Description of rank four PPT entangled states of two qutrits
  It is known that some two qutrit entangled states of rank 4 with positive partial transpose [PPT] can be built from the unextendible product bases [UPB]. We show that this fact is indeed universal, namely all such states can be constructed from UPB. We also classify the 5-dimensional subspaces of two qutrits which contain only finitely many product states (up to scalar multiple), and in particular those spanned by a UPB. 
Lin Chen and Dragomir Z. Djokovic
quant-ph math-ph math.MP