D\'eveloppements asymptotiques combin\'es et points tournants d'\'equations diff\'erentielles singuli\`erement perturb\'ees

TL;DR

This paper introduces combined asymptotic expansions (DAC) for solutions of singularly perturbed differential equations near turning points, linking classical methods and providing new theoretical and practical insights.

Contribution

It develops the theory of DAC, proving their existence with Gevrey estimates, and demonstrates their application to scalar differential equations with potential for broader use.

Findings

DAC effectively describe solutions near turning points

Ramis-Sibuya type result guarantees DAC existence with Gevrey estimates

Applications show DAC's utility in singular perturbation problems

Abstract

We develop the theory of a new type of asymptotic expansions for functions of two variables the coefficients of which contain functions of one of the variables as well as functions of the quotient of these two variables. These combined asymptotic expansions (DAC) are particularly well suited for the description of solutions of singularly perturbed ordinary differential equations in the neighborhood of turning points. We describe the relations with the method of matched asymptotic expansions and with the classical composite asymptotic expansions used for boundary layers. We present a result of Ramis-Sibuya type that proves the existence of these DAC and provides Gevrey estimates. Three applications are given, two of which depend on Gevrey estimates for the DAC. In our article, we only apply the theory to scalar ordinary differential equations, but we are convinced that they will be very useful for systems of differential equations and other types of functional equations as well.