What do we know about cosmography
  In the present paper, we investigate the cosmographic problem using the
bias-variance trade-off. We find that both the z-redshift and the
$y=z/(1+z)$-redshift can present a small bias estimation. It means that the
cosmography can describe the supernova data more accurately. Minimizing risk,
it suggests that cosmography up to the second order is the best approximation.
Forecasting the constraint from future measurements, we find that future
supernova and redshift drift can significantly improve the constraint, thus
having the potential to solve the cosmographic problem. We also exploit the
values of cosmography on the deceleration parameter and equation of state of
dark energy $w(z)$. We find that supernova cosmography cannot give stable
estimations on them. However, much useful information was obtained, such as
that the cosmography favors a complicated dark energy with varying $w(z)$, and
the derivative $dw/dz<0$ for low redshift. The cosmography is helpful to model
the dark energy.
