THE SPIN-SPIN CORRELATION-FUNCTION IN THE 2-DIMENSIONAL ISING-MODEL IN A MAGNETIC-FIELD AT TAU=TAU(C)

The form-factor bootstrap approach is used to compute the exact contributions in the large-distance expansion of the correlation function (sigma(x)sigma(0)) of the two-dimensional Ising model in a magnetic field at T = T-c, The matrix elements of the magnetization operator sigma(x) present a rich analytic structure induced by the (multi-) scattering processes of the eight massive particles of the model. The spectral representation series has a fast rate of convergence and perfectly agrees with the numerical determination of the correlation function