The equation div$u$+$\langle a, u \rangle=f$

Abstract: We study the solutions $u$ to the equation $$\begin{cases} \operatorname{div} u + \langle a , u \rangle = f & \textrm{ in } \Omega,\ u=0 & \textrm{ on } \partial \Omega, \end{cases}$$ where $a$ and $f$ are given. We significantly improve the existence results of [Csat\'o and Dacorogna, A Dirichlet problem involving the divergence operator, \textit{Ann. Inst. H. Poincar\'e Anal. Non Lin\'eaire}, 33 (2016), 829--848], where this equation has been considered for the first time. In particular, we prove the existence of a solution under essentially sharp regularity assumptions on the coefficients. The condition that we require on the vector field $a$ is necessary and sufficient. Finally, our results cover the whole scales of Sobolev and H\"older spaces.