2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/45247Within 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) in an asymptotically free theory is completely determined by the one-loop structure of the theory. This exact result constrains the qualitative shape of SF's, and implies striking phenomenological effects near phase transitions.5 pages using world_sci.sty, which is appended to the file. Full postscript version available via anonymous ftp from risc0.ca.infn.it:/pub/private/lissia/infnca-th-94-18.ps (192.84.132.4) To appear in the Proceedings of the ``Workshop on Quantum Infrared Physics'', Paris, France, 6-10 June 1994High Energy Physics - PhenomenologyHigh Energy Physics - LatticeNuclear Theorytext