Improvement of perturbation theory in QCD for e^+e^- -> hadrons and the problem of α_s freezing

We develope the method of improvement of perturbative theory in QCD, applied to any polarization operator. The case of polarization operator \Pi(q^2), corresponding to the process e^+e^- -> hadrons is considered in details. Using the analytical properties of \Pi(q^2) and perturbative expansion of \Pi(q^2) at q^2<0, Im\Pi(q^2) at q^2>0 is determined in such a way, that the infared pole is eliminated. The convergence of perturbative series for R(q^2)=\sigma(e^+e^- -> hadrons)/(e^+e^- -> \mu^+\mu^-) is improved. After substitution of R(q^2) into dispersion relation the improved Adler function D(q^2) with no infrared pole and frozen \alpha_s(q^2) has been obtained. A good agreement with experiment has been achieved.