A Class of Special Matrices and Quantum Entanglement
  We present a kind of construction for a class of special matrices with at most two different eigenvalues, in terms of some interesting multiplicators which are very useful in calculating eigenvalue polynomials of these matrices. This class of matrices defines a special kind of quantum states -- $d$-computable states. The entanglement of formation for a large class of quantum mixed states is explicitly presented. 
Shao-Ming Fei and Xianqing Li-Jost