An analytical approximation of the growth function in Friedmann-Lema\^itre universes

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.