and in the zero-field case which dictate the behaviour of the system close to were derived to be which are independent of the dimension of d when it is greater than four, the dimension being able to take any real value. References R. J. Baxter, Exactly solved models in statistical mechanics, London, Academic Press, 1982; 2007 Dover reprint, with a new chapter "Subsequent Developments" Further reading Lattice models Exactly solvable models