Constructive Approaches to QP-Time-Dependent KAM Theory for Lagrangian Tori in Hamiltonian Systems

In this paper, we prove a KAM theorem in a-posteriori format, using the parameterization method to look invariant tori in non-autonomous Hamiltonian systems with $n$ degrees of freedom that depend periodically or quasi-periodically (QP) on time, with $\ell$ external frequencies. Such a system is described by a Hamiltonian function in the $2n$-dimensional phase space, $\mathscr{M}$, that depends also on $\ell$ angles, $\varphi\in \mathbb{T}^\ell$. We take advantage of the fibbered structure of the extended phase space $\mathscr{M} \times \mathbb{T}^\ell$. As a result of our approach, the parameterization of tori requires the last $\ell$ variables, to be precise $\varphi$, while the first $2n$ components are determined by an invariance equation. This reduction decreases the dimension of the problem where the unknown is a parameterization from $2(n+\ell)$ to $2n$. We employ a quasi-Newton method, in order to prove the KAM theorem. This iterative method begins with an initial parameterization of an approximately invariant torus, meaning it approximately satisfies the invariance equation. The approximation is refined by applying corrections that reduce quadratically the invariance equation error. This process converges to a torus in a complex strip of size $\rho_\infty$, provided suitable Diophantine $(\gamma,\tau)$ conditions and a non-degeneracy condition on the torsion are met. Given the nature of the proof, this provides a numerical method that can be effectively implemented on a computer, the details are given in the companion paper [CHP25]. This approach leverages precision and efficiency to compute invariant tori.