Bell Inequalities Classifying Bi-separable Three-qubit States
  We present a set of Bell inequalities that gives rise to a finer classification of the entanglement for tripartite systems. These inequalities distinguish three possible bi-separable entanglements for three-qubit states. The three Bell operators we employed constitute an external sphere of the separable cube. 
Bao-Zhi Sun, Shao-Ming Fei