# Asymptotic Expansion with Boundary Layer Analysis for Strongly Anisotropic Elliptic Equations

## TL;DR

This paper develops an asymptotic expansion method with boundary layer analysis for strongly anisotropic elliptic equations, addressing ill-posedness and boundary conditions to improve solution approximation accuracy.

## Contribution

It introduces a systematic approach combining mean-fluctuation decomposition and boundary layer analysis for higher-order asymptotic expansions in anisotropic elliptic problems.

## Key findings

- Derived asymptotic expansions up to arbitrary order.
- Handled ill-posedness via solution decomposition.
- Provided composite approximation for the leading order.

## Abstract

In this article, we derive the asymptotic expansion, up to an arbitrary order in theory, for the solution of a two-dimensional elliptic equation with strongly anisotropic diffusion coefficients along different directions, subject to the Neumann boundary condition and the Dirichlet boundary condition on specific parts of the domain boundary, respectively. The ill-posedness arising from the Neumann boundary condition in the strongly anisotropic diffusion limit is handled by the decomposition of the solution into a mean part and a fluctuation part. The boundary layer analysis due to the Dirichlet boundary condition is conducted for each order in the expansion for the fluctuation part. Our results suggest that the leading order is the combination of the mean part and the composite approximation of the fluctuation part for the general Dirichlet boundary condition.