A dispersion-theory approach provides an exact representation of the two-point correlation function $G(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}},t)=C{t}^{\ensuremath{-}\ensuremath{\gamma}}g({q}^{2}{\ensuremath{\xi}}^{2})$, where $t=\frac{(T\ensuremath{-}{T}_{c})}{{T}_{c}}$ and $\ensuremath{\xi}$ is the correlation length, which describes the critical scattering near a second-order phase transition. The threshold property of the spectral weight function $F(\frac{x}{3})\ensuremath{\propto}\mathrm{Im}{g}^{\ensuremath{-}1}(\ensuremath{-}{x}^{2})$ is used to motivate an approximation to the scaling function
g(x2) based on the asymptotically exact Fisher-Langer approximant ${g}_{\mathrm{FL}}({x}^{2})=(\frac{{C}_{1}}{{x}^{2\ensuremath{-}\ensuremath{\eta}}})(1+\frac{{C}_{2}}{{x}^{\frac{(1\ensuremath{-}\ensuremath{\alpha})}{\ensuremath{\nu}}}}+\frac{{C}_{3}}{{x}^{\frac{1}{\ensuremath{\nu}}}})$, where $\ensuremath{\alpha}$, $\ensuremath{\eta}$, $\ensuremath{\nu}$ are the usual critical exponents. The approximation consists of truncating the spectral function associated with ${g}_{\mathrm{FL}}^{\ensuremath{-}1}({x}^{2})$ in a manner designed to simulate the known threshold property of the exact spectral function, namely
F(3x)=0 for $x\ensuremath{\le}3$. The new approximant is checked on the two-dimensional Ising model and shown to agree with the exact result to better than 0.03% for all
x. Near four dimensions, the agreement with exact results is also excellent. A phenomenological approach to the intermediate three-dimensional case is presented, and shown to be in good agreement with high-temperature series and $\ensuremath{\epsilon}$-expansion results. The dispersion-theory approach is also shown to provide a convenient framework for calculations within the $\ensuremath{\epsilon}$ expansion and is used to compute
g(x2) to $O({\ensuremath{\epsilon}}^{3})$, a new result.
Some exact results are derived relating quantities computable by the so-called damage spreading method and thermodynamic functions for the N -color Ashkin–Teller model. The results are valid for any ergodic dynamics. Since we restrict our analysis to the ferromagnetic case, the results are also valid for any translationally invariant lattice. The relations derived can be used in order to determine the N -color Ashkin–Teller critical exponents numerically, providing better accuracy and requiring less computational effort than standard Monte Carlo simulations.