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Multisummability of formal power series solutions of linear ordinary differential equations. (English) Zbl 0754.34057
The authors consider the differential equation $${\mathcal L}x=0$$ when $${\mathcal L}=dx/dz-A(z)$$, $$x$$ is an $$n\times n$$ matrix, $$z$$ is a complex independently variable and $$A$$ is an $$n\times n$$ matrix of functions meromorphic at $$z=0$$, and prove these two theorems.
Theorem 4.1. Let $${1\over 2}\leq h_ q<h_{q-1}<\cdots<h_ 2<h_ 1<+\infty$$ be all the positive slopes of the Newton polygon $$N({\mathcal L})$$ of the operator $${\mathcal L}$$. If a formal power series $$f\in\mathbb{C}[[z]]^ n$$ satisfies the condition $${\mathcal L}[f]\in(\mathbb{C}\{z\}[z^{-1}])^ n$$ and if a direction $$\arg z=d$$ is not singular, then there exist $$f_ \ell\in(\mathbb{C}\{z\}_{h_ \ell d})^ n$$ $$(\ell=1,2,\ldots,q)$$ such that $$f=\sum_{q\geq\ell\geq 1}f_{\ell}$$, $${\mathcal L}[f_ \ell]\in(\mathbb{C}\{z\}[z^{-1}])^ n$$ $$(\ell=1,2,\ldots,q)$$. In the case when $$0<h_ \ell<{1\over 2}$$ for some $$\ell$$, we change the independent variable $$z$$ by $$z=\zeta^ m$$ so that $$mh_ \ell\geq{1\over 2}$$ for all $$\ell$$.
Theorem 6.1. For any given direction $$\arg z=d$$, there exists an admissible system $$\{X_ \nu(z)\mid\nu\in\mathbb{Z}\}$$ of fundamental solutions of equations (5.5) such that the corresponding Stokes multipliers $$\{V_ \nu\mid\nu\in\mathbb{Z}\}$$ satisfy the condition $$V_ \nu^{(j-1)}+{\mathbf I}$$ for every pair $$(j,\nu)$$ such that $$d-\pi/(2k_ j)<\tau_{\nu-1}<\tau_ \nu<d+\pi/(2k_ j)$$, where $$j=1,2,\ldots,p$$.
Reviewer: H.S.Nur (Fresno)

##### MSC:
 34E15 Singular perturbations for ordinary differential equations 34M99 Ordinary differential equations in the complex domain 30B10 Power series (including lacunary series) in one complex variable 30D05 Functional equations in the complex plane, iteration and composition of analytic functions of one complex variable 34A25 Analytical theory of ordinary differential equations: series, transformations, transforms, operational calculus, etc. 34A30 Linear ordinary differential equations and systems