Remarks on 2-q-bit states
  We distinguish six classes of families of locally equivalent states in a straightforward scheme for classifying all 2-q-bit states; four of the classes consist of two subclasses each. The simple criteria that we stated recently for checking a given state's positivity and separability are justified, and we discuss some important properties of Lewenstein-Sanpera decompositions. An upper bound is conjectured for the sum of the degree of separability of a 2-q-bit state and its concurrence. 
Berthold-Georg Englert and Nasser Metwally