# Existence of stationary fronts in a system of two coupled wave equations with spatial inhomogeneity

## TL;DR

This paper studies stationary fronts in a coupled sine-Gordon system with a spatially varying potential, demonstrating bifurcations and stability changes through numerical and analytical methods, including geometric singular perturbation theory.

## Contribution

It introduces a novel analysis of bifurcations of stationary fronts in coupled inhomogeneous sine-Gordon equations, combining numerical and analytical approaches.

## Key findings

- Stable stationary fronts exist in the uncoupled system.
- Strong coupling causes loss of stability and bifurcation of fronts.
- Analytical proof of pitchfork bifurcation using piecewise approximation.

## Abstract

We investigate the existence of stationary fronts in a coupled system of two sine-Gordon equations with a smooth, "hat-like" spatial inhomogeneity. The spatial inhomogeneity corresponds to a spatially dependent scaling of the sine-Gordon potential term. The uncoupled inhomogeneous sine-Gordon equation has stable stationary front solutions that persist in the coupled system. Carrying out a numerical investigation it is found that these inhomogeneous sine-Gordon fronts loose stability, provided the coupling between the two inhomogeneous sine-Gordon equations is strong enough, with new stable fronts bifurcating. In order to analytically study the bifurcating fronts, we first approximate the smooth spatial inhomogeneity by a piecewise constant function. With this approximation, we prove analytically the existence of a pitchfork bifurcation. To complete the argument, we prove that transverse fronts for a piecewise constant inhomogeneity persist for the smooth "hat-like" spatial inhomogeneity by introducing a fast-slow structure and using geometric singular perturbation theory.