  We recall the fundamental fact that Cooper pairs defined without ignoring
two- hole pairs along with two-particle ones leads to a purely imaginary pair
energy when the problem is based on the ideal Fermi gas sea. However, bound
finite- lifetime pairs are recovered when it is based on the BCS ground-state
Fermi sea. Their excitation energy is gapped at zero total momenta and then
rises linearly in either 2D or 3D, making them similar to 3D plasmons which
however rise quadratically.
