  Observations of fluctuations in the redshifted 21 cm radiation from neautral
hydrogen (HI) are perceived to be an important future probe of the universe at
high redshifts. Under the assumption that at redshifts z less than 6
(Post-Reionization Era), the HI traces the underlying dark matter with a
possible bias, we investigate the possibility of using observations of
redshifted 21 cm radiation to detect the bispectrum arising from non-linear
gravitational clustering and from non-linear bias. We find that the expected
signal is ~ 0.1 mJy at 325 MHz (z=3.4) for the small baselines at the GMRT, the
strength being a few times larger at higher frequencies 610 MHz (z=1.3).
Further, the magnitude of the signal from the bispectrum is predicted to be
comparable to that from the power spectrum, allowing a detection of both in
roughly the same integration time. The HI signal is found to be uncorrelated
beyond frequency separations of 1.3 MHz whereas the continuum sources of
continuum are expected to be correlated across much larger frequencies.
  This signature can in principle be used to distinguish the HI signal from the
contamination. We also consider the possibility of using observations of the
bispectrum to determine the linear and quadratic bias parameters of the HI at
high redshifts, this having possible implications for theories of galaxy
formation.
