Dear all,

I am looking for a good reference for elliptic regularity in $L^1$. To be more precise

Let $\Omega\subset\mathrm{R}^n$ be a bounded smooth domain, let $A$ be a properly elliptic differential operator in $\Omega$ and let $B$ be a first order differential operator on $\partial\Omega$ satisfying the complementing condition. For $f\in L^1(\Omega)$ consider the problem

$$Au=f, \mbox{ in }\Omega,\qquad Bu=0, \mbox{ on }\partial\Omega.$$

It seems to be well-known that there is at least a weak solution $u\in W^{1,1}(\Omega)$. Is it possible to give a reference for this? Is it possible to state more regularity for $u$?

I would be very grateful for any useful hints on this problem.

Richard