Solutions to the $σ_k$-Loewner-Nirenberg problem on annuli are locally Lipschitz and not differentiable

Abstract: We show for $k \geq 2$ that the locally Lipschitz viscosity solution to the $\sigma_k$-Loewner-Nirenberg problem on a given annulus ${a < |x| < b}$ is $C^{1,\frac{1}{k}}_{\rm loc}$ in each of ${a < |x| \leq \sqrt{ab}}$ and ${\sqrt{ab} \leq |x| < b}$ and has a jump in radial derivative across $|x| = \sqrt{ab}$. Furthermore, the solution is not $C^{1,\gamma}_{\rm loc}$ for any $\gamma > \frac{1}{k}$. Optimal regularity for solutions to the $\sigma_k$-Yamabe problem on annuli with finite constant boundary values is also established.