@article{ARM_1989_25_4_a5,
     author = {Mawhin, Jean},
     title = {A simple proof of a {semi-Fredholm} principle for periodically forced systems with homogeneous nonlinearities},
     journal = {Archivum mathematicum},
     pages = {235--238},
     year = {1989},
     volume = {25},
     number = {4},
     mrnumber = {1188067},
     zbl = {0713.34045},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_1989_25_4_a5/}
}