On PPT States in KxMxN Composite Quantum Systems
  We study the general representations of positive partial transpose (PPT) states in ${\cal C}^K \otimes {\cal C}^M \otimes {\cal C}^N$. For the PPT states with rank-$N$ a canonical form is obtained, from which a sufficient separability condition is presented. 
Xiao-Hong Wang, Shao-Ming Fei, Zhi-Xi Wang and Ke Wu