Conditional stability for backward parabolic equations with $\mathrm{L}\mathrm{o}\mathrm{g}{\mathrm{L}\mathrm{i}\mathrm{p}}_t\times {\mathrm{L}\mathrm{i}\mathrm{p}}_x$-coefficients

In this paper we present an improvement of [Math. Ann. 345 (2009), 213–243], where the authors proved a result concerning continuous dependence for backward parabolic operators whose coefficients are Log-Lipschitz in $t$ and $C^2$ in $x$. The $C^2$ regularity with respect to $x$ had to be assumed for technical reasons. Here we remove this assumption, replacing it with Lipschitz-continuity. The main tools in the proof are Littlewood-Paley theory and Bony’s paraproduct as well as a result of Coifman and Meyer [Ast'erisque 57, 1978, Th. 35].