Modification of weakly nonlinear mode couplings due to the periodic boundary condition

The periodic boundary condition is usually imposed on cosmological N-body s imulations. We investigate effects of this artificial condition on the weak ly nonlinear mode couplings induced by gravity at scales comparable to the side length of a simulation box. Using the framework of the higher order Eu lerian perturbation theory, we examine modifications of the skewness parame ters and the second-order power spectra for power-law initial fluctuations. We show quantitatively that these modifications depend on the initial spec tral index as well as the spatial scab relative to the side length.