  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.
