Statefinder hierarchy exploration of the extended Ricci dark energy
  We apply the statefinder hierarchy plus the fractional growth parameter to
explore the extended Ricci dark energy (ERDE) model, in which there are two
independent coefficients $\alpha$ and $\beta$. By adjusting them, we plot
evolution trajectories of some typical parameters, including Hubble expansion
rate $E$, deceleration parameter $q$, the third and fourth order hierarchy
$S_3^{(1)}$ and $S_4^{(1)}$ and fractional growth parameter $\epsilon$,
respectively, as well as several combinations of them. For the case of variable
$\alpha$ and constant $\beta$, in the low-redshift region the evolution
trajectories of $E$ are in high degeneracy and that of $q$ separate somewhat.
However, the $\Lambda$CDM model is confounded with ERDE in both of these two
cases. $S_3^{(1)}$ and $S_4^{(1)}$, especially the former, perform much better.
They can differentiate well only varieties of cases within ERDE except
$\Lambda$CDM in the low-redshift region. For high-redshift region, combinations
$\{S_n^{(1)},\epsilon\}$ can break the degeneracy. Both of
$\{S_3^{(1)},\epsilon\}$ and $\{S_4^{(1)},\epsilon\}$ have the ability to
discriminate ERDE with $\alpha=1$ from $\Lambda$CDM, of which the degeneracy
cannot be broken by all the before-mentioned parameters. For the case of
variable $\beta$ and constant $\alpha$, $S_3^{(1)}(z)$ and $S_4^{(1)}(z)$ can
only discriminate ERDE from $\Lambda$CDM. Nothing but pairs
$\{S_3^{(1)},\epsilon\}$ and $\{S_4^{(1)},\epsilon\}$ can discriminate not only
within ERDE but also ERDE from $\Lambda$CDM. Finally we find that $S_3^{(1)}$
is surprisingly a better choice to discriminate within ERDE itself, and ERDE
from $\Lambda$CDM as well, rather than $S_4^{(1)}$.
