  The Bogoliubov approach to superconductivity provides a strong mathematical
support to the wave function ansatz proposed by Bardeen, Cooper and Schrieffer
(BCS). Indeed, this ansatz --- with all pairs condensed into the same state ---
corresponds to the ground state of the Bogoliubov Hamiltonian. Yet, this
Hamiltonian only is part of the BCS Hamiltonian. As a result, the BCS ansatz
definitely differs from the BCS Hamiltonian ground state. This can be directly
shown either through a perturbative approach starting from the Bogoliubov
Hamiltonian, or better by analytically solving the BCS Schr\"{o}dinger equation
along Richardson-Gaudin exact procedure. Still, the BCS ansatz leads not only
to the correct extensive part of the ground state energy for an arbitrary
number of pairs in the energy layer where the potential acts --- as recently
obtained by solving Richardson-Gaudin equations analytically --- but also to a
few other physical quantities such as the electron distribution, as here shown.
The present work also considers arbitrary filling of the potential layer and
evidences the existence of a super dilute and a super dense regime of pairs,
with a gap \emph{different} from the usual gap. These regimes constitute the
lower and upper limits of density-induced BEC-BCS cross-over in Cooper pair
systems.
