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The pendulum equation is forced by a continuous and -periodic function with zero average. In these conditions it is well known that there exist at least two -periodic solutions. We construct some examples where there are exactly two -periodic solutions and both are unstable.
Cite this article
Rafael Ortega, A forced pendulum equation without stable periodic solutions of a fixed period. Port. Math. 71 (2014), no. 3, pp. 193–216