Constraints of the equation of state of dark energy from current and
  future observational data by piecewise parametrizations
  The model-independent piecewise parametrizations (0-spline, linear-spline and
cubic-spline) are used to estimate constraints of equation of state of dark
energy ($w_{de}$) from current observational data (including SNIa, BAO and
Hubble parameter) and the simulated future data. A combination of fitting
results of $w_{de}$ from these three spline methods reveal essential properties
of real equation of state $w_{de}$. It is shown that $w_{de}$ beyond redshift
$z\sim0.5$ is poorly constrained from current data, and the mock future
$\sim2300$ supernovae data give poor constraints of $w_{de}$ beyond $z\sim1$.
The fitting results also indicate that there might exist a rapid transition of
$w_{de}$ around $z\sim0.5$. The difference between three spline methods in
reconstructing and constraining $w_{de}$ has also been discussed.
