[5] Gaines, R. E., Mawhin, J. L.: 
Coincidence Degree, and Nonlinear Differential Equations. Lecture Notes in Math., Vol. 568. Springer Berlin (1977). 
DOI 10.1007/BFb0089537 | 
MR 0637067[8] Jiang, D., Nieto, J. J., Zuo, W.: 
On monotone method for first and second order periodic boundary value problems and periodic solutions of functional differential equations. J. Math. Anal. Appl. 289 (2004), 691-699. 
DOI 10.1016/j.jmaa.2003.09.020 | 
MR 2026934 | 
Zbl 1134.34322[9] Li, X., Lin, X., Jiang, D., Zhang, X.: 
Existence and multiplicity of positive periodic solutions to functional differential equations with impulse effects. Nonlinear Anal., Theory Methods Appl. 62 (2005), 683-701. 
DOI 10.1016/j.na.2005.04.005 | 
MR 2149910 | 
Zbl 1084.34071[14] Liu, Y., Bai, Z., Gui, Z., Ge, W.: 
Positive periodic solutions of impulsive delay differential equations with sign-changing coefficients. Port. Math. (N.S.) 61 (2004), 177-191. 
MR 2066673[15] Liu, Y., Ge, W.: 
Stability theorems and existence results for periodic solutions of nonlinear impulsive delay differential equations with variable coefficients. Nonlinear Anal., Theory Methods Appl. 57A (2004), 363-399. 
MR 2064097 | 
Zbl 1064.34051[20] Pierson-Gorez, C.: 
Impulsive differential equations of first order with periodic boundary conditions. Differ. Equ. Dyn. Syst. 1 (1993), 185-196. 
MR 1258896 | 
Zbl 0868.34007