Meson contributions in the Nambu–Jona-Lasinio model

We discuss the problem of calculating corrections to the mean-field approximation in the Nambu–Jona-Lasinio model. To calculate such corrections, we propose using the method of the Legendre transformation with respect to a bilocal source, which allows taking symmetry constraints related to the chiral Ward identity into account effectively. Using the proposed method, we determine the corrections to the quark propagator and the two-particle quark function.