Moduli space for generic unfolded differential linear systems

TL;DR

This paper characterizes the moduli space of generic unfoldings of nonresonant linear differential systems with irregular singularities, providing conditions for realizability and explicit constructions of such systems.

Contribution

It precisely identifies which moduli can be realized by rational systems and establishes conditions for their realization, advancing the understanding of the structure of unfolded differential systems.

Findings

Identified the moduli space for generic unfoldings of linear systems with irregular singularities.

Established necessary and sufficient conditions for the realizability of moduli.

Constructed explicit rational systems realizing any admissible modulus.

Abstract

In this paper, we identify the moduli space for germs of generic unfoldings of nonresonant linear differential systems with an irregular singularity of Poincar\'e rank $k$ at the origin, under analytic equivalence. The modulus of a given family was determined in \cite{HLR}: it comprises a formal part depending analytically on the parameters, and an analytic part given by unfoldings of the Stokes matrices. These unfoldings are given on "Douady-Sentenac" (DS) domains in the parameter space covering the generic values of the parameters corresponding to Fuchsian singular points. Here we identify exactly which moduli can be realized. A necessary condition on the analytic part, called compatibility condition, is saying that the unfoldings define the same monodromy group (up to conjugacy) for the different presentations of the modulus on the intersections of DS domains. With the additional requirement that the corresponding cocycle is trivial and good limit behavior at some boundary points of the DS domains, this condition becomes sufficient. In particular we show that any modulus can be realized by a $k$-parameter family of systems of rational linear differential equations over $\mathbb C\mathbb P^1$ with $k+1$, $k+2$ or $k+3$ singular points (with multiplicities). Under the generic condition of irreducibility, there are precisely $k+2$ singular points which are Fuchsian as soon as simple. This in turn implies that any unfolding of an irregular singularity of Poincar\'e rank $k$ is analytically equivalent to a rational system of the form $y'=\frac{A(x)}{p_\epsilon(x)}\cdot y$, with $A(x)$ polynomial of degree at most $k$ and $p_\epsilon(x)$ is the generic unfolding of the polynomial $x^{k+1}$.