Repulsion in an instanton gas, formation of a core in the nucleon, and the method of Bogolyubov hierarchies

The effective action of the instanton gas in QCD with allowance for repulsion between the instantons is found. This repulsion leads to the formation of a repulsive core of the constituent quarks. The structure of the effective action is similar to an operator expansion. The method of Bogolyubov hierarchies is used to find the coefficient functions.