Doubly-resonant saddle-nodes in $(\mathbb{C}^{3},0)$ and the fixed singularity at infinity in the Painlev\'e equations (part III): local analytic classification

TL;DR

This paper classifies certain doubly-resonant saddle-node singularities in complex three-dimensional vector fields, especially those arising from Painlevé equations, using sectorial normalizations and Stokes phenomena.

Contribution

It provides an analytic classification of these singularities via sectorial normalizing maps and Stokes diffeomorphisms, extending classical results to more complex resonant cases.

Findings

Constructs sectorial normalizing maps as Gevrey-1 sums of formal maps

Establishes classification under fibered diffeomorphisms

Connects local singularity analysis to Painlevé equations

Abstract

In this work, following [Bit15] and [Bit16a], we consider analytic singular vector fields in $(\mathbb{C}^{3},0)$ with an isolated and doubly-resonant singularity of saddle-node type at the origin. Such vector fields come from irregular two-dimensional differential systems with two opposite non-zero eigenvalues, and appear for instance when studying the irregular singularity at infinity in Painlev{\'e} equations (P\_j), j=I...V , for generic values of the parameters. Under suitable assumptions, we provide an analytic classification under the action of fibered diffeomorphisms, based on the study of the Stokes diffeomorphisms obtained by comparing consecutive sectorial normalizing maps {\`a} la Martinet-Ramis / Stolovitch. These normalizing maps over sectorial domains are obtained in the main theorem of [Bit16a], which is analogous to the classical one due to Hukuhara-Kimura-Matuda for saddle-nodes in $\mathbb{C}^{3}$. We also prove that these maps are in fact the Gevrey-1 sums of the formal normalizing map, the existence of which has been proved in [Bit15].