Nonperturbative vertex corrections for superconductivity in the Frohlich model

A nonperturbative calculation of the vertex corrections for the electron-ph onon Frohlich model beyond the Migdal regime is performed within a modified version of the conventional gauge technique. We find that they show tenden cy to depress the critical temperature and other relevant quantities increa sing the Migdal parameter nr. Moreover, a comparison with the results recen tly obtained for the BCS pairing model using the same method suggests that, varying the coupling constant lambda at fixed m, the sign of the vertex co rrections may depend sensibly on the effective pairing interaction between the charge carriers. (C) 2000 Published by Elsevier Science B.V. All rights reserved.