Duality in the Kondo model and perturbative approach to strong coupling theory

We investigate the duality in the Kondo model. Starting from the s-d model with the coupling constant $J$, the strong coupling model with the constant of 1/J is identified. The model shows the unitary limit of the conductance, $G=2 e^{2}/h$ at zero temperature. The perturbation theory of the model gives qualitative agreement with the results of numerical renormalization group and Bethe ansatz near $T=T_{K}$.