  The gravitational evolution of the cosmic one-point Probability Distribution
Function (PDF) can be estimated using an analytic approximation that combines
gravitational Perturbation Theory (PT) with the Edgeworth expansion around a
Gaussian PDF. We present an alternative to the Edgeworth series based on an
expansion around the Gamma PDF, which is more appropriate to describe a
realistic PDF. The Gamma expansion converges when the PDF exhibits exponential
tails, which are predicted by PT and N-body simulations in the weakly
non-linear regime (i.e, when the variance, $\sigma^2$, is small). We compare
both expansions to N-body simulations and find that the Gamma expansion yields
a better overall match to the numerical results.
