A lower bound of concurrence for multipartite quantum systems

We present a lower bound of concurrence for four-partite systems in terms of the concurrence for M (2 <= M <= 3) part quantum systems and give an analytical lower bound for 2 circle times 2 circle times 2 circle times 2 mixed quantum sates. It is shown that these lower bounds are able to improve the existing bounds and detect entanglement better. Furthermore, our approach can be generalized to multipartite quantum systems.