where η η is a critical exponent, which does not have any simple relation with the non-critical exponent ϑ ϑ introduced above. For example, the exact solution of the two-dimensional Ising model (with short-ranged ferromagnetic interactions) gives precisely at criticality η η = 1 4 , but above criticality ϑ ϑ = 1 2 and below criticality ϑ ϑ = 2 .