@article{MM2_1971__5_154067,
     author = {Paul H. Rabinowitz},
     title = {Time {Periodic} {Solutions} of {Nonlinear} {Wave} {Equations.}},
     journal = {Manuscripta mathematica},
     pages = {165--194},
     publisher = {mathdoc},
     volume = {5},
     year = {1971},
     zbl = {0219.35062},
     url = {http://geodesic.mathdoc.fr/item/MM2_1971__5_154067/}
}