Superconductors weak-coupling limit

As the mechanism of superconductivity in these doped fullerites was not clear, the greatest attention was directed to the standard mechanism of superconductivity. The conventional theory is based on the electron-phonon interaction and generalized in the Eliashberg equations which include the Bardeen, Cooper and Schrieffer (BCS) equation as a weak-coupling limit [78]. For real superconductors the ratio for the weak-coupling limit is close to the value of 3.52, and for stronger coupling materials it increases up to about 5.1... 

In the so called weak coupling limit VogF 1 which is justified a posteriori since it is reasonably obeyed by classical superconductors, the above expression reduces to... 

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