Nonperturbative effects from the resummation of perturbation theory

Using the general argument in Borel resummation of perturbation theory that links the divergent perturbation theory to the nonperturbative effect I argue that the nonperturbative effect associated with the perturbation theory should have a branch cut only along the positive real axis in the complex coupling plane. The component in the weak coupling expansion of the nonperturbative amplitude that gives rise to the branch cut can be calculated in principle from the perturbation theory combined with the exactly calculable properties of the nonperturbative effect. The realization of this mechanism is demonstrated in the double well potential and the two-dimensional O(N) nonlinear sigma model. In these models the leading term in the weak coupling expansion of the nonperturbative effect can be obtained with a good accuracy from the first terms of perturbation theory. Applying this mechanism to the infrared renormalon induced nonperturbative effect in QCD, I suggest some of the QCD condensate effects can be calculated in principle from the perturbation theory.