Denisova, I. V., and V. A. Solonnikov. “$L_2$-Theory for Two Incompressible Fluids Separated by a Free Interface”. Topological Methods in Nonlinear Analysis, vol. 52, no. 1, Aug. 2018, pp. 213-38, https://apcz.umk.pl/TMNA/article/view/TMNA.2018.019.