Quantum field theory and the two-dimensional Ising model

A relationship between the two-point correlation function of the Ising model in the critical region and the $〈sin\sqrt{\ensuremath{\pi}}\ensuremath{\varphi}(0)sin\sqrt{\ensuremath{\pi}}\ensuremath{\varphi}(\ensuremath{\rho})〉$ Green's function of a sine-Gordon field is constructed. This relationship is tested by means of a mass perturbation expansion with an infrared cutoff and shown to agree, at the leading-logarithm approximation and up to the second order, with the exact result of the Ising model.