  We present an analytical approximation formula for the growth function in a
spatially flat cosmology with dust and a cosmological constant. Our approximate
formula is written simply in terms of a rational function. We also show the
approximate formula in a dust cosmology without a cosmological constant,
directly as a function of the scale factor in terms of a rational function. The
single rational function applies for all, open, closed and flat universes.
  Our results involve no elliptic functions, and have very small relative error
of less than 0.2 per cent over the range of the scale factor $1/1000 \la a \lid
1$ and the density parameter $0.2 \la \Omega_{\rmn{m}} \lid 1$ for a flat
cosmology, and less than $0.4$ per cent over the range $0.2 \la
\Omega_{\rmn{m}} \la 4$ for a cosmology without a cosmological constant.
