@article{PORMA_1982__41_1-4_115484,
     author = {Eid, M.H. and Anwar, M.N. and El-Serafi, S.A.},
     title = {Approximate methods for calculating the period of oscillations for the generalized {Lienard's} differential equation},
     journal = {Portugaliae mathematica},
     pages = {71--81},
     publisher = {mathdoc},
     volume = {41},
     number = {1-4},
     year = {1982},
     mrnumber = {0766840},
     zbl = {0564.34038},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PORMA_1982__41_1-4_115484/}
}