Let a tilde represent the change in thermodynamic variables when , , and are all increased by the same amount , i.e.
The nicest feature of an extensive system is that the number of parameters required for a complete specification of the thermodynamic state can be reduced by one, and in such a way that only intensive thermodynamic variables remain. To see this, let , in which case
We can think of a given relation as the equation of state, to be determined in the flat, tangent space at each point of the manifold, or, physically, small enough patches across which the changes in the gravitational field are negligible, but also large enough to contain a large number of particles. For example, for a neutron star Glendenning  has reasoned that the relative change in the metric over the size of a nucleon with respect to the change over the entire star is about , and thus one must consider many internucleon spacings before a substantial change in the metric occurs. In other words, it is sufficient to determine the properties of matter in special relativity, neglecting effects due to spacetime curvature. The equation of state is the major link between the microphysics that governs the local fluid behavior and global quantities (such as the mass and radius of a star).
In what follows we will use a thermodynamic formulation that satisfies the fundamental scaling relation, meaning that the local thermodynamic state (modulo entrainment, see later) is a function of the variables , , etc. This is in contrast to the fluid formulation of “MTW” . In their approach one fixes from the outset the total number of particles , meaning that one simply sets in the first law of thermodynamics. Thus without imposing any scaling relation, one can write
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