One-point functions in integrable quantum field theory at finite temperature

We determine the form factor expansion of the one-point functions in integr able quantum field theory at finite temperature and find that it is simpler than previously conjectured. We show that no singularities are left in the final expression provided that the operator is local with respect to the p articles and argue that the divergences arising in the non-local case are r elated to the absence of spontaneous symmetry breaking on the cylinder. As a specific application, we give the first terms of the low-temperature expa nsion of the one-point functions for the Ising model in a magnetic field.