  We propose a compact perturbative approach that reveals the physical origin
of the singularity occurring in the density dependence of correlation energy:
like fermions, elementary bosons have a singular correlation energy which comes
from the accumulation, through Feynman "bubble" diagrams, of the same non-zero
momentum transfer excitations from the free particle ground state, that is, the
Fermi sea for fermions and the Bose-Einstein condensate for bosons. This
understanding paves the way toward deriving the correlation energy of composite
bosons like atomic dimers and semiconductor excitons, by suggesting Shiva
diagrams that have similarity with Feynman "bubble" diagrams, the previous
elementary boson approaches, which hide this physics, being difficult to do so.
