Entanglement or separability: the choice of how to factorize the algebra of a density matrix W. Thirring, R.A. Bertlmann, P. Köhler and H. Narnhofer Eur. Phys. J. D, 64 2-3 (2011) 181-196 Published online: 22 September 2011 DOI: 10.1140/epjd/e2011-20452-1