Emptiness formation probability of the XXZ spin-1/2 Heisenberg chain at   Delta=1/2
  Using a multiple integral representation for the correlation functions, we compute the emptiness formation probability of the XXZ spin-1/2 Heisenberg chain at anisotropy Delta=1/2. We prove it is expressed in term of the number of alternating sign matrices. 
N. Kitanine, J.M. Maillet, N.A. Slavnov, V. Terras