calculate the tensor perturbation of this model. In Einstein frame where the model behaves like two-field inflation model, the tensor power spectrum is the same as that of GR, which is \[tensor\] [P]{}\_[T]{}= . One can also define the tensor-scalar ratio of the model, which is $r\equiv{\cal P}_{\cal T}/{\cal P}_{\cal R}$.

Perturbations at large scales
-----------------------------

What we concerned more about is the values of perturbations that reenters the horizon. In single-field case, there is only curvature perturbation which is conserved at superhubble scales, so it is reasonable to take the Hubble-exit values of perturbation the same as those of the Hubble-reenter values, and thus the calculations in the above subsection is enough. However, in multi-field case, there is also isocurvature perturbation which will source the curvature one and the latter will evolve even after Hubble-exit, which we must take into account. We in this subsection give the formulation of perturbations at large scales as briefly as possible, while more detailed calculations can be found in the preceding works [@Lalak:2007vi; @Langlois:2008wt]. From the equations of motion of the field perturbation (\[eomQsigma\]) and (\[eomQs\]), one can get the varying of the curvature and isocurvature perturbations at large scales as: \[eqlargescale\] AH[S]{} 