Exact Sum Rules at Finite Temperature and Chemical Potential and Their   Application to QCD
  Within the framework of the operator product expansion (OPE) and the renormalization group equation (RGE), we show that the temperature and chemical potential dependence of the zeroth moment of a spectral function (SF) is completely determined by the one-loop structure of an asymptotically free theory. This exact result constrains the shape of SF's, and implies a highly non-trivial functional form for the SF near second order, or weak first order, phase transitions. Phenomenological parameterizations of the SF, often used in applications such as the analysis of lattice QCD data or QCD sum rule calculations at finite temperature and baryon density, must satisfy these constraints. 
Suzhou HUANG and Marcello LISSIA