Breakdown of Zero--Mode Approximation in the Instanton Vacuum
  A new approach to effective theory of quarks in the instanton vacuum is presented. Exact equations for the quark propagator and Lagrangian are derived which contain contributions of all quark modes with known coefficients. The resulting effective Lagrangian resembles that of the Nambu--Jona--Lasinio model. The commonly used zero--mode approximation is shown to be invalid in the chiral limit. 
B. O. Kerbikov, D. S. Kuzmenko and Yu. A. Simonov (ITEP, Moscow)