The axial anomaly and the conversion of gluons into photons
  The axial anomaly in the divergence of the singlet axial current in QCD $+$ QED leads to low-energy theorems for the matrix element of this operator equation over vacuum and two--photon states and for the matrix element over vacuum and two--gluon states. The solution of these theorems is related only to the nonperturbative phenomena. These matrix elements are calculated in instanton vacuum generated N-JL type quark model for arbitrary $N_f .$ It is shown that this model does satisfy the low-energy theorems. 
M.M. Musakhanov, F. C. Khanna