Modelling the post-reionization neutral hydrogen (${\rm HI}$) 21-cm
  bispectrum
  Measurements of the post-reionization 21-cm bispectrum $B_{{\rm
HI}}(\mathbf{k_1},\mathbf{k_2},\mathbf{k_3})$ using various upcoming intensity
mapping experiments hold the potential for determining the cosmological
parameters at a high level of precision. In this paper we have estimated the
21-cm bispectrum in the $z$ range $1 \le z \le 6$ using semi-numerical
simulations of the neutral hydrogen (${\rm HI}$) distribution. We determine the
$k$ and $z$ range where the 21-cm bispectrum can be adequately modelled using
the predictions of second order perturbation theory, and we use this to predict
the redshift evolution of the linear and quadratic ${\rm HI}$ bias parameters
$b_1$ and $b_2$ respectively. The $b_1$ values are found to decreases nearly
linearly with decreasing $z$, and are in good agreement with earlier
predictions obtained by modelling the 21-cm power spectrum $P_{{\rm HI}}(k)$.
The $b_2$ values fall sharply with decreasing $z$, becomes zero at $z \sim 3$
and attains a nearly constant value $b_2 \approx - 0.36$ at $z<2$. We provide
polynomial fitting formulas for $b_1$ and $b_2$ as functions of $z$. The
modelling presented here is expected to be useful in future efforts to
determine cosmological parameters and constrain primordial non-Gaussianity
using the 21-cm bispectrum.
