  Fluctuations of conserved quantum numbers are associated with the
corresponding susceptibilities because of the symmetry of the system. The
underlying fact is that these fluctuations as defined through the static
correlators become identical to the direct calculation of these
susceptibilities defined through the thermodynamic derivatives, due to the
fluctuation-dissipation theorem. Through a rigorous exercise we explicitly show
that a diagrammatic calculation of the static correlators associated with the
conserved quark number fluctuations and the corresponding susceptibilities are
possible in case of mean field theories, if the implicit dependence of the mean
fields on the quark chemical potential are taken into account appropriately. As
an aside we also give an analytical prescription for obtaining the implicit
dependence of the mean fields on the quark chemical potential.
