On the effect of the degeneracy between w_0 and w_a
  The dynamics of scalar fields as dark energy is well approximated by some
general relations between the equation of state parameter $w(z)$ and the
fraction energy density $\Omega_\phi$. Based on the approximation, for
slowly-rolling scalar fields, we derived the analytical expressions of $w(z)$
which reduce to the popular Chevallier-Polarski-Linder parametrization with
explicit degeneracy relation between $w_0$ and $w_a$. The models approximate
the dynamics of scalar fields well and help eliminate the degeneracies among
$w_a$, $w_0$ and $\Omega_{m0}$. With the explicit degeneracy relations, we test
their effects on the constraints of cosmological parameters. We find that: (1)
The analytical relations between $w_0$ and $w_a$ for the two models are
consistent with observational data; (2) The degeneracies have little effect on
$\Omega_{m0}$; (3) The $1\sigma$ error of $w_0$ was reduced about 30% with the
degeneracy relations.
