Dynamic response function in Ising systems below T-c

With the aid of Nelson's method we derive expressions for the frequency-, momentum-, and temperature-dependent response function below a critical point in the Ising-type system. The scaling function is given within the renormalization-group formalism at one-loop order for zero external field. The comparison with the corresponding expressions above T-c and with the mean-field approximation is made. We also discuss the dynamic correlation function, focusing on the deviations from the Gaussian expression. The comparison with field-theoretical calculations is also given.