A common approach to singular perturbation and homogenization III: Nonlinear periodic homogenization with localized defects

We consider periodic homogenization with localized defects for semilinear elliptic equations and systems of the type $$\nabla\cdot\Big(\Big(A(x/\varepsilon)+B(x/\varepsilon)\Big)\nabla u(x)+c(x,u(x)\Big)=d(x,u(x)) \mbox{ in } \Omega$$ with Dirichlet boundary conditions. For small $\varepsilon>0$ we show existence of weak solutions $u=u_\varepsilon$ as well as their local uniqueness for $\|u-u_0\|_\infty \approx 0$, where $u_0$ is a given non-degenerate weak solution to the homogenized problem. Moreover, we prove that $\|u_\varepsilon-u_0\|_\infty\to 0$ for $\varepsilon \to 0$, and we estimate the corresponding rate of convergence. Our assumptions are, roughly speaking, as follows: $\Omega$ is a bounded Lipschitz domain, $A$, $B$, $c(\cdot,u)$ and $d(\cdot,u)$ are bounded and measurable, $c(x,\cdot)$ and $d(x,\cdot)$ are $C^1$-smooth, $A$ is periodic, and $B$ is a localized defect. Neither global uniqueness is supposed nor growth restriction for $c(x,\cdot)$ or $d(x,\cdot)$. The main tool of the proofs is an abstract result of implicit function theorem type which permits a common approach to nonlinear singular perturbation and homogenization.