  Extending the BCS theory towards the strong electron-phonon interaction
(EPI), a charged Bose liquid of small bipolarons has been predicted by us with
a further prediction that the highest superconducting critical temperature is
found in the crossover region of the EPI strength from the BCS-like to
bipolaronic superconductivity. Later on we have shown that the unscreened
(infinite-range) Froehlich EPI combined with the strong Coulomb repulsion
create \emph{superlight} small bipolarons, which are several orders of
magnitude lighter than small bipolarons in the Holstein-Hubbard model (HHM)
with a zero-range EPI. The analytical and numerical studies of this
Coulomb-Froehlich model (CFM) provide the following recipes for
room-temperature superconductivity: (a) The parent compound should be an ionic
insulator with light ions to form high-frequency optical phonons, (b) The
structure should be quasi two-dimensional to ensure poor screening of
high-frequency phonons polarized perpendicular to the conducting planes, (c) A
triangular lattice is required in combination with strong, on-site
  Coulomb repulsion to form the small superlight bipolaron, (d) Moderate
carrier densities are required to keep the system of small bipolarons close to
the Bose-Einstein condensation regime. Clearly most of these conditions are
already met in the cuprates.
