Existence of Positive Solution of a Class of Semi-linear Sub-elliptic   Equation in the Entire Space $H^n$
  In this paper, we study the following problem $$ \{{ll} \Delta_{H^n} u-u+u^p=0 & in H^n u>0& in H^n u(x)\to 0 &\rho(x)\to\infty}. $$ where $1<p < \frac{Q+2}{Q-2}$, Q is the homogeneous dimension of Heisenberg group $H^n$. Our main result is that this problem has at least one positive solution. 
Zhu-Jun Zheng, Xiu-Fang Feng