Noetherianity and length of Melnikov functions
We study foliations in $\mathbb{C}^2$ given by polynomial deformations of the form $dH+εη=0$, with $γ(t)\subset H^{-1}(t)$ a family of cycles. The \emph{Poincaré first return map} is of the form $P(t)=t+\sum_j ε^j M_j^γ(t).$ The functions $M_j^γ$ are called \emph{Melnikov functions} and are given by \emph{iterated integrals of orbit length} at most $j$. We show that, for each $k\in\mathbb{N}$, there exists a \emph{universal Noetherianity index} $n_{\scriptscriptstyle H,γ}(k)$, independent of the deformation $η$, such that, if $M_j^γ\equiv0$, for $j=1,\ldots,n_{ H,γ}(k)$, then $M_j^γ$ is of orbit length $j-k$, for any Melnikov function $M_j^γ$. We call the smallest index with this property just the \emph{Noetherianity index} $ν_{\scriptscriptstyle H,γ}(k)$. In order to prove this theorem, we develop a structure theorem for Melnikov functions and use the Ritt-Raudenbush differential algebra theorem. We calculate the universal Noetherianity index $n_{H,γ}(k)$ in various nontrivial examples.