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Assuming the Hubble parameter is a continuous and differentiable function of comoving time, we investigate necessary conditions for the quintessence to phantom phase transition in the quintom model. For power-law and exponential potential examples, we study the behavior of dynamical dark energy fields and the Hubble parameter near the transition time, and show that the phantom-divide line =-1 is crossed in these models.
Sadjadi et al. (Mon,) studied this question.