Reduction of multipartite qubit density matrixes to bipartite qubit   density matrixes and criteria of partial separability of multipartite qubit   density matrixes
  The partial separability of multipartite qubit density matrixes is strictly defined. We give a reduction way from N-partite qubit density matrixes to bipartite qubit density matrixes, and prove a necessary condition that a N-partite qubit density matrix to be partially separable is its reduced density matrix to satisfy PPT condition. 
Zai-Zhe Zhong