Lower Bound of Concurrence for Qubit Systems
  We study the concurrence of four-qubit quantum states and provide analytical lower bounds of concurrence in terms of the monogamy inequality of concurrence for qubit systems. It is shown that these lower bounds are able to improve the existing bounds and detect entanglement better. The approach is generalized to arbitrary qubit systems. 
Xue-Na Zhu, Shao-Ming Fei
quant-ph