| Abstract
| - For a completely general anisotropic order parameter (including changes of sign), we show that weak-coupling theory is incompatible with high values of the maximum $\Delta _{\rm M} $ of the zero-temperature gap as compared to the critical temperature Tc, such as those found experimentally in $\rm Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta }$ where $2 \Delta _{\rm M} /T_{\rm c } \approx 7.5$. This gives evidence for strong-coupling effects. In particular this comes as a major support for a spin fluctuation mechanism with strong coupling, if one assumes that only a repulsive pairing interaction is at work in high- $T_{\rm c } $ superconductors.
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