Quantitative uniqueness estimates for second order elliptic equations with unbounded drift

Abstract: In this paper we derive quantitative uniqueness estimates at infinity for solutions to an elliptic equation with unbounded drift in the plane. More precisely, let $u$ be a real solution to $\Delta u+W\cdot\nabla u=0$ in ${\mathbf R}^2$, where $W$ is real vector and $|W|{L^p({\mathbf R}^2)}\le K$ for $2\le p<\infty$. Assume that $|u|{L^{\infty}({\mathbf R}^2)}\le C_0$ and satisfies certain a priori assumption at $0$. Then $u$ satisfies the following asymptotic estimates at $R\gg 1$ [ \inf_{|z_0|=R}\sup_{|z-z_0|<1}|u(z)|\ge \exp(-C_1R^{1-2/p}\log R)\quad\text{if}\quad 2<p<\infty \] and \[ \inf_{|z_0|=R}\sup_{|z-z_0|\<1}|u(z)|\ge R^{-C_2}\quad\text{if}\quad p=2, \] where $C_1\>0$ depends on $p, K, C_0$, while $C_2>0$ depends on $K, C_0$ . Using the scaling argument in [BK05], these quantitative estimates are easy consequences of estimates of the maximal vanishing order for solutions of the local problem. The estimate of the maximal vanishing order is a quantitative form of the strong unique continuation property.