  The replica method for a quenched disordered system is considered in a
perturbative field theory. Since correction in a finite-order perturbation is
given in a polynomial of the replica number $n$, the zero-replica limit $n \to
0$ is regarded as extracting the constant term from the polynomial, which
mathematically makes sense. The meaning of the extraction is clarified
comparing with a direct calculation.

Disordered Systems and Neural Networks;Statistical Mechanics