A Liouville theorem for high order degenerate elliptic equations
  In this paper, we apply the moving plane method to the following high order degenerate elliptic equation,\begin{equation*} (-A)^p u=u^\alpha\text{ in } \mathbb R^{n+1}_+,n\geq 1, \end{equation*}where the operator $A=y\partial_y^2+a\partial_y+\Delta_x,a\geq 1$. We get a Liouville theorem for subcritical case and classify the solutions for the critical case. 
Genggeng Huang and Congming Li
math.AP