The square root problem for second order, divergence form operators with   mixed boundary conditions on $L^p$
  We show that, under general conditions, the operator $\bigl (-\nabla \cdot \mu \nabla +1\bigr)^{1/2}$ with mixed boundary conditions provides a topological isomorphism between $W^{1,p}_D(\Omega)$ and $L^p(\Omega)$, for $p \in {]1,2[}$ if one presupposes that this isomorphism holds true for $p=2$. The domain $\Omega$ is assumed to be bounded, the Dirichlet part $D$ of the boundary has to satisfy the well-known Ahlfors-David condition, whilst for the points from $\overline {\partial \Omega \setminus D}$ the existence of bi-Lipschitzian boundary charts is required. 
Pascal Auscher (LM-Orsay), Nadine Badr (ICJ), Robert   Haller-Dintelmann, Joachim Rehberg (WIAS)
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