Entanglement of 2 x K quantum systems

We derive an analytical expression for the lower bound of the concurrence of mixed quantum states of composite 2 x K systems. In contrast to other, implicitly defined entanglement measures, the numerical evaluation of our bound is straightforward. We explicitly evaluate its tightness for general mixed states of 2 x 3 systems, and identify a large class of states where our expression gives the exact value of the concurrence.