Multiple solutions of the forced double pendulum equation
Annales de l'I.H.P. Analyse non linéaire, Tome S6 (1989), pp. 259-281. 
@article{AIHPC_1989__S6__259_0,
     author = {Fournier, G. and Willem, M.},
     title = {Multiple solutions of the forced double pendulum equation},
     journal = {Annales de l'I.H.P. Analyse non lin\'eaire},
     pages = {259--281},
     publisher = {Gauthier-Villars},
     volume = {S6},
     year = {1989},
     mrnumber = {1204018},
     zbl = {0683.70022},
     language = {en},
     url = {http://www.numdam.org/item/AIHPC_1989__S6__259_0/}
}
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Fournier, G.; Willem, M. Multiple solutions of the forced double pendulum equation. Annales de l'I.H.P. Analyse non linéaire, Tome S6 (1989), pp. 259-281. http://www.numdam.org/item/AIHPC_1989__S6__259_0/

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