Entanglement witnesses and characterizing entanglement properties of some PPT states M. A. Jafarizadeh, N. Behzadi and Y. Akbari Eur. Phys. J. D, 55 1 (2009) 197-203 Published online: 23 June 2009 DOI: 10.1140/epjd/e2009-00176-7