Correlation Function of the Spin-1/2 XXX Antiferromagnet
  We consider a special correlation function in the isotropic spin-$\half$ Heisenberg antiferromagnet. It is the probability of finding a ferromagnetic string of (adjacent) spins in the antiferromagnetic ground state. We give two different representations for this correlation function. Both of them are exact at any distance, but one becomes more effective for the description of long distance behaviour, the other for the description of short distance behaviour. 
Vladimir E. Korepin, Anatoli G. Izergin, Fabian H.L. Essler and Denis   B. Uglov