The Correlator of Topological Charge Densities at low $Q^2$ in QCD
  The correlator of topological charge densities $\chi(Q^2)$ in QCD is calculated in the domain of low $Q^2$. Basing on the Ward identities the low energy theorems are proved. The value of the first derivative $\chi^{\prime}(0)$ found recently by QCD sum rules \cite{1} is used. The contributions of pseudoscalar quasi-Goldstone bosons as intermediate states in the correlator are calculated with the account of $\eta-\pi$ mixing. 
B.L.Ioffe