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nonuniqueness; time-periodical solutions; semilinear equation; irrational periods; dual variational method

References:

[1] J. M. Coron: **Periodic solutions of a nonlinear wave equation without assumption of monotonicity**. Math. Ann. 262 (1983), 273-285. DOI 10.1007/BF01455317 | MR 0690201 | Zbl 0489.35061

[2] D. G. Costa M. Willem: **Multiple critical points of invariant functional and applications**. Séminaire de Mathématique 2-éme Semestre Université Catholique de Louvain.

[3] I. Ekeland R. Temam: **Convex analysis and variational problems**. North-Holland Publishing Company 1976. MR 0463994

[4] N. Krylová O. Vejvoda: **A linear and weakly nonlinear equation of a beam: the boundary value problem for free extremities and its periodic solutions**. Czechoslovak Math. J. 21 (1971), 535-566. MR 0289918