HI intensity mapping with FAST
  We discuss the detectability of large-scale HI intensity fluctuations using
the FAST telescope. We present forecasts for the accuracy of measuring the
Baryonic Acoustic Oscillations and constraining the properties of dark energy.
The FAST $19$-beam L-band receivers ($1.05$--$1.45$ GHz) can provide
constraints on the matter power spectrum and dark energy equation of state
parameters ($w_{0},w_{a}$) that are comparable to the BINGO and CHIME
experiments. For one year of integration time we find that the optimal survey
area is $6000\,{\rm deg}^2$. However, observing with larger frequency coverage
at higher redshift ($0.95$--$1.35$ GHz) improves the projected errorbars on the
HI power spectrum by more than $2~\sigma$ confidence level. The combined
constraints from FAST, CHIME, BINGO and Planck CMB observations can provide
reliable, stringent constraints on the dark energy equation of state.
