theorem order_parameter_exponent_one (J : ℕ) (ρ T Tstar : ℝ)
(hTs : 0 < Tstar) (hT : T < Tstar) :
effectiveCurvatureKeff J ρ T Tstar =
curvatureK J ρ * ((Tstar - T) / Tstar) := by
rw [keff_below_critical J ρ T Tstar hTs hT]
congr 1; field_simpPhase Transition at T* (Gap #8)