On $C^1$ regularity for degenerate elliptic equations in the plane

We show that Lipschitz solutions $u$ of $\mathrm{div}\, G(\nabla u)=0$ in $B_1\subset\mathbb R^2$ are $C^1$, for strictly monotone vector fields $G\in C^0(\mathbb R^2;\mathbb R^2)$ satisfying a mild ellipticity condition. If $G=\nabla F$ for a strictly convex function $F$, and $0\leq \lambda(\xi)\leq \Lambda(\xi)$ are the two eigenvalues of $\nabla^2 F(\xi)$, our assumption is that the set $\lbrace\lambda=0\rbrace \cap \lbrace \Lambda=\infty\rbrace$, where ellipticity degenerates $both$ from below and from above, is finite. This extends results by De Silva and Savin (Duke Math. J. 151, No. 3, p.487-532, 2010), which assumed either that set empty, or the larger set $\lbrace \lambda=0\rbrace$ finite. Our main new input is to transfer estimates in $\lbrace \lambda > 0 \rbrace$ to estimates in $\lbrace \Lambda <\infty\rbrace$ by means of a conjugate equation. When $G$ is not a gradient, the ellipticity assumption needs to be interpreted in a specific way, and we highlight the nontrivial effect of the antisymmetric part of $\nabla G$.