Global well-posedness for a system of quasilinear wave equations on a product space

We consider a system of quasilinear wave equations on the product space ${ℝ}^{1+3}\times {\mathbb{𝕊}}^1$, which we want to see as a toy model for the Einstein equations with additional compact dimensions. We show global existence of solutions for small and regular initial data with polynomial decay at infinity. The method combines energy estimates on hyperboloids inside the light cone and weighted energy estimates outside the light cone.