The Influence of Strong Interaction on the Pionium Wave Functions at   Small Distances
  The influence of strong $\pi^+\pi^-$ interaction of the behaviour of pionium nS-state wave functions at small distance are investigated both analytically (perturbatively) and so numerically. It is shown that in the whole the accounting of strong interaction results in multiplying of pure Coulomb pionium wave functions by some function practically independent on value of principal quantum number n. Due to this reason the n-independence of probability of $\pi^+\pi^-$ atom production in nS-state remains the same as in case of pure Coulomb $\pi^+\pi^-$ interaction 
I. Amirkhanov, I. Puzynin, A. Tarasov, O. Voskresenskaya and O.   Zeinalova