  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
