@article{EJDE_2011__2011__a57,
     author = {Pan,  Lijun},
     title = {Existence of periodic solutions for second order delay differential equations with impulses},
     journal = {Electronic journal of differential equations},
     year = {2011},
     volume = {2011},
     zbl = {1216.34078},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_2011__2011__a57/}
}