The entanglement criterion of N qubit system

In previous paper (Int.J.Mod.Phys.B. 33 (18):1950197(2019); 34 (5):2050022(2020)),we present a method to judge the entanglement of 2 qubit system.The necessary and sufficient conditions for the 2 qubit system being separable are presented by the involved coefficients being positive and the principal density matrix being separable,then the system is separable,otherwise the system is entangled.Now in this paper,we try to generalize this method to the entanglement criterion of 3 qubit system,and then,we furthermore generalize 3 qubit system to N qubit system.This is a complicated and interesting problem.