@article{MS2_1977__41_166561,
     author = {Frank Hansen and Dorte Olesen},
     title = {Perturbations of centre-fixing dynamical systems.},
     journal = {Mathematica Scandinavica},
     pages = {295--307},
     publisher = {mathdoc},
     volume = {41},
     year = {1977},
     zbl = {0374.46054},
     url = {http://geodesic.mathdoc.fr/item/MS2_1977__41_166561/}
}