A criterion of integrability for perturbed nonresonant harmonic oscillators. ?Wick ordering? of the perturbations in classical mechanics and invariance of the frequency spectrum
1982 ◽
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pp. 365-383
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1934 ◽
Vol 147
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pp. 594-599
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2011 ◽
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pp. 819-832
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1940 ◽
Vol 86
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pp. 297-306
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2018 ◽
Vol 14
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pp. 5708-5733
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