= Renormalization-group fixed point
A renormalization-group fixed point is a set of dimensionless couplings unchanged by the stated coarse-graining and rescaling transformation. Its beta functions vanish, and its long-distance description has scale-invariant correlation laws.
Linearizing the <renormalization-group flow> around $g_*$ gives $d\delta g_i/d\ell=\sum_jM_{ij}\delta g_j$. With $\ell$ increasing toward long distances, positive eigenvalues identify perturbations that grow, negative ones identify perturbations that decay, and zero eigenvalues require nonlinear analysis. A physical critical system must tune its relevant perturbations appropriately. The fixed point therefore describes universal critical behavior, while the stability directions describe how to reach or leave it. A zero of a beta function alone does not specify whether it attracts infrared or ultraviolet flows.
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