“…The search for persistence of KAM tori and reduction problems of finitely differentiable Hamilton's systems goes back to the work of Kolmogorov-Arnold-Moser, and attracts great attention over years. For the Hamilton's equations ẋ = H y (x, y), ẏ = −H x (x, y), (x, y) ∈ R 2n , where H ∈ C ℓ , it was shown by Pöschel [30,34] and Cheng [7] that the hypotheses ℓ > 2n is optimal for the existence of KAM torus. When considering the Hamiltonian (1.5) h ǫ (I, θ, z, z) = h 0 (I, z, z) + ǫq(I, θ, z, z), (I, θ, z, z) ∈ R n × T n × C 2d , where q is C ℓ in θ, Chierchia-Qian [8] proved the persistence of lower dimensional tori with ℓ > 6n + 5 and Sun-Li-Xie [35] considered the reduction problem with ℓ ≥ 200n.…”