  We study response of liquid to a scale transformation, which generates a
change of the liquid density, and obtain a set of differential equations for
correlation functions. The set of equations, which we call density
renormalization group equations (DRGEs), is similar to the BBGKY hierarchy as
it relates different multiple-point correlation functions. In particular, we
derive DRGEs for one-particle irreducible vertex functions of liquid by
performing Legendre transformations, which enables us to calculate properties
of liquid at higher density in terms of correlation functions at lower density.
