A number of relations are obtained connecting the critical exponents of the Ashkin-Teller model. For the special case of the four-state Potts model these relations predict alpha = 2 / 3 , beta = 1 / 12 , gamma = 7 / 6 , delta =15. Series estimates are in agreement with these predictions.
For pt. II, see abstr. A64455 of 1973. The critical-point exponents of two Ising models with pure three-spin coupling terms are examined using low-temperature (T<T c ) series expansions for the thermodynamic functions. In particular the critical isotherm exponent delta is examined by applying numerical extrapolation methods to the critical isotherm. The two models are (a) a two-dimensional triangular lattice and (b) a three-dimensional face-centred cubic lattice. Direct numerical estimates of alpha ', beta and delta for (b) show that the scaling relations hold for this model and it is found that alpha '=0.655, beta =0.0538, gamma =1.24, nu =0.448, eta =-0.76, and delta =24 on the basis of the scaling relations. The numerical accuracy of the exponents beta and delta for model (a) does not permit an independent verification of the scaling relations; assuming scaling to hold here also, and using the exact result alpha '= 2 / 3 obtained by Baxter and Wu, it is found that gamma '=1.187+or-0.006, nu = 2 / 3 , eta =0.219+or-0.009, beta =0.073+or-0.003, and delta =17.3+or-0.8.
A series expansion for the free energy of the q-state Potts model on a square lattice is used to estimate the specific heat critical exponents. The analysis is based on a series transformation which was suggested by the known solution of the two-state Potts (Ising) model, and which makes optimum use of the duality theorem. The transformed series is quite smooth. Neville tables yield the estimates alpha (2)=0.0001+or-0.0003 for the two-state model, alpha (3)=0.296+or-0.002 for the three-state model, and alpha (4)=0.45+or-0.02 for the four-state model. The value for alpha (3) differs considerably from one reported by Straley and Fisher (1973), and substantially improves compliance with the Rushbrooke inequality.