Ratio between π+π−-atom and free π+π− pair production rates with account of the strong interaction in final states

The ratio of π+π−-atom production rate to free π+π− pairs is obtained with account of the strong interaction in final states in the frame of the perturbation theory. It is shown that this ratio is expressed via a squared ratio of the well-known Coulomb wave functions and thus can be calculated with a high accuracy.