SYSTEMATIC ANALYSIS OF THE RATIO 2-DELTA TC FROM ELIASHBERG THEORY/

In the calculation of 2DELTA/T(c) from Eliashberg theory, it is possib le to map all the boson spectra on the subset of two Einstein peaks sp ectra. Indeed they give within of a few percent the same result as the original spectra provided a proper choice of the parameters is made. This leads to a systematic analysis of this ratio and a classification of all the possible outcome. In particular one can extract in this wa y the maximum information on the boson spectrum from the knowledge of 2DELTA/T(c). We make a specific application of our results to YBa2Cu3O 7 and to Rb3C60.