Quantitative Homogenization and Convergence of Moving Averages

We study homogenization it its most basic form $$-\left(a\left(\frac{x}{\varepsilon}\right) u_{\varepsilon}'(x)\right)' = f(x) \quad \mbox{for} ~x \in (0,1),$$ where $a(\cdot)$ is a positive $1-$periodic continuous function, $f$ is smooth and $u_{\varepsilon}$ is subjected to Dirichlet boundary conditions. Classically, there is a homogenized equation with $a(\cdot)$ replaced by a constant coefficient $\overline{a} > 0$ whose solution $u$ satisfies $\|u-u_{\varepsilon}\|_{L^{\infty}} \lesssim \varepsilon$. We show that local averages can result in faster convergence: for example, if $a(x) = a(1-x)$, then for $x \in (\varepsilon, 1-\varepsilon)$ $$\left| \frac{1}{\varepsilon} \int_{x-\varepsilon/2}^{x+\varepsilon/2}{ u_{\varepsilon}(y) dy} - u(x) \right| \lesssim_{a, f} \varepsilon^2.$$ If the condition on $a(\cdot)$ is not satisfied, then subtracting an explicitly given linear function (depending on $a(\cdot),f$) results in the same bound. We also describe another approach to quantitative homogenization problems and illustrate it on the same example.