Inserted: 25 mar 2019
The study of what we now call Sobolev inequalities has now been considered for more than a century by physicists, while it has been eighty years since Sobolev's seminal mathematical contributions. Yet there are still things we don't understand about the action of integral operators on functions. This is no more apparent than in the $L^1$ setting, where only recently have optimal inequalities been obtained on the Lebesgue and Lorentz scale for scalar functions, while the full resolution of similar estimates for vector-valued functions is incomplete. The purpose of this paper is to discuss how some often overlooked estimates for the classical Poisson equation give an entry into these questions, to the present state of the art of what is known, and to survey some open problems on the frontier of research in the area.