## Abstract

In this paper we study linear differential systems (1) x′ = A ̃(θ + ωt)x, where A ̃(θ) is an (n × n) matrix-valued function defined on the k-torus T^{k} and (θ, t) → θ + ωt is a given irrational twist flow on T^{k}. First, we show that if A ε{lunate} C^{N}(T^{k}), where N ε{lunate} {0, 1, 2,...; ∞; ω}, then the spectral subbundles are of class C^{N} on T^{k}. Next we assume that Ã is sufficiently smooth on T^{k} and ω satisfies a suitable "small divisors" inequality. We show that if (1) satisfies the "full spectrum" assumption, then there is a quasi-periodic linear change of variables x = P(t)y that transforms (1) to a constant coefficient system y′ =By. Finally, we study the case where the matrix A ̃(θ + ωt) in (1) is the Jacobian matrix of a nonlinear vector field f{hook}(x) evaluated along a quasi-periodic solution x = φ(t) of (2) x′ = f{hook}(x). We give sufficient conditions in terms of smoothness and small divisors inequalities in order that there is a coordinate system (z, θ{symbol}) defined in the vicinity of Ω = H(φ), the hull of φ, so that the linearized system (1) can be represented in the form z′ = Dz, θ{symbol}′ = ω, where D is a constant matrix. Our results represent substantial improvements over known methods because we do not require that Ã be "close to" a constant coefficient system.

Original language | English (US) |
---|---|

Pages (from-to) | 262-288 |

Number of pages | 27 |

Journal | Journal of Differential Equations |

Volume | 41 |

Issue number | 2 |

DOIs | |

State | Published - Aug 1981 |

### Bibliographical note

Funding Information:* This research ,was supported in part by NSF Grant MCS 79-01998. This work was done while the first author was visiting at the University of Minnesota.