 This method indicated that, as temperature is decreased, the effective coupling between the spin and the band, formula_5, increases without limit.As this method is perturbative in J, it becomes invalid when J becomes large, so this method did not truly solve the Kondo problem, although it did hint at the way forward.The Kondo problem was finally solved when Kenneth Wilson applied the numerical renormalization group to the Kondo model and showed that the resistivity goes to a constant as temperature goes to zero.