Renormalized perturbation theory flow equations for the Anderson impurity model

We apply the renormalized perturbation theory (RPT) to the symmetric Anderson impurity model. Within the RPT framework exact results for physical observables such as the spin and charge susceptibility can be obtained in terms of the renormalized values $\tilde{\boldsymbol\mu} = (\tilde{\Delta}, \tilde{U})$ of the hybridization $\Delta$ and Coulomb interaction $U$ of the model. The main difficulty in the RPT approach usually lies in the calculation of the renormalized values themselves. In the present work we show how this can be accomplished by deriving differential flow equations describing the evolution of $\tilde{\boldsymbol \mu} (\Delta)$ with $\Delta$. By exploiting the fact that $\tilde{\boldsymbol \mu} (\Delta)$ can be determined analytically in the limit $\Delta \rightarrow \infty$ we solve the flow equations numerically to obtain estimates for the renormalized parameters in the range $0<U/\pi\Delta<3.5$.