@article{EJDE_2001__2001__a159,
     author = {Cheng,  Sui Sun and Zhang,  Guang},
     title = {Existence of positive periodic solutions for non-autonomous functional differential equations},
     journal = {Electronic journal of differential equations},
     year = {2001},
     volume = {2001},
     zbl = {1003.34059},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_2001__2001__a159/}
}