A necessary and sufficient criterion for multipartite separable states
  We present a necessary and sufficient condition for the separability of multipartite quantum states, this criterion also tells us how to write a multipartite separable state as a convex sum of separable pure states. To work out this criterion, we need to solve a set of equations, actually it is easy to solve these quations analytically if the density matrix of the given quantum state has few nonzero eigenvalues. 
Shengjun Wu, Xuemei Chen, Yongde Zhang