Global second order Sobolev-regularity of $p$-harmonic functions

We prove a global version of the classical result that $p$-harmonic functions belong to $W_{loc}^{2,2}$ for $1<p<3+\frac{2}{n-2}$. The proof relies on Cordes’ matrix inequalities [7] and techniques from the work of Cianchi and Maz’ya [5,6].