Coupled Quintessence in a Power-Law Case and the Cosmic Coincidence   Problem
  The problem of the cosmic coincidence is a longstanding puzzle. This conundrum may be solved by introducing a coupling between the two dark sectors. In this Letter, we study a coupled quintessence scenario in which the scalar field evolves in a power law potential and the mass of dark matter particles depends on a power law function of $\phi$. It is shown that this scenario has a stable attractor solution and can thus provide a natural solution to the cosmic coincidence problem. 
Xin Zhang