We consider Dirichlet problems for linear elliptic equations of second order in divergence form on a bounded or exterior smooth domain $Omega$ in $mathbb{R}^n$, $n ge 3$, with drifts $mathbf{b}$ in the critical weak $L^n$-space $L^{n,infty}(Omega ; mathbb{R}^n )$. First, assuming that the drift $mathbf{b}$ has nonnegative weak divergence in $L^{n/2, infty }(Omega )$, we establish existence and uniqueness of weak solutions in $W^{1,p}(Omega )$ or $D^{1,p}(Omega )$ for any $p$ with $n = n/(n-1)< p < n$. By duality, a similar result also holds for the dual problem. Next, we prove $W^{1,n+varepsilon}$ or $W^{2, n/2+delta}$-regularity of weak solutions of the dual problem for some $varepsilon, delta >0$ when the domain $Omega$ is bounded. By duality, these results enable us to obtain a quite general uniqueness result as well as an existence result for weak solutions belonging to $bigcap_{p< n }W^{1,p}(Omega )$. Finally, we prove a uniqueness result for exterior problems, which implies in particular that (very weak) solutions are unique in both $L^{n/(n-2),infty}(Omega )$ and $L^{n,infty}(Omega )$.
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