@article{10_4064_ap_36_1_29_41,
     author = {G. J. Butler and H. I. Freedman},
     title = {Periodic solutions of x" + f(\ensuremath{\mu}, x) = 0},
     journal = {Annales Polonici Mathematici},
     pages = {29--41},
     publisher = {mathdoc},
     volume = {36},
     number = {1},
     year = {1979},
     doi = {10.4064/ap-36-1-29-41},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-36-1-29-41/}
}