2-LOOP EFFECTIVE ACTION FOR THEORIES WITH FERMIONS

On the basis of a new approach for the calculation of the effective ac tion developed in our previous works we calculate the effective action (up to two-loop level) for some models containing fermion fields. Thi s method allows us to calculate the fermionic part of the effective ac tion properly. The two-loop contribution to the effective potential fo r the Nambu-Jona-Lasinio model is calculated and is shown to vanish. ( C) 1995 Academic Press, Inc.