@article{10_1007_s10492_012_0036_5,
     author = {Zhang, Tailei and Liu, Junli and Teng, Zhidong},
     title = {Existence of positive periodic solutions of an {SEIR} model with periodic coefficients},
     journal = {Applications of Mathematics},
     pages = {601--616},
     publisher = {mathdoc},
     volume = {57},
     number = {6},
     year = {2012},
     doi = {10.1007/s10492-012-0036-5},
     mrnumber = {3010239},
     zbl = {1274.34150},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.1007/s10492-012-0036-5/}
}