Characteristic solution of the massless Callan-Symanzik equation ($G_n(\lambda (\mu ),\mu )=e^{-n{\int }_{{\mu }_0}^{\mu }{\gamma }_{\varphi }(\lambda (\nu ))d\nu /\nu }{G}_n({\lambda }_0,{\mu }_0)$)

For fixed external momenta, the massless Callan-Symanzik equation is $(\mu {\partial }_{\mu }+\beta (\lambda ){\partial }_{\lambda }+n{\gamma }_{\varphi })G_n=0$. Choose a running coupling satisfying $d\lambda /d\log \mu =\beta (\lambda )$ and $\lambda ({\mu }_0)={\lambda }_0$. The chain rule reduces the equation along this curve to $d{G}_n/d\log \mu =-n{\gamma }_{\varphi }{G}_n$, whose integrating factor gives the displayed solution. The reference correlation function at ${\mu }_0$ supplies boundary data; the renormalization group alone does not determine these data.

If $\beta =b{\lambda }^2$, then $\lambda (\mu )={\lambda }_0/[1-b{\lambda }_0\log (\mu /{\mu }_0)]$. If in addition the anomalous dimension is ${\gamma }_{\varphi }=c{\lambda }^2$, the integral in the exponent is $(c/b)(\lambda (\mu )-{\lambda }_0)$. These formulas use the stipulated truncated renormalization group functions and apply on a branch on which the running solution remains defined.