/S0362-546X(03)00227-X | 
		
				| [9] | Wang F,An Y.Nonnegative doubly periodic solutions for nonlinear telegraph system[J].J Math Anal Appl, 2008,338(1):91-100. doi:  10.1016/j.jmaa.2007.05.008 | 
		
				| [10] | Davis J M,Eloe P W,Henderson J.Triple positive solutions and dependence on high order derivatives[J].J Math Anal Appl, 1999, 237(2):710-720. doi:  10.1006/jmaa.1999.6500 | 
		
				| [11] | Sun J,Li W.Multiple positive solutions of a discrete difference system[J].Appl Math Comput, 2003,143(2):213-221. doi:  10.1016/S0096-3003(02)00354-5 | 
		
				| [12] | Leggett R W, Williams L R.Multiple positive fixed points of nonlinear operators on ordered Banach spaces[J].Indiana Univ Math J, 1979, 28(4):673-688. doi:  10.1512/iumj.1979.28.28046 |