C-theorem
In quantum field theory, the C-theorem states that there exists a positive real function, C ( g i , μ ) {\displaystyle C(g_{i}^{},\mu )} , depending on the coupling constants of the quantum field theory considered, g i {\displaystyle g_{i}^{}} , and on the energy scale, μ {\displaystyle \mu _{}^{}} , which has the following properties: C ( g i , μ ) {\displaystyle C(g_{i}^{},\mu )} decreases monotonically under the renormalization group (RG) flow. At fixed points of the RG flow, which are specified by a set of fixed-point couplings g i ∗ {\displaystyle g_{i}^{*}} , the function C ( g i ∗ , μ ) = C ∗ {\displaystyle C(g_{i}^{*},\mu )=C_{*}} is a constant, independent of energy scale.