%0 Journal Article %T Modulational and Parametric Instabilities of the Discrete Nonlinear Schr£żdinger Equation %A Z. Rapti %A P. G. Kevrekidis %A A. Smerzi %A A. R. Bishop %J Physics %D 2004 %I arXiv %R 10.1088/0953-4075/37/7/070 %X We examine the modulational and parametric instabilities arising in a non-autonomous, discrete nonlinear Schr{\"o}dinger equation setting. The principal motivation for our study stems from the dynamics of Bose-Einstein condensates trapped in a deep optical lattice. We find that under periodic variations of the heights of the interwell barriers (or equivalently of the scattering length), additionally to the modulational instability, a window of parametric instability becomes available to the system. We explore this instability through multiple-scale analysis and identify it numerically. Its principal dynamical characteristic is that, typically, it develops over much larger times than the modulational instability, a feature that is qualitatively justified by comparison of the corresponding instability growth rates. %U http://arxiv.org/abs/cond-mat/0404600v1