A lower bound of concurrence for multipartite quantum systems. (arXiv:1801.00078v1 [quant-ph])
We present a lower bound of concurrence for four-partite systems in terms of
the concurrence for $M\, (2\leq M\leq 3)$ part quantum systems and give an
analytical lower bound for $2\otimes2\otimes2\otimes2$ 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.