Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 303 / 2 / b / ii / Solution

For $d<4$, $y_g=4-d>0$, so the quartic coupling is a relevant direction of a fixed point. Generic Ising-like systems therefore flow away from the Gaussian point toward the interacting Wilson-Fisher fixed point, which controls the Ising universality class below four dimensions.

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