@article{MZ_1933__36_168396,
     author = {Underwood. F.},
     title = {Some conditions for the existence of two periodic integrals of the homogeneous linear differential equation of the seoond order},
     journal = {Mathematische Zeitschrift},
     pages = {195--201},
     publisher = {mathdoc},
     volume = {36},
     year = {1933},
     zbl = {58.0457.01},
     url = {http://geodesic.mathdoc.fr/item/MZ_1933__36_168396/}
}