@article{EJDE_2005__2005__a149,
     author = {Huang,  Chuangxia and Huang,  Lihong and Meng,  Yimin},
     title = {Periodic solutions for planar systems with time-varying delays},
     journal = {Electronic journal of differential equations},
     year = {2005},
     volume = {2005},
     zbl = {1075.34063},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_2005__2005__a149/}
}