Tracking k-essence -: art. no. 063514

We derive a condition for converging a common evolutionary track for k-essence (a scalar field dark energy with noncanonical kinetic terms). For the Lagrangian density V(phi)W(X) with X=phi(2)/2, we find that tracker solutions with w(phi)<w(B) exist if Gammaequivalent toV"V/(V')(2)>3/2. Here w(phi) (w(B)) is the equation of state of the scalar field (background radiation or matter). Our condition may be useful for examining the existence of attractorlike behavior in cosmology with k-essence (for example, a rolling tachyon).