$\mathcal{R}$-sectoriality of higher-order elliptic systems on general bounded domains
On bounded domains $Ω\subset \mathbb{R}^d , d \geq 2$, reaching far beyond the scope of Lipschitz domains, we consider an elliptic system of order $2 m$ in divergence form with complex $\mathrm{L}^{\infty}$-coefficients complemented with homogeneous mixed Dirichlet/Neumann boundary conditions. We prove that the $\mathrm{L}^p$-realization of the corresponding operator $A$ is $\mathcal{R}$-sectorial of angle $ω\in [0 , \fracπ{2})$, where in the case $2m < d$, $p \in (\frac{2d}{d + 2 m} - \varepsilon , \frac{2d}{d - 2 m} + \varepsilon)$ for some $\varepsilon > 0$, and where $p \in (1 , \infty)$ in the case $2m \geq d$. To perform this proof, we generalize the $\mathrm{L}^p$-extrapolation theorem of Shen to the Banach space valued setting and to arbitrary Lebesgue-measurable underlying sets.