A compactness theorem of $n$-harmonic maps
  For $n\ge 3$, let $\Omega$ be a bounded domain in $R^n$ and $N$ be a compact Riemannian manifold in $R^L$ without boundary. Suppose that $u_n\in W^{1,n}(\Omega,N)$ are the Palais-Smale sequences of the Dirichlet $n$-energy functional and $u_n$ converges weakly in $W^{1,n}$ to a map $u\in W^{1,n}(\Omega,N)$. Then $u$ is a $n$-harmonic map. In particular, the space of $n$-harmonic maps is sequentially compact for the weak $W^{1,n}$-topology. 
Changyou Wang