  By taking advantage of a stability criterion established recently, the
critical temperature $T_c$ is reckoned with help of the microscopic parameters,
characterising the normal and superconducting electrons, namely the
independent-electron band structure and a \textit{repulsive} two-electron
force. The emphasis is laid on the sharp $T_c$ dependence upon electron
concentration and inter-electron coupling, which might offer a practical route
toward higher $T_c$ values and help to understand why high-$T_c$ compounds
exhibit such remarkable properties.
