2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/198184We analyze the way in which duality constrains the exact beta function and correlation length in single-coupling spin systems. A consistency condition we propose shows very concisely the relation between self-dual points and phase transitions, and implies that the correlation length must be duality invariant. These ideas are then tested on the 2-d Ising model, and used towards finding the exact beta function of the $q$-state Potts model. Finally, a generic procedure is given for identifying a duality symmetry in other single-coupling models with a continuous phase transition.LaTeX, 6 pagesCondensed MatterHigh Energy Physics - LatticeHigh Energy Physics - Theorytext