  Hamilton et al. have suggested an invaluable scaling formula which describes
how the power spectra of density fluctuations evolve into the nonlinear regime
of hierarchical clustering. This paper presents an extension of their method to
low-density universes and universes with nonzero cosmological constant. We pay
particular attention to models with large negative spectral indices, and give a
spectrum-dependent fitting formula which is of significantly improved accuracy
by comparison with an earlier version of this work. The tendency of nonlinear
effects to increase power on small scales is stronger for spectra with more
negative spectral indices, and for lower densities. However, for low-density
models with a cosmological constant, the nonlinear effects are less strong than
for an open universe of the same $\Omega$.
