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# Mathematical Research Letters

## Volume 19 (2012)

### Number 4

### $C^1$–boundary regularity of planar infinity harmonic functions

Pages: 823 – 835

DOI: http://dx.doi.org/10.4310/MRL.2012.v19.n4.a7

#### Authors

#### Abstract

We prove that if $\Omega\subset \Rset ^2$ is a bounded domain with $C^2$-boundary and $g\in C^2(\Rset ^2)$, then any viscosity solution $u\in C(\overline\Omega)$ of the infinity Laplacian equation (\ref{eq}) is $C^1(\overline \Omega)$. The interior $C^1$ and $C^{1,\alpha}$-regularity of $u$ in dimension two has been proved by Savin \cite{S}, and Evans and Savin \cite{ES}, respectively. We also show that for any $n\ge 3$, if $\Omega\subset \Rset ^n$ is a bounded domain with $C^1$-boundary and $g\in C^1(\Rset ^n)$, then the solution $u$ of equation (\ref{eq}) is differentiable on $\partial\Omega$. This can be viewed as a supplementary result to the much deeper interior differentiability theorem by Evans and Smart.