Abstract
We consider a perturbed Hill's equation of the form +(p0(t)+ɛp1(t))ϕ=0, where p0 is real analytic and periodic, p1 is real analytic and quasi-periodic and ɛ ∈ℝ is ``small''. Assuming Diophantine conditions on the frequencies of the decoupled system, i.e. the frequencies of the external potentials p0 and p1 and the proper frequency of the unperturbed (ɛ=0) Hill's equation, but without making any assumptions on the perturbing potential p1 other than analyticity, we prove that quasi-periodic solutions of the unperturbed equation can be continued into quasi-periodic solutions if ɛ lies in a Cantor set of relatively large measure in where ɛ0 is small enough. Our method is based on a resummation procedure of a formal Lindstedt series obtained as a solution of a generalized Riccati equation associated to Hill's problem.
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Gentile, G., Cortez, D. & Barata, J. Stability for Quasi-Periodically Perturbed Hill's Equations. Commun. Math. Phys. 260, 403–443 (2005). https://doi.org/10.1007/s00220-005-1413-7
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DOI: https://doi.org/10.1007/s00220-005-1413-7