Density matrix of a finite sub-chain of the Heisenberg anti-ferromagnet
  We consider a finite sub-chain on an interval of the infinite XXX model in the ground state. The density matrix for such a subsystem was described in our previous works for the model with inhomogeneous spectral parameters. In the present paper, we give a compact formula for the physically interesting case of the homogeneous model. 
H. Boos, M. Jimbo, T. Miwa, F. Smirnov, Y. Takeyama