  We assume that the most important quark correlations are pairwise at all
baryon densities. We introduce correlated pairs by means of Bogoliubov
transformations which are functions of time and spatial gauge fields, in the
formalism of the transfer matrix with lattice regularization. The dependence on
time and gauge fields allows us to enforce gauge invariance and other
symmetries term by term in the transformed quantities. The resulting action
should be suitable for the description of multiquark mesons and baryons as
states of a quark and a diquark. We derive the quark contribution to the free
energy at finite chemical potential in a certain approximation. Its expression
cannot be evaluated analytically, but it has a definite sign.
