A Simplified and Obvious Expression of Concurrence in Wootters‘ Measure of Entanglement of a Pair of Qubits

We obtain a simplified and obvious expression of“concurrence” in Wootters‘ measure of entanglement of a pair of qubits that have no more than two non-zero eigenvalues in terms of the concurrences of eigenstates and theirsimple combinations. It not only simplifies the calculation of Wootters‘ measure of entanglement, but also revealssome general and important features. Our conclusions are helpful for the further understanding and applicationof quantum entanglement.