Title: Eulerian and Lagrangian solutions to the continuity and Euler equations with $L^1$ vorticity,
Abstract: In the first part of this paper we establish a uniqueness result for
continuity equations with velocity field whose derivative can be represented by
a singular integral operator of an $L^1$ function, extending the Lagrangian
theory in \cite{BouchutCrippa13}. The proof is based on a combination of a
stability estimate via optimal transport techniques developed in \cite{Seis16a}
and some tools from harmonic analysis introduced in \cite{BouchutCrippa13}. In
the second part of the paper, we address a question that arose in
\cite{FilhoMazzucatoNussenzveig06}, namely whether 2D Euler solutions obtained
via vanishing viscosity are renormalized (in the sense of DiPerna and Lions)
when the initial data has low integrability. We show that this is the case even
when the initial vorticity is only in~$L^1$, extending the proof for the $L^p$
case in \cite{CrippaSpirito15}.