A useful strong lower bound on two-qubit concurrence

A new strong lower bound on concurrence for two-qubit states is derived. Its equality with the concurrence itself for the pure- and X-states is proved analytically; while extensive numerical computations show that equality for a general mixed state may also exist. Being a very simple function and easy to calculate, it is more convenient and practical than the exact value in some cases, including entanglement investigations in spin chains. We study thermal localizable entanglement in spin chains as an example, to demonstrate the convenience of this bound.