THE GAP OVER CRITICAL-TEMPERATURE RATIO 2-DELTA T-C IN ELIASHBERG THEORY/

We point out that, in the calculation of 2 Delta/T-c from Eliashberg t heory of superconductivity, all the boson spectra can be mapped system atically on the subset of two Einstein peaks spectra which, with prope r choice of the parameters, give within of a few percent the same resu lt as the original spectra. This allows to extract from 2 Delta/T-c th e maximum information on the boson spectrum. We classify all the possi ble outcome and show that 2 Delta/T-c is strongly dependent on the spe ctral shape. We apply our results to YBa2Cu3O7.