Tracking quintessence and k-essence in a general cosmological background
  We derive conditions for stable tracker solutions for both quintessence and k-essence in a general cosmological background, H^2 \propto f(\rho). We find that tracker solutions are possible only when \eta = d ln f /d ln \rho is constant, aside from a few special cases, which are enumerated. Expressions for the quintessence or k-essence equation of state are derived as a function of \eta and the equation of state of the dominant background component. 
Rupam Das, Thomas W. Kephart, and Robert J. Scherrer