where k → → is the comoving wavenumber and the hats denote Fourier transforms. Since g → → = − − ∇ ∇ → → Φ Φ , the gravitational field can now be found by multiplying by − − i k → → and computing the inverse Fourier transform (or computing the inverse transform and then using some other method). Since this method is limited by the mesh size, in practice a smaller mesh or some other technique (such as combining with a tree or simple particle-particle algorithm) is used to compute the small-scale forces. Sometimes an adaptive mesh is used, in which the mesh cells are much smaller in the denser regions of the simulation.