@article{PORMA_2004__61_2_51355,
     author = {Liu, Yuji and Bai, Zhanbing and Gui, Zhanjie and Ge, Weigao},
     title = {Positive periodic solutions of impulsive delay differential equations with sign-changing coefficient.},
     journal = {Portugaliae mathematica},
     pages = {177--191},
     publisher = {mathdoc},
     volume = {61},
     number = {2},
     year = {2004},
     zbl = {1064.34050},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PORMA_2004__61_2_51355/}
}