Central Charge and Universal Combinations of Amplitudes in Two-Dimensional Theories Away from Criticality

For a general isotropic two-dimensional theory near criticality, the universal singular free energy per correlation volume fₛξ² is equal to -(c/12π)(2-α)(1-α)^-1, where c is the central charge of the theory at criticality, α is the usual specific-heat exponent, and ξ is defined (for α>0) in terms of the second moment of the energy correlations. Some generalizations of this result are also noted.