Universal bound independent of geometry for solution to symmetric diffusion equation in exterior domain with boundary flux

Abstract: Fix $R>0$ and let $B_R$ denote the ball of radius $R$ centered at the origin in $R^d$, $d\ge2$. Let $D\subset B_R$ be an open set with smooth boundary and such that $R^d-\bar D$ is connected, and let $$ L=\sum_{i,j=1}^da_{i,j}\frac{\partial^2}{\partial x_i\partial x_j}-\sum_{i=1}^db_i\frac{\partial}{\partial x_i} $$ be a second order elliptic operator. Consider the following linear heat equation in the exterior domain $R^d-\bar D$ with boundary flux: \begin{equation*} \begin{aligned} &L u=0 \ \text{in}\ R^d-\bar D;\ &a\nabla u\cdot \bar n=-h\ \text{on}\ \partial D;\ &u>0 \ \text{is minimal}, \end{aligned} \end{equation*} where $h\gneq0$ is continuous, and where $\bar n$ is the unit inward normal to the domain $R^d-\bar D$. The operator $L$ must possess a Green's function in order that a solution $u$ exist. An important feature of the equation is that there is no a priori bound on the supremum $\sup_{x\in R^d-\bar D}u(x)$ of the solution exclusively in terms of the boundary flux $h$, the hyper-surface measure of $\partial D$ and the coefficients of $L$; rather the geometry of $D\subset B_R$ plays an essential role. However, we prove that in the case that $L$ is a \it symmetric operator\rm\ with respect to some reference measure, then \it outside of\rm\ $\bar B_R$, the solution to \eqref{LHE} is uniformly bounded, independent of the particular choice of $D\subset B_R$. The proof uses a combination of analytic and probabilistic techniques.