An explicit formula for the polynomial entanglement measures of degree 2   of even-N qubits mixed states
  Characterization of the multipartite mixed state entanglement is still a challenging problem. Since due to the fact that the entanglement for the mixed states, in general, is defined by a convex-roof extension. That is the entanglement measure of a mixed state \r{ho} of a quantum system can be defined as the minimum average entanglement of an ensemble of pure states. In this Letter, we give an explicit formula for the polynomial entanglement measures of degree 2 of even-N qubits mixed states that is similar to Wooters formula in [1]. Then we discuss our findings in the framework of X density matrices and show that our formula for this type of density matrices is in the full agreement with the genuine multipartite (GM) entanglement of these states. 
M.A. Jafarizadeh, M. Yahyavi, A.Heshmati, N.Karimi, A. Mohamadzadeh,   F. Eghbalifam and S. Nami
quant-ph